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charset="utf-8" The IOThread's AioContext is currently created in iothread_init() where it's easy to propagate errors before spawning the thread that runs iothread_run(). However, this means that aio_context_new() is called from the main loop thread rather than from the IOThread. In order to use Linux io_uring's IORING_SETUP_SINGLE_ISSUER feature in the next commit, only one thread can use the io_uring context and therefore iothread.c must call aio_context_new() from iothread_run() instead of iothread_init(). Extract the iothread_run() arguments into an IOThreadRunArgs struct where an Error *error field can be used to report back initialization errors. This works pretty well thanks to the init_done_sem semaphore that is already used by iothread_init() to wait for iothread_run() to initialize. Move iothread_run() further down for proximity with iothread_init() and to avoid adding a function prototype for iothread_set_aio_context_params(). Signed-off-by: Stefan Hajnoczi --- include/system/iothread.h | 1 - iothread.c | 140 +++++++++++++++++++++----------------- 2 files changed, 78 insertions(+), 63 deletions(-) diff --git a/include/system/iothread.h b/include/system/iothread.h index d95c17a645..ec4e798d5e 100644 --- a/include/system/iothread.h +++ b/include/system/iothread.h @@ -29,7 +29,6 @@ struct IOThread { bool run_gcontext; /* whether we should run gcontext */ GMainContext *worker_context; GMainLoop *main_loop; - QemuSemaphore init_done_sem; /* is thread init done? */ bool stopping; /* has iothread_stop() been called? */ bool running; /* should iothread_run() continue? */ int thread_id; diff --git a/iothread.c b/iothread.c index 8810376dce..c6547779d0 100644 --- a/iothread.c +++ b/iothread.c @@ -36,46 +36,6 @@ #define IOTHREAD_POLL_MAX_NS_DEFAULT 0ULL #endif =20 -static void *iothread_run(void *opaque) -{ - IOThread *iothread =3D opaque; - - rcu_register_thread(); - /* - * g_main_context_push_thread_default() must be called before anything - * in this new thread uses glib. - */ - g_main_context_push_thread_default(iothread->worker_context); - qemu_set_current_aio_context(iothread->ctx); - iothread->thread_id =3D qemu_get_thread_id(); - qemu_sem_post(&iothread->init_done_sem); - - while (iothread->running) { - /* - * Note: from functional-wise the g_main_loop_run() below can - * already cover the aio_poll() events, but we can't run the - * main loop unconditionally because explicit aio_poll() here - * is faster than g_main_loop_run() when we do not need the - * gcontext at all (e.g., pure block layer iothreads). In - * other words, when we want to run the gcontext with the - * iothread we need to pay some performance for functionality. - */ - aio_poll(iothread->ctx, true); - - /* - * We must check the running state again in case it was - * changed in previous aio_poll() - */ - if (iothread->running && qatomic_read(&iothread->run_gcontext)) { - g_main_loop_run(iothread->main_loop); - } - } - - g_main_context_pop_thread_default(iothread->worker_context); - rcu_unregister_thread(); - return NULL; -} - /* Runs in iothread_run() thread */ static void iothread_stop_bh(void *opaque) { @@ -104,7 +64,6 @@ static void iothread_instance_init(Object *obj) =20 iothread->poll_max_ns =3D IOTHREAD_POLL_MAX_NS_DEFAULT; iothread->thread_id =3D -1; - qemu_sem_init(&iothread->init_done_sem, 0); /* By default, we don't run gcontext */ qatomic_set(&iothread->run_gcontext, 0); } @@ -135,7 +94,6 @@ static void iothread_instance_finalize(Object *obj) g_main_loop_unref(iothread->main_loop); iothread->main_loop =3D NULL; } - qemu_sem_destroy(&iothread->init_done_sem); } =20 static void iothread_init_gcontext(IOThread *iothread, const char *thread_= name) @@ -176,47 +134,105 @@ static void iothread_set_aio_context_params(EventLoo= pBase *base, Error **errp) base->thread_pool_max, errp); } =20 +typedef struct { + IOThread *iothread; + const char *thread_name; + QemuSemaphore init_done_sem; /* is thread init done? */ + Error *error; /* filled in before init_done_sem is posted */ +} IOThreadRunArgs; =20 -static void iothread_init(EventLoopBase *base, Error **errp) +static void *iothread_run(void *opaque) { - Error *local_error =3D NULL; - IOThread *iothread =3D IOTHREAD(base); - g_autofree char *thread_name =3D NULL; + IOThreadRunArgs *args =3D opaque; + IOThread *iothread =3D args->iothread; =20 - iothread->stopping =3D false; - iothread->running =3D true; - iothread->ctx =3D aio_context_new(errp); + rcu_register_thread(); + + iothread->ctx =3D aio_context_new(&args->error); if (!iothread->ctx) { - return; + goto out; } =20 - thread_name =3D g_strdup_printf("IO %s", - object_get_canonical_path_component(OBJECT(base))); + iothread_set_aio_context_params(EVENT_LOOP_BASE(iothread), &args->erro= r); + if (args->error) { + aio_context_unref(iothread->ctx); + iothread->ctx =3D NULL; + goto out; + } =20 /* * Init one GMainContext for the iothread unconditionally, even if * it's not used */ - iothread_init_gcontext(iothread, thread_name); + iothread_init_gcontext(iothread, args->thread_name); =20 - iothread_set_aio_context_params(base, &local_error); - if (local_error) { - error_propagate(errp, local_error); - aio_context_unref(iothread->ctx); - iothread->ctx =3D NULL; - return; + /* + * g_main_context_push_thread_default() must be called before anything + * in this new thread uses glib. + */ + g_main_context_push_thread_default(iothread->worker_context); + qemu_set_current_aio_context(iothread->ctx); + + iothread->stopping =3D false; + iothread->running =3D true; + + iothread->thread_id =3D qemu_get_thread_id(); + qemu_sem_post(&args->init_done_sem); + + while (iothread->running) { + /* + * Note: from functional-wise the g_main_loop_run() below can + * already cover the aio_poll() events, but we can't run the + * main loop unconditionally because explicit aio_poll() here + * is faster than g_main_loop_run() when we do not need the + * gcontext at all (e.g., pure block layer iothreads). In + * other words, when we want to run the gcontext with the + * iothread we need to pay some performance for functionality. + */ + aio_poll(iothread->ctx, true); + + /* + * We must check the running state again in case it was + * changed in previous aio_poll() + */ + if (iothread->running && qatomic_read(&iothread->run_gcontext)) { + g_main_loop_run(iothread->main_loop); + } } =20 + g_main_context_pop_thread_default(iothread->worker_context); +out: + rcu_unregister_thread(); + return NULL; +} + +static void iothread_init(EventLoopBase *base, Error **errp) +{ + IOThread *iothread =3D IOTHREAD(base); + g_autofree char *thread_name =3D NULL; + IOThreadRunArgs args =3D { + .iothread =3D iothread, + }; + + qemu_sem_init(&args.init_done_sem, 0); + + thread_name =3D g_strdup_printf("IO %s", + object_get_canonical_path_component(OBJECT(base))); + args.thread_name =3D thread_name; + /* This assumes we are called from a thread with useful CPU affinity f= or us * to inherit. */ - qemu_thread_create(&iothread->thread, thread_name, iothread_run, - iothread, QEMU_THREAD_JOINABLE); + qemu_thread_create(&iothread->thread, thread_name, iothread_run, &args, + QEMU_THREAD_JOINABLE); =20 /* Wait for initialization to complete */ while (iothread->thread_id =3D=3D -1) { - qemu_sem_wait(&iothread->init_done_sem); + qemu_sem_wait(&args.init_done_sem); } + + qemu_sem_destroy(&args.init_done_sem); + error_propagate(errp, args.error); } =20 typedef struct { --=20 2.50.1 From nobody Sat Nov 15 09:23:59 2025 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass(p=quarantine dis=none) header.from=redhat.com ARC-Seal: i=1; a=rsa-sha256; t=1753390118; cv=none; d=zohomail.com; s=zohoarc; b=cU73tTQh5uUnCsup9/q6yaMx57CkLqo8/IVXb/iZETcd6q0gdCh6AQnuuZyQ8bCaCZVQwMPWEtCmGpaxHJDOpN28CHnCgtPMZ3pHojrw01sg9zOgUqBMh8+/k8nGx4qeQnxuaK4D9IyDYQa6pbRsvOIrB4hWAVDiCS9DikMsQZg= ARC-Message-Signature: i=1; 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charset="utf-8" IORING_SETUP_SINGLE_ISSUER enables optimizations in the host Linux kernel's io_uring code when the io_uring context is only used from a single thread. This is true is QEMU because io_uring SQEs are submitted from the same thread that processes the CQEs. Signed-off-by: Stefan Hajnoczi --- util/fdmon-io_uring.