Re: [RFC] time: xtime_lock is held too long

Thomas Gleixner
2011-05-09T08:40:39Z

[-- Attachment #1: Type: TEXT/PLAIN, Size: 1731 bytes --]

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