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    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.



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