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

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Message-ID: <ZoXEiCvdNRr_tj2N@google.com>
Date: Wed, 3 Jul 2024 14:37:12 -0700
From: Isaac Manjarres <isaacmanjarres@...gle.com>
To: Mateusz Guzik <mjguzik@...il.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 Tue, Jun 25, 2024 at 07:58:43PM +0200, Mateusz Guzik wrote:
> On Thu, Jun 06, 2024 at 10:28:11AM -0700, Isaac J. Manjarres wrote:
> > +static atomic_t wakesource_create_id  = ATOMIC_INIT(0);
> >  static const struct file_operations eventpoll_fops;
> >  
> >  static inline int is_file_epoll(struct file *f)
> > @@ -1545,15 +1546,21 @@ static int ep_create_wakeup_source(struct epitem *epi)
> >  {
> >  	struct name_snapshot n;
> >  	struct wakeup_source *ws;
> > +	pid_t task_pid;
> > +	int id;
> > +
> > +	task_pid = task_pid_nr(current);
> >  
> >  	if (!epi->ep->ws) {
> > -		epi->ep->ws = wakeup_source_register(NULL, "eventpoll");
> > +		id = atomic_inc_return(&wakesource_create_id);
> > +		epi->ep->ws = wakeup_source_register(NULL, "epoll:%d:%d", id, task_pid);
> 
> How often does this execute? Is it at most once per task lifespan?
Thank you for your feedback! This can execute multiple times throughout
a task's lifespan. However, I haven't seen it execute that often.

> The var probably wants to be annotated with ____cacheline_aligned_in_smp
> so that it does not accidentally mess with other stuff.
> 
> I am assuming there is no constant traffic on it.
Right, I don't see much traffic on it. Can you please elaborate a bit
more on what interaction you're concerned with here? If it's a
concern about false sharing, I'm worried that we may be prematurely
optimizing this.

--Isaac