lkml.org 
[lkml]   [2011]   [Dec]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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



\
 
 \ /
  Last update: 2011-12-02 23:45    [from the cache]
©2003-2011 Jasper Spaans