linux-kernel - Re: [PATCH v5] fs: Improve eventpoll logging to stop indicting timerfd

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Date: Tue, 25 Jun 2024 10:43:41 -0700
From: Isaac Manjarres <isaacmanjarres@...gle.com>
To: John Stultz <jstultz@...gle.com>
Cc: tglx@...utronix.de, "Rafael J. Wysocki" <rafael@...nel.org>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	saravanak@...gle.com, Manish Varma <varmam@...gle.com>,
	Kelly Rossmoyer <krossmo@...gle.com>, kernel-team@...roid.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5] fs: Improve eventpoll logging to stop indicting
 timerfd

On Mon, Jun 24, 2024 at 11:03:43AM -0700, John Stultz wrote:
> On Thu, Jun 6, 2024 at 10:28 AM 'Isaac J. Manjarres' via kernel-team
> <kernel-team@...roid.com> wrote:
> >
> > From: Manish Varma <varmam@...gle.com>
> >
> > 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.
> 
> While your explanation above is understandable, I feel like it might
> benefit from a more concrete example to show why this is problematic?
> It feels like the description gets into the weeds pretty quickly and
> makes it hard to understand the importance of the change.

> 
> Again the N:P.F mapping is clear, but maybe including a specific
> before and after example would help?
> 
> Additionally, once you have this better named wakesource, can you
> provide a specific example to illustrate a bit on how this
> specifically helps the troubleshooting that was difficult before?
> 
> thanks
> -john

Hi John,

Thanks for your feedback on this! I'm more than happy to add more
details to the commit text. I'll go ahead and add an example to
showcase a scenario where the proposed changes make debugging easier.

I'll send out v6 of the patch soon.

--Isaac