Re: [RFC] eventfd: add EFD_AUTORESET flag

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Feb 04, 2020 at 03:40:35PM +0000, Stefan Hajnoczi wrote:
> 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@redhat.com>
> > ---
> > 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?

Ping?

I would appreciate an indication of whether EFD_AUTORESET is an
acceptable new feature.  Thanks!

> > 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
> >