linux-kernel - Re: [PATCH v2] fs: Improve eventpoll logging to stop indicting timerfd

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Date:   Thu, 1 Apr 2021 02:28:55 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Manish Varma <varmam@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, Kelly Rossmoyer <krossmo@...gle.com>
Subject: Re: [PATCH v2] fs: Improve eventpoll logging to stop indicting
 timerfd

On Wed, Mar 31, 2021 at 07:16:45PM -0700, Manish Varma wrote:
> timerfd doesn't create any wakelocks, but eventpoll can.  When it does,
> it names them after the underlying file descriptor, and since all
> timerfd file descriptors are named "[timerfd]" (which saves memory on
> systems like desktops with potentially many timerfd instances), all
> wakesources created as a result of using the eventpoll-on-timerfd idiom
> are called... "[timerfd]".
> 
> However, it becomes impossible to tell which "[timerfd]" wakesource is
> affliated with which process and hence troubleshooting is difficult.
> 
> This change addresses this problem by changing the way eventpoll
> wakesources are named:
> 
> 1) the top-level per-process eventpoll wakesource is now named "epoll:P"
> (instead of just "eventpoll"), where P, is the PID of the creating
> process.
> 2) individual per-underlying-filedescriptor eventpoll wakesources are
> now named "epollitemN:P.F", where N is a unique ID token and P is PID
> of the creating process and F is the name of the underlying file
> descriptor.
> 
> All together that should be splitted up into a change to eventpoll and
> timerfd (or other file descriptors).

FWIW, it smells like a variant of wakeup_source_register() that would
take printf format + arguments would be a good idea.  I.e. something
like

> +		snprintf(buf, sizeof(buf), "epoll:%d", task_pid);
> +		epi->ep->ws = wakeup_source_register(NULL, buf);

		... = wakeup_source_register(NULL, "epoll:%d", task_pid);

etc.