linux-kernel - Re: [PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX

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Message-ID: <ZxJNb0UNE6yxMNNe@localhost.localdomain>
Date: Fri, 18 Oct 2024 13:58:39 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Benjamin Segall <bsegall@...gle.com>
Cc: linux-kernel@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

Le Thu, Oct 17, 2024 at 01:09:08PM -0700, Benjamin Segall a écrit :
> Frederic Weisbecker <frederic@...nel.org> writes:
> 
> > On Wed, Oct 16, 2024 at 04:59:08PM -0700, Benjamin Segall wrote:
> >> When we clone a new thread, we do not inherit its posix_cputimers, and
> >> clear them with posix_cputimers_init. However, this does not clear the
> >> tick dependency it creates in tsk->tick_dep_mask, and the handler does
> >> not reach the code to clear the dependency if there were no timers to
> >> begin with.
> >> 
> >> Thus if a thread has a cputimer running before cloneing/forking, that
> >> hierarchy will prevent nohz_full unless they create a cputimer of their
> >> own.
> >> 
> >> Process-wide timers do not have this problem because fork does not copy
> >> signal_struct as a baseline, it creates one from scratch.
> >> 
> >> Fixes: b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")
> >> Signed-off-by: Ben Segall <bsegall@...gle.com>
> >> Cc: stable@...r.kernel.org
> >> ---
> >>  kernel/fork.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >> 
> >> diff --git a/kernel/fork.c b/kernel/fork.c
> >> index df8e4575ff01..b57cd63cfcd1 100644
> >> --- a/kernel/fork.c
> >> +++ b/kernel/fork.c
> >> @@ -2290,10 +2290,11 @@ __latent_entropy struct task_struct *copy_process(
> >>  
> >>  	task_io_accounting_init(&p->ioac);
> >>  	acct_clear_integrals(p);
> >>  
> >>  	posix_cputimers_init(&p->posix_cputimers);
> >> +	tick_dep_clear_task(p, TICK_DEP_BIT_POSIX_TIMER);
> >
> > Yes but we don't need the expensive atomic_fetch_andnot(). Also more
> > generally the task tick dependency should be 0 upon creation.
> >
> > So something like this?
> 
> Yeah, the only other uses are contained in rcu_do_batch and rcutorture
> tests, which won't end up here anyways.
> 
> Up to you if you want to send this or I can send out a v2.

Sounds good, please send a v2.

Thanks!