linux-kernel - Re: [RFC PATCH] timer: Improve itimers scalability

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Message-ID: <20150807120114.GB16897@redhat.com>
Date:	Fri, 7 Aug 2015 14:01:14 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Mike Galbraith <umgwanakikbuti@...il.com>, terry.rudd@...com,
	Rik van Riel <riel@...hat.com>,
	Waiman Long <Waiman.Long@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [RFC PATCH] timer: Improve itimers scalability

On 08/06, Jason Low wrote:
>
> On Thu, 2015-08-06 at 16:18 +0200, Oleg Nesterov wrote:
> > > +	/*
> > > +	 * Check for the special case process timers.
> > > +	 */
> > > +	check_cpu_itimer(tsk, &sig->it[CPUCLOCK_PROF], &prof_expires, ptime,
> > > +			 SIGPROF);
> > > +	check_cpu_itimer(tsk, &sig->it[CPUCLOCK_VIRT], &virt_expires, utime,
> > > +			 SIGVTALRM);
> > > +
> >
> > Not sure I understand this part... looks wrong actually, please note
> > that RLIMIT_CPU block above may need to update prof_expires _after_
> > check_cpu_itimer(), or I am totally confused.
>
> This change isn't critical to the patch, so we can delete this from the
> patch. Though from my understanding, the purpose of prof_expires is to
> collect the earliest prof expire time for when we update
> "sig->cputime_expires.prof_exp". So I think it wouldn't matter which
> order prof_expire gets updated (as long as check_timers_list() is called
> first, since prof_expires gets directly assigned there).

Yes, I missed that check_cpu_itimer() checks "it->expires < *expires"
before it updates *expires.

Thanks for correcting me!

Oleg.

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