linux-kernel - Re: [Query] Can we use normal timers (kernel/timer.c) while in NO_HZ

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Message-ID: <20140604140512.GD13827@localhost.localdomain>
Date:	Wed, 4 Jun 2014 16:05:14 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Kevin Hilman <khilman@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linaro Networking <linaro-networking@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [Query] Can we use normal timers (kernel/timer.c) while in
 NO_HZ_FULL mode?

On Wed, Jun 04, 2014 at 07:14:43PM +0530, Viresh Kumar wrote:
> On 4 June 2014 18:53, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > Right, if we enqueue a timer when the tick is stopped, we call wake_up_nohz_cpu().
> 
> Ahh, I looked at __mod_timer() :)
> 
> BTW, shouldn't we do something similar for mod_timer_pinned()
> as well ?

Ah that looks right. In fact we should probably move the wake_up_nohz_cpu()
to internal_add_timer().

And that concerns nohz idle as well. If mod_timer() selects a CPU that doesn't
appear to be idle in get_nohz_timer_target() but then becomes idle afterward
before we lock its timer base, the CPU may well miss the timer.
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