Some applications simply use eventfd for inter-thread notifications without requiring counter or semaphore semantics. They wait for the eventfd to become readable using poll(2)/select(2) and then call read(2) to reset the counter. This patch adds the EFD_AUTORESET flag to reset the counter when f_ops->poll() finds the eventfd is readable, eliminating the need to call read(2) to reset the counter. This results in a small but measurable 1% performance improvement with QEMU virtio-blk emulation. Each read(2) takes 1 microsecond execution time in the event loop according to perf. Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx> --- Does this look like a reasonable thing to do? I'm not very familiar with f_ops->poll() or the eventfd internals, so maybe I'm overlooking a design flaw. I've tested this with QEMU and it works fine: https://github.com/stefanha/qemu/commits/eventfd-autoreset --- fs/eventfd.c | 99 +++++++++++++++++++++++++---------------- include/linux/eventfd.h | 3 +- 2 files changed, 62 insertions(+), 40 deletions(-) diff --git a/fs/eventfd.c b/fs/eventfd.c index 8aa0ea8c55e8..208f6b9e2234 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -116,45 +116,62 @@ static __poll_t eventfd_poll(struct file *file, poll_table *wait) poll_wait(file, &ctx->wqh, wait); - /* - * All writes to ctx->count occur within ctx->wqh.lock. This read - * can be done outside ctx->wqh.lock because we know that poll_wait - * takes that lock (through add_wait_queue) if our caller will sleep. - * - * The read _can_ therefore seep into add_wait_queue's critical - * section, but cannot move above it! add_wait_queue's spin_lock acts - * as an acquire barrier and ensures that the read be ordered properly - * against the writes. The following CAN happen and is safe: - * - * poll write - * ----------------- ------------ - * lock ctx->wqh.lock (in poll_wait) - * count = ctx->count - * __add_wait_queue - * unlock ctx->wqh.lock - * lock ctx->qwh.lock - * ctx->count += n - * if (waitqueue_active) - * wake_up_locked_poll - * unlock ctx->qwh.lock - * eventfd_poll returns 0 - * - * but the following, which would miss a wakeup, cannot happen: - * - * poll write - * ----------------- ------------ - * count = ctx->count (INVALID!) - * lock ctx->qwh.lock - * ctx->count += n - * **waitqueue_active is false** - * **no wake_up_locked_poll!** - * unlock ctx->qwh.lock - * lock ctx->wqh.lock (in poll_wait) - * __add_wait_queue - * unlock ctx->wqh.lock - * eventfd_poll returns 0 - */ - count = READ_ONCE(ctx->count); + if (ctx->flags & EFD_AUTORESET) { + unsigned long flags; + __poll_t requested = poll_requested_events(wait); + + spin_lock_irqsave(&ctx->wqh.lock, flags); + count = ctx->count; + + /* Reset counter if caller is polling for read */ + if (count != 0 && (requested & EPOLLIN)) { + ctx->count = 0; + events |= EPOLLOUT; + /* TODO is a EPOLLOUT wakeup necessary here? */ + } + + spin_unlock_irqrestore(&ctx->wqh.lock, flags); + } else { + /* + * All writes to ctx->count occur within ctx->wqh.lock. This read + * can be done outside ctx->wqh.lock because we know that poll_wait + * takes that lock (through add_wait_queue) if our caller will sleep. + * + * The read _can_ therefore seep into add_wait_queue's critical + * section, but cannot move above it! add_wait_queue's spin_lock acts + * as an acquire barrier and ensures that the read be ordered properly + * against the writes. The following CAN happen and is safe: + * + * poll write + * ----------------- ------------ + * lock ctx->wqh.lock (in poll_wait) + * count = ctx->count + * __add_wait_queue + * unlock ctx->wqh.lock + * lock ctx->qwh.lock + * ctx->count += n + * if (waitqueue_active) + * wake_up_locked_poll + * unlock ctx->qwh.lock + * eventfd_poll returns 0 + * + * but the following, which would miss a wakeup, cannot happen: + * + * poll write + * ----------------- ------------ + * count = ctx->count (INVALID!) + * lock ctx->qwh.lock + * ctx->count += n + * **waitqueue_active is false** + * **no wake_up_locked_poll!** + * unlock ctx->qwh.lock + * lock ctx->wqh.lock (in poll_wait) + * __add_wait_queue + * unlock ctx->wqh.lock + * eventfd_poll returns 0 + */ + count = READ_ONCE(ctx->count); + } if (count > 0) events |= EPOLLIN; @@ -400,6 +417,10 @@ static int do_eventfd(unsigned int count, int flags) if (flags & ~EFD_FLAGS_SET) return -EINVAL; + /* Semaphore semantics don't make sense when autoreset is enabled */ + if ((flags & EFD_SEMAPHORE) && (flags & EFD_AUTORESET)) + return -EINVAL; + ctx = kmalloc(sizeof(*ctx), GFP_KERNEL); if (!ctx) return -ENOMEM; diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h index ffcc7724ca21..27577fafc553 100644 --- a/include/linux/eventfd.h +++ b/include/linux/eventfd.h @@ -21,11 +21,12 @@ * shared O_* flags. */ #define EFD_SEMAPHORE (1 << 0) +#define EFD_AUTORESET (1 << 6) /* aliases O_CREAT */ #define EFD_CLOEXEC O_CLOEXEC #define EFD_NONBLOCK O_NONBLOCK #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK) -#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE) +#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE | EFD_AUTORESET) struct eventfd_ctx; struct file; -- 2.24.1
