The AioContext event loop uses ppoll(2) or epoll_wait(2) to monitor file descriptors or until a timer expires. In cases like virtqueues, Linux AIO, and ThreadPool it is technically possible to wait for events via polling (i.e. continuously checking for events without blocking). Polling can be faster than blocking syscalls because file descriptors, the process scheduler, and system calls are bypassed. The main disadvantage to polling is that it increases CPU utilization. In classic polling configuration a full host CPU thread might run at 100% to respond to events as quickly as possible. This patch implements a timeout so we fall back to blocking syscalls if polling detects no activity. After the timeout no CPU cycles are wasted on polling until the next event loop iteration. The run_poll_handlers_begin() and run_poll_handlers_end() trace events are added to aid performance analysis and troubleshooting. If you need to know whether polling mode is being used, trace these events to find out. Note that the AioContext is now re-acquired before disabling notify_me in the non-polling case. This makes the code cleaner since notify_me was enabled outside the non-polling AioContext release region. This change is correct since it's safe to keep notify_me enabled longer (disabling is an optimization) but potentially causes unnecessary event_notifer_set() calls. I think the chance of performance regression is small here. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20161201192652.9509-4-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
415 lines
10 KiB
C
415 lines
10 KiB
C
/*
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* QEMU System Emulator
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*
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* Copyright (c) 2003-2008 Fabrice Bellard
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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#include "qemu/osdep.h"
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#include "qapi/error.h"
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#include "qemu-common.h"
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#include "block/aio.h"
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#include "block/thread-pool.h"
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#include "qemu/main-loop.h"
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#include "qemu/atomic.h"
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#include "block/raw-aio.h"
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/***********************************************************/
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/* bottom halves (can be seen as timers which expire ASAP) */
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struct QEMUBH {
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AioContext *ctx;
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QEMUBHFunc *cb;
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void *opaque;
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QEMUBH *next;
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bool scheduled;
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bool idle;
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bool deleted;
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};
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void aio_bh_schedule_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
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{
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QEMUBH *bh;
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bh = g_new(QEMUBH, 1);
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*bh = (QEMUBH){
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.ctx = ctx,
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.cb = cb,
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.opaque = opaque,
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};
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qemu_mutex_lock(&ctx->bh_lock);
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bh->next = ctx->first_bh;
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bh->scheduled = 1;
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bh->deleted = 1;
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/* Make sure that the members are ready before putting bh into list */
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smp_wmb();
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ctx->first_bh = bh;
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qemu_mutex_unlock(&ctx->bh_lock);
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aio_notify(ctx);
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}
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QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
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{
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QEMUBH *bh;
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bh = g_new(QEMUBH, 1);
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*bh = (QEMUBH){
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.ctx = ctx,
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.cb = cb,
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.opaque = opaque,
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};
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qemu_mutex_lock(&ctx->bh_lock);
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bh->next = ctx->first_bh;
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/* Make sure that the members are ready before putting bh into list */
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smp_wmb();
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ctx->first_bh = bh;
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qemu_mutex_unlock(&ctx->bh_lock);
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return bh;
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}
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void aio_bh_call(QEMUBH *bh)
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{
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bh->cb(bh->opaque);
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}
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/* Multiple occurrences of aio_bh_poll cannot be called concurrently */
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int aio_bh_poll(AioContext *ctx)
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{
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QEMUBH *bh, **bhp, *next;
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int ret;
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ctx->walking_bh++;
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ret = 0;
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for (bh = ctx->first_bh; bh; bh = next) {
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/* Make sure that fetching bh happens before accessing its members */
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smp_read_barrier_depends();
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next = bh->next;
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/* The atomic_xchg is paired with the one in qemu_bh_schedule. The
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* implicit memory barrier ensures that the callback sees all writes
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* done by the scheduling thread. It also ensures that the scheduling
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* thread sees the zero before bh->cb has run, and thus will call
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* aio_notify again if necessary.
