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bh=meRVHQbFdBhSIhJPbJEZ5sqaW1AaKZ7lh6KoSI5vmn0=; b=OGtXTz6sAkX6nqk198UV+xlASwRZTDw+2Uv7ew2fDRPe/eV+ekIqGFqAYLSQPogZOSw64M 2UYBs6ZdNASD81j2a+MJBTyTcdJhCVntOkAUIV8uUWvBkefr/ghh/bkNBt3fFpd6Rd2Unb t1aqmNaufo1sxCtjhvwtZv8jwT1pl/4= X-MC-Unique: PA6NytbLNsudN8-8Gr1Xeg-1 From: Stefan Hajnoczi To: qemu-devel@nongnu.org, Peter Maydell Subject: [PULL 3/3] aio-posix: keep aio_notify_me disabled during polling Date: Mon, 17 Aug 2020 16:16:34 +0100 Message-Id: <20200817151634.35563-4-stefanha@redhat.com> In-Reply-To: <20200817151634.35563-1-stefanha@redhat.com> References: <20200817151634.35563-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0.002 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; 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charset="utf-8" Polling only monitors the ctx->notified field and does not need the ctx->notifier EventNotifier to be signalled. Keep ctx->aio_notify_me disabled while polling to avoid unnecessary EventNotifier syscalls. This optimization improves virtio-blk 4KB random read performance by 18%. The following results are with an IOThread and the null-co block driver: Test IOPS Error Before 244518.62 =C2=B1 1.20% After 290706.11 =C2=B1 0.44% Signed-off-by: Stefan Hajnoczi Reviewed-by: Paolo Bonzini Reviewed-by: Philippe Mathieu-Daud=C3=A9 Message-id: 20200806131802.569478-4-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi --- util/aio-posix.c | 47 +++++++++++++++++++++++++---------------------- 1 file changed, 25 insertions(+), 22 deletions(-) diff --git a/util/aio-posix.c b/util/aio-posix.c index 1b2a3af65b..f7f13ebfc2 100644 --- a/util/aio-posix.c +++ b/util/aio-posix.c @@ -464,9 +464,6 @@ static bool remove_idle_poll_handlers(AioContext *ctx, = int64_t now) * * Polls for a given time. * - * Note that ctx->notify_me must be non-zero so this function can detect - * aio_notify(). - * * Note that the caller must have incremented ctx->list_lock. * * Returns: true if progress was made, false otherwise @@ -476,7 +473,6 @@ static bool run_poll_handlers(AioContext *ctx, int64_t = max_ns, int64_t *timeout) bool progress; int64_t start_time, elapsed_time; =20 - assert(ctx->notify_me); assert(qemu_lockcnt_count(&ctx->list_lock) > 0); =20 trace_run_poll_handlers_begin(ctx, max_ns, *timeout); @@ -520,8 +516,6 @@ static bool run_poll_handlers(AioContext *ctx, int64_t = max_ns, int64_t *timeout) * @timeout: timeout for blocking wait, computed by the caller and updated= if * polling succeeds. * - * ctx->notify_me must be non-zero so this function can detect aio_notify(= ). - * * Note that the caller must have incremented ctx->list_lock. * * Returns: true if progress was made, false otherwise @@ -556,6 +550,7 @@ bool aio_poll(AioContext *ctx, bool blocking) AioHandlerList ready_list =3D QLIST_HEAD_INITIALIZER(ready_list); int ret =3D 0; bool progress; + bool use_notify_me; int64_t timeout; int64_t start =3D 0; =20 @@ -566,33 +561,39 @@ bool aio_poll(AioContext *ctx, bool blocking) */ assert(in_aio_context_home_thread(ctx)); =20 - /* aio_notify can avoid the expensive event_notifier_set if + qemu_lockcnt_inc(&ctx->list_lock); + + if (ctx->poll_max_ns) { + start =3D qemu_clock_get_ns(QEMU_CLOCK_REALTIME); + } + + timeout =3D blocking ? aio_compute_timeout(ctx) : 0; + progress =3D try_poll_mode(ctx, &timeout); + assert(!(timeout && progress)); + + /* + * aio_notify can avoid the expensive event_notifier_set if * everything (file descriptors, bottom halves, timers) will * be re-evaluated before the next blocking poll(). This is * already true when aio_poll is called with blocking =3D=3D false; * if blocking =3D=3D true, it is only true after poll() returns, * so disable the optimization now. */ - if (blocking) { + use_notify_me =3D timeout !=3D 0; + if (use_notify_me) { atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2); /* - * Write ctx->notify_me before computing the timeout - * (reading bottom half flags, etc.). Pairs with + * Write ctx->notify_me before reading ctx->notified. Pairs with * smp_mb in aio_notify(). */ smp_mb(); - } - - qemu_lockcnt_inc(&ctx->list_lock); =20 - if (ctx->poll_max_ns) { - start =3D qemu_clock_get_ns(QEMU_CLOCK_REALTIME); + /* Don't block if aio_notify() was called */ + if (atomic_read(&ctx->notified)) { + timeout =3D 0; + } } =20 - timeout =3D blocking ? aio_compute_timeout(ctx) : 0; - progress =3D try_poll_mode(ctx, &timeout); - assert(!(timeout && progress)); - /* If polling is allowed, non-blocking aio_poll does not need the * system call---a single round of run_poll_handlers_once suffices. */ @@ -600,12 +601,14 @@ bool aio_poll(AioContext *ctx, bool blocking) ret =3D ctx->fdmon_ops->wait(ctx, &ready_list, timeout); } =20 - if (blocking) { + if (use_notify_me) { /* Finish the poll before clearing the flag. */ - atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me)= - 2); - aio_notify_accept(ctx); + atomic_store_release(&ctx->notify_me, + atomic_read(&ctx->notify_me) - 2); } =20 + aio_notify_accept(ctx); + /* Adjust polling time */ if (ctx->poll_max_ns) { int64_t block_ns =3D qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - star= t; --=20 2.26.2