linux-kernel - Re: [PATCH] posix-timers: RCU conversion

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Date:	Tue, 05 Apr 2011 10:56:22 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Avi Kivity <avi@...hat.com>, ben@...u.net,
	Eric Dumazet <eric.dumazet@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	KVM list <kvm@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>,
	Richard Cochran <richard.cochran@...cron.at>
Subject: Re: [PATCH] posix-timers: RCU conversion

On Tue, 2011-04-05 at 10:48 +0200, Peter Zijlstra wrote:
> On Tue, 2011-03-22 at 12:35 +0200, Avi Kivity wrote:
> >> Here's top with 96 idle guests running:
> 
> On some hacked up 2.6.38 kernel...
> 
> > > Start of perf report -g
> > >      55.26%            kvm  [kernel.kallsyms]     [k] __ticket_spin_lock
> > >                        |
> > >                        --- __ticket_spin_lock
> > >                           |
> > >                           |--94.68%-- _raw_spin_lock
> > >                           |          |
> > >                           |          |--97.55%-- double_rq_lock
> > >                           |          |          load_balance
> > >                           |          |          idle_balance
> > >                           |          |          schedule
> > >                           |          |          |
> > >                           |          |          |--60.56%-- schedule_hrtimeout_range_clock
> > >                           |          |          |          schedule_hrtimeout_range
> > >                           |          |          |          poll_schedule_timeout
> > >                           |          |          |          do_select
> > >                           |          |          |          core_sys_select
> > >                           |          |          |          sys_select
> > >                           |          |          |          system_call_fastpath
> 
> Looks like your workload and idle balancing don't much like each-other.
> 
> What I think is happening is that all your 'idle' qemu thingies keep
> waking up frequently and because you've got like twice the number of
> qemu instances as you've got cpus there's a fair chance you'll have a
> cpu with a pending task while another one goes idle.
> 
> (Why does qemu keep waking if its idle? broken NOHZ?)
> 
> So idle balancing is called when the cpu goes idle (context switch to
> the idle thread) and tries to steal a pending task from another cpu,
> clearly it keeps finding these tasks otherwise it wouldn't try to take
> that lock.
> 
> Mike, you build in some idle balance throttle logic, but that seems
> defeated here (possible because it keeps finding pending tasks to
> migrate? still need morning juice).

Hm.  Maybe someone set sysctl_sched_migration_cost to 0, or wakeups
aren't arriving that frequently is all that pops to mind atm.

	-Mike


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