linux-kernel - Re: [PATCH] specific do_timer

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Date:	Fri, 2 Dec 2011 23:42:39 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dimitri Sivanich <sivanich@....com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode

On Fri, 2 Dec 2011, Dimitri Sivanich wrote:
> On Thu, Dec 01, 2011 at 02:56:23PM -0800, Andrew Morton wrote:
> Well, we do have to take the write_seqlock() in tick_periodic, so there's
> no danger of do_timer running exactly concurrently.
> 
> But yes, we may end up with 2 jiffies ticks occurring close together
> (when 5 runs do_timer while 4 waits for the seqlock), or we might end up
> missing a jiffies update for almost a full tick (when it changes from 5
> to 4 immediately after 4 has done the 'tick_do_timer_cpu == cpu' check).
> 
> So at that time, we could be off +- almost a tick.  The question is, how
> critical is that?  When you down a cpu, the same sort of thing could 
> happen via tick_handover_do_timer(), which itself does nothing more than
> change tick_do_timer_cpu.

It's uncritical as long as you are not using clocksource=jiffies. With
all other clocksources you just miss a jiffies update, which does not
affect timekeeping at all. It just might expire your network timeout a
jiffie earlier or later. So there is no damage to expect.

Thanks,

	tglx

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