Messages in this thread | | | Date | Fri, 2 Dec 2011 23:42:39 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH] specific do_timer_cpu value for nohz off mode |
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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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