linux-kernel - Re: [RFC] time: xtime

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Date:	Mon, 9 May 2011 10:40:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	john stultz <johnstul@...ibm.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Anton Blanchard <anton@...ba.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [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