linux-kernel - Re: [PATCH 2/3] posix_timers: Defer per process timer stop after timers processing

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Date:	Tue, 30 Apr 2013 14:54:24 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Olivier Langlois <olivier@...llion01.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/3] posix_timers: Defer per process timer stop after
 timers processing

On Fri, Apr 26, 2013 at 12:27:59AM -0400, Olivier Langlois wrote:
> On Fri, 2013-04-19 at 14:47 +0200, Frederic Weisbecker wrote:
> 
> > 
> > >
> > > I might be mistaken but I believe that firing timers are not rescheduled
> > > in the current interrupt context. They are going to be rescheduled later
> > > from the task context handling the timer generated signal.
> > 
> > No, when the timer fires, it might generate a signal. But it won't
> > execute that signal right away in the same code path. Instead, after
> > signal generation, it may reschedule the timer if necessary then look
> > at the next firing timer in the list. This is all made from the same
> > timer interrupt context from the same call to run_posix_cpu_timers().
> > The signal itself is executed asynchronously. Either by interrupting a
> > syscall, or from the irq return path.
> > 
> Frederic, be careful with the interpretation, there are 2 locations from
> where posix_cpu_timer_schedule() can be called.
> 
> Call to posix_cpu_timer_schedule() from cpu_timer_fire() only happens if
> the signal isn't sent because it is ignored by the recipient.
> 
> Maybe the condition around the posix_cpu_timer_schedule() block inside
> cpu_timer_fire() could even be a good candidate for 'unlikely'
> qualifier.

Well, cpu_timer_fire() is probably not a fast path. So helping branch
prediction there probably won't have much measurable effect in practice.

> 
> IMO, a more likely scenario, posix_cpu_timer_schedule() will be called
> from dequeue_signal() which will be from from a different context than
> the interrupt context.

Oh you're right! I misunderstood that. So I need to take into consideration
for the nohz code.

> 
> At best, you have an interesting race!
> 
> dequeue_signal() is called when delivering a signal, not when it is
> generated, right?

Yeah you're right, sorry for the confusion. I'll reconsider your patches
with that in mind.

Thanks.
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