linux-kernel - Re: [PATCH linux-next] perf/ring_buffer: Add EPOLLRDNORM flag for poll

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Message-ID: <Z9MRWT5PWtPaY-pl@gmail.com>
Date: Thu, 13 Mar 2025 18:09:45 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
	namhyung@...nel.org, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH linux-next] perf/ring_buffer: Add EPOLLRDNORM flag for
 poll


* Tao Chen <chen.dylane@...ux.dev> wrote:

> 在 2025/3/13 18:05, Ingo Molnar 写道:
> > 
> > * Tao Chen <chen.dylane@...ux.dev> wrote:
> > 
> > > The poll man page says POLLRDNORM is equivalent to POLLIN,
> > > so add EPOLLRDNORM here.
> > > 
> > > Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
> > > ---
> > >   kernel/events/ring_buffer.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > > index 59a52b1a1..5130b119d 100644
> > > --- a/kernel/events/ring_buffer.c
> > > +++ b/kernel/events/ring_buffer.c
> > > @@ -19,7 +19,7 @@
> > >   static void perf_output_wakeup(struct perf_output_handle *handle)
> > >   {
> > > -	atomic_set(&handle->rb->poll, EPOLLIN);
> > > +	atomic_set(&handle->rb->poll, EPOLLIN | EPOLLRDNORM);
> > 
> > So what does EPOLLRDNORM mean to begin with? There doesn't seem to be
> > separate/specific handling of it anywhere in the kernel that I can
> > see...
> > 
> 
> It seems that if user set pollfd with POLLRDNORM, perf_poll will not return
> until timeout even if perf_output_wakeup called, whereas POLLIN returns.

Mind adding this to the changelog, and explain that this patch fixes 
this particular poll() functionality and semantics for userspace?

Thanks,

	Ingo