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*/
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if (atomic_xchg(&bh->scheduled, 0)) {
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/* Idle BHs don't count as progress */
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if (!bh->idle) {
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ret = 1;
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}
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bh->idle = 0;
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aio_bh_call(bh);
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}
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}
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ctx->walking_bh--;
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/* remove deleted bhs */
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if (!ctx->walking_bh) {
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qemu_mutex_lock(&ctx->bh_lock);
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bhp = &ctx->first_bh;
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while (*bhp) {
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bh = *bhp;
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if (bh->deleted && !bh->scheduled) {
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*bhp = bh->next;
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g_free(bh);
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} else {
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bhp = &bh->next;
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}
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}
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qemu_mutex_unlock(&ctx->bh_lock);
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}
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return ret;
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}
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void qemu_bh_schedule_idle(QEMUBH *bh)
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{
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bh->idle = 1;
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/* Make sure that idle & any writes needed by the callback are done
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* before the locations are read in the aio_bh_poll.
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*/
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atomic_mb_set(&bh->scheduled, 1);
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}
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void qemu_bh_schedule(QEMUBH *bh)
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{
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AioContext *ctx;
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ctx = bh->ctx;
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bh->idle = 0;
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/* The memory barrier implicit in atomic_xchg makes sure that:
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* 1. idle & any writes needed by the callback are done before the
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* locations are read in the aio_bh_poll.
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* 2. ctx is loaded before scheduled is set and the callback has a chance
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* to execute.
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*/
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if (atomic_xchg(&bh->scheduled, 1) == 0) {
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aio_notify(ctx);
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}
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}
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/* This func is async.
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*/
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void qemu_bh_cancel(QEMUBH *bh)
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{
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bh->scheduled = 0;
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}
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/* This func is async.The bottom half will do the delete action at the finial
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* end.
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*/
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void qemu_bh_delete(QEMUBH *bh)
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{
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bh->scheduled = 0;
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bh->deleted = 1;
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}
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int64_t
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aio_compute_timeout(AioContext *ctx)
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{
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int64_t deadline;
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int timeout = -1;
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QEMUBH *bh;
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for (bh = ctx->first_bh; bh; bh = bh->next) {
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if (bh->scheduled) {
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if (bh->idle) {
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/* idle bottom halves will be polled at least
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* every 10ms */
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timeout = 10000000;
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} else {
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/* non-idle bottom halves will be executed
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* immediately */
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return 0;
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}
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}
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}
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deadline = timerlistgroup_deadline_ns(&ctx->tlg);
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if (deadline == 0) {
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return 0;
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} else {
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return qemu_soonest_timeout(timeout, deadline);
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}
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}
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static gboolean
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aio_ctx_prepare(GSource *source, gint *timeout)
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{
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AioContext *ctx = (AioContext *) source;
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atomic_or(&ctx->notify_me, 1);
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/* We assume there is no timeout already supplied */
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*timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));
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if (aio_prepare(ctx)) {
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*timeout = 0;
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}
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return *timeout == 0;
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}
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static gboolean
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aio_ctx_check(GSource *source)
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{
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AioContext *ctx = (AioContext *) source;
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QEMUBH *bh;
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atomic_and(&ctx->notify_me, ~1);
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aio_notify_accept(ctx);
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for (bh = ctx->first_bh; bh; bh = bh->next) {
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if (bh->scheduled) {
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return true;
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}
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}
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return aio_pending(ctx) || (timerlistgroup_deadline_ns(&ctx->tlg) == 0);
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}
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static gboolean
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aio_ctx_dispatch(GSource *source,
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GSourceFunc callback,
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gpointer user_data)
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{
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AioContext *ctx = (AioContext *) source;
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assert(callback == NULL);
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aio_dispatch(ctx, true);
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return true;
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}
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static void
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aio_ctx_finalize(GSource *source)
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{
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AioContext *ctx = (AioContext *) source;
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thread_pool_free(ctx->thread_pool);
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#ifdef CONFIG_LINUX_AIO
