linux-kernel - Re: [RFC PATCH] epoll: Add synchronous wakeup support for ep_poll

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Message-ID: <ZxAOgj9RWm4NTl9d@google.com>
Date: Wed, 16 Oct 2024 15:05:38 -0400
From: Brian Geffon <bgeffon@...gle.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, jack@...e.cz,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
	cmllamas@...gle.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, ke.wang@...soc.com,
	jing.xia@...soc.com, xuewen.yan94@...il.com,
	viro@...iv.linux.org.uk, mingo@...hat.com, peterz@...radead.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	Brian Geffon <bgeffon@...gle.com>, stable@...r.kernel.org,
	lizeb@...gle.com
Subject: Re: [RFC PATCH] epoll: Add synchronous wakeup support for
 ep_poll_callback

On Wed, Oct 16, 2024 at 03:10:34PM +0200, Christian Brauner wrote:
> On Fri, 26 Apr 2024 16:05:48 +0800, Xuewen Yan wrote:
> > Now, the epoll only use wake_up() interface to wake up task.
> > However, sometimes, there are epoll users which want to use
> > the synchronous wakeup flag to hint the scheduler, such as
> > Android binder driver.
> > So add a wake_up_sync() define, and use the wake_up_sync()
> > when the sync is true in ep_poll_callback().
> > 
> > [...]
> 
> Applied to the vfs.misc branch of the vfs/vfs.git tree.
> Patches in the vfs.misc branch should appear in linux-next soon.
> 
> Please report any outstanding bugs that were missed during review in a
> new review to the original patch series allowing us to drop it.
> 
> It's encouraged to provide Acked-bys and Reviewed-bys even though the
> patch has now been applied. If possible patch trailers will be updated.
> 
> Note that commit hashes shown below are subject to change due to rebase,
> trailer updates or similar. If in doubt, please check the listed branch.
> 
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
> branch: vfs.misc

This is a bug that's been present for all of time, so I think we should:

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") 
Cc: stable@...r.kernel.org

I sent a patch which adds a benchmark for nonblocking pipes using epoll:
https://lore.kernel.org/lkml/20241016190009.866615-1-bgeffon@google.com/

Using this new benchmark I get the following results without this fix
and with this fix:

$ tools/perf/perf bench sched pipe -n
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

     Total time: 12.194 [sec]

      12.194376 usecs/op
          82005 ops/sec


$ tools/perf/perf bench sched pipe -n
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

     Total time: 9.229 [sec]

       9.229738 usecs/op
         108345 ops/sec

> 
> [1/1] epoll: Add synchronous wakeup support for ep_poll_callback
>       https://git.kernel.org/vfs/vfs/c/2ce0e17660a7