On Wed, Jan 29, 2020 at 05:20:10PM +0000, Stefan Hajnoczi wrote: > 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. Ping? > 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 >
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