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if (ctx->linux_aio) {
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laio_detach_aio_context(ctx->linux_aio, ctx);
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laio_cleanup(ctx->linux_aio);
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ctx->linux_aio = NULL;
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}
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#endif
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qemu_mutex_lock(&ctx->bh_lock);
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while (ctx->first_bh) {
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QEMUBH *next = ctx->first_bh->next;
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/* qemu_bh_delete() must have been called on BHs in this AioContext */
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assert(ctx->first_bh->deleted);
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g_free(ctx->first_bh);
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ctx->first_bh = next;
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}
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qemu_mutex_unlock(&ctx->bh_lock);
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aio_set_event_notifier(ctx, &ctx->notifier, false, NULL, NULL);
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event_notifier_cleanup(&ctx->notifier);
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qemu_rec_mutex_destroy(&ctx->lock);
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qemu_mutex_destroy(&ctx->bh_lock);
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timerlistgroup_deinit(&ctx->tlg);
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}
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static GSourceFuncs aio_source_funcs = {
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aio_ctx_prepare,
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aio_ctx_check,
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aio_ctx_dispatch,
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aio_ctx_finalize
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};
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GSource *aio_get_g_source(AioContext *ctx)
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{
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g_source_ref(&ctx->source);
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return &ctx->source;
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}
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ThreadPool *aio_get_thread_pool(AioContext *ctx)
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{
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if (!ctx->thread_pool) {
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ctx->thread_pool = thread_pool_new(ctx);
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}
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return ctx->thread_pool;
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}
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#ifdef CONFIG_LINUX_AIO
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LinuxAioState *aio_get_linux_aio(AioContext *ctx)
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{
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if (!ctx->linux_aio) {
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ctx->linux_aio = laio_init();
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laio_attach_aio_context(ctx->linux_aio, ctx);
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}
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return ctx->linux_aio;
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}
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#endif
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void aio_notify(AioContext *ctx)
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{
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/* Write e.g. bh->scheduled before reading ctx->notify_me. Pairs
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* with atomic_or in aio_ctx_prepare or atomic_add in aio_poll.
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*/
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smp_mb();
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if (ctx->notify_me) {
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event_notifier_set(&ctx->notifier);
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atomic_mb_set(&ctx->notified, true);
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}
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}
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void aio_notify_accept(AioContext *ctx)
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{
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if (atomic_xchg(&ctx->notified, false)) {
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event_notifier_test_and_clear(&ctx->notifier);
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}
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}
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static void aio_timerlist_notify(void *opaque)
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{
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aio_notify(opaque);
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}
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static void event_notifier_dummy_cb(EventNotifier *e)
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{
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}
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/* Returns true if aio_notify() was called (e.g. a BH was scheduled) */
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static bool event_notifier_poll(void *opaque)
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{
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EventNotifier *e = opaque;
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AioContext *ctx = container_of(e, AioContext, notifier);
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return atomic_read(&ctx->notified);
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}
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AioContext *aio_context_new(Error **errp)
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{
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int ret;
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AioContext *ctx;
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ctx = (AioContext *) g_source_new(&aio_source_funcs, sizeof(AioContext));
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aio_context_setup(ctx);
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ret = event_notifier_init(&ctx->notifier, false);
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if (ret < 0) {
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error_setg_errno(errp, -ret, "Failed to initialize event notifier");
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goto fail;
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}
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g_source_set_can_recurse(&ctx->source, true);
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aio_set_event_notifier(ctx, &ctx->notifier,
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false,
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(EventNotifierHandler *)
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event_notifier_dummy_cb,
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event_notifier_poll);
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#ifdef CONFIG_LINUX_AIO
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ctx->linux_aio = NULL;
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#endif
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ctx->thread_pool = NULL;
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qemu_mutex_init(&ctx->bh_lock);
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qemu_rec_mutex_init(&ctx->lock);
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timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
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ctx->poll_max_ns = 0;
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return ctx;
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fail:
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g_source_destroy(&ctx->source);
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return NULL;
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}
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void aio_context_ref(AioContext *ctx)
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{
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g_source_ref(&ctx->source);
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}
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void aio_context_unref(AioContext *ctx)
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{
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g_source_unref(&ctx->source);
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}
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void aio_context_acquire(AioContext *ctx)
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{
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qemu_rec_mutex_lock(&ctx->lock);
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}
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void aio_context_release(AioContext *ctx)
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{
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qemu_rec_mutex_unlock(&ctx->lock);
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}
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