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c index 28b93c8ab9..4798439097 100644 --- a/util/fdmon-io_uring.c +++ b/util/fdmon-io_uring.c @@ -456,13 +456,30 @@ static const FDMonOps fdmon_io_uring_ops =3D { .add_sqe =3D fdmon_io_uring_add_sqe, }; =20 +static inline bool is_creating_iothread(void) +{ + return qemu_get_thread_id() !=3D getpid(); +} + void fdmon_io_uring_setup(AioContext *ctx, Error **errp) { int ret; + /* TODO only enable these flags if they are available in the host's ke= rnel headers */ + unsigned flags =3D 0; =20 ctx->io_uring_fd_tag =3D NULL; =20 - ret =3D io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uri= ng, 0); + /* + * The main thread's AioContexts are created from the main loop thread= but + * may be accessed from multiple threads (e.g. vCPUs or the migration + * thread). IOThread AioContexts are only accessed from the IOThread + * itself. + */ + if (is_creating_iothread()) { + flags =3D IORING_SETUP_SINGLE_ISSUER; + } + + ret =3D io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uri= ng, flags); if (ret !=3D 0) { error_setg_errno(errp, -ret, "Failed to initialize io_uring"); return; --=20 2.50.1 From nobody Sat Nov 15 09:23:59 2025 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass(p=quarantine dis=none) header.from=redhat.com ARC-Seal: i=1; a=rsa-sha256; t=1753390074; cv=none; d=zohomail.com; s=zohoarc; b=FJS9ndoUeZe/Wy375KotYnbrfMt183d+PYTkXjsDd0rO2L7J3IkHA2JXOaMZxmgqmoQAnZopSHAWCUItY0vsqsbVEU6MwD2bgk9GewiGlFVJIcCfh34w2MRYWST0V4FXTR9fb3D/Cv2OYlAS0hvLpn9L2EZWl+8jVOiFvwsxj90= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; 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charset="utf-8" The IORING_SETUP_COOP_TASKRUN flag reduces interprocessor interrupts when an io_uring event occurs on a different CPU. The idea is that the QEMU thread will wait for a CQE anyway, so there is no need to interrupt the CPU that it is on. The IORING_SETUP_TASKRUN_FLAG ensures that QEMU's io_uring CQ ring polling still works with COOP_TASKRUN. The kernel will set a flag in the SQ ring (this is not a typo, the flag is located in the SQ ring even though it pertains to the CQ ring) that can be polled from userspace. Signed-off-by: Stefan Hajnoczi --- util/fdmon-io_uring.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c index 4798439097..649dc18907 100644 --- a/util/fdmon-io_uring.c +++ b/util/fdmon-io_uring.c @@ -428,13 +428,16 @@ static int fdmon_io_uring_wait(AioContext *ctx, AioHa= ndlerList *ready_list, =20 static bool fdmon_io_uring_need_wait(AioContext *ctx) { + struct io_uring *ring =3D &ctx->fdmon_io_uring; + /* Have io_uring events completed? */ - if (io_uring_cq_ready(&ctx->fdmon_io_uring)) { + if (io_uring_cq_ready(ring) || + IO_URING_READ_ONCE(*ring->sq.kflags) & IORING_SQ_TASKRUN) { return true; } =20 /* Are there pending sqes to submit? */ - if (io_uring_sq_ready(&ctx->fdmon_io_uring)) { + if (io_uring_sq_ready(ring)) { return true; } =20 @@ -465,7 +468,7 @@ void fdmon_io_uring_setup(AioContext *ctx, Error **errp) { int ret; /* TODO only enable these flags if they are available in the host's ke= rnel headers */ - unsigned flags =3D 0; + unsigned flags =3D IORING_SETUP_COOP_TASKRUN | IORING_SETUP_TASKRUN_FL= AG; =20 ctx->io_uring_fd_tag =3D NULL; =20 --=20 2.50.1