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    SubjectRe: [RFC] time: xtime_lock is held too long
    On Fri, 6 May 2011, john stultz wrote:
    > On Sat, 2011-05-07 at 01:00 +0200, Eric Dumazet wrote:
    > > Le vendredi 06 mai 2011 à 15:46 -0700, john stultz a écrit :
    > > > On Sat, 2011-05-07 at 00:30 +0200, Eric Dumazet wrote:
    > > > > I can see many cpus entering tick_do_update_jiffies64() and all are
    > > > > calling write_seqlock(&xtime_lock);
    > > > >
    > > > > Only first one can perform the work, but all others are waiting on the
    > > > > spinlock, get it, change seqcount, and realize they have nothing to
    > > > > do...
    > > >
    > > > Huh. So who is calling tick_do_update_jiffies64 in your case? I know the
    > > > sched_tick_timer and tick_nohz_handler checks to make sure
    > > > tick_do_timer_cpu == cpu to avoid exactly the thundering heard problem
    > > > on the jiffies update.
    > > >
    > > > There's other spots that call tick_do_update_jiffies64, but I thought
    > > > those were more rare. So there may be something else wrong going on
    > > > here.
    > > >
    > >
    > > That I can answer :
    > [snip]
    > > (I added do_timestamp1/do_timestamp2) after/before write_seqlock()/write_sequnlock()
    > >
    > > <idle>-0 [003] 920.355377: do_timestamp1 <-tick_do_update_jiffies64
    > > <idle>-0 [006] 920.355377: tick_do_update_jiffies64 <-tick_sched_timer
    > > <idle>-0 [003] 920.355378: do_timestamp2 <-tick_do_update_jiffies64
    > > <idle>-0 [000] 920.355657: tick_do_update_jiffies64 <-tick_check_idle
    > > <idle>-0 [000] 920.355660: tick_do_update_jiffies64 <-tick_nohz_restart_sched_tick
    >
    > Thomas, any clues why this would be getting hammered?

    Hmm, tick-sched code grew quite a few unconditional callsites which
    i'm not sure of whether they are correct.

    Thanks,

    tglx
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