linux-kernel - Re: [PATCH] stopmachine: add stopmachine

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Date:	Mon, 14 Jul 2008 23:20:26 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Zachary Amsden <zach@...are.com>
Subject: Re: [PATCH] stopmachine: add stopmachine_timeout

On Mon, Jul 14, 2008 at 11:56:18AM -0700, Jeremy Fitzhardinge wrote:
> Rusty Russell wrote:
> > On Monday 14 July 2008 21:51:25 Christian Borntraeger wrote:
> >> Am Montag, 14. Juli 2008 schrieb Hidetoshi Seto:
> >>     
> >>> +	/* Wait all others come to life */
> >>> +	while (cpus_weight(prepared_cpus) != num_online_cpus() - 1) {
> >>> +		if (time_is_before_jiffies(limit))
> >>> +			goto timeout;
> >>> +		cpu_relax();
> >>> +	}
> >>> +
> >>>       
> >> Hmm. I think this could become interesting on virtual machines. The
> >> hypervisor might be to busy to schedule a specific cpu at certain load
> >> scenarios. This would cause a failure even if the cpu is not really locked
> >> up. We had similar problems with the soft lockup daemon on s390.
> > 5 seconds is a fairly long time.  If all else fails we could have a config 
> > option to simply disable this code.

Hmm.. probably a stupid question: but what could happen that a real cpu
(not virtual) becomes unresponsive so that it won't schedule a MAX_RT_PRIO-1
prioritized task for 5 seconds?

> >> It would be good to not-use wall-clock time, but really used cpu time
> >> instead. Unfortunately I have no idea, if that is possible in a generic
> >> way. Heiko, any ideas?
> >
> > Ah, cpu time comes up again.  Perhaps we should actually dig that up again; 
> > Zach and Jeremy CC'd.
> 
> Hm, yeah. But in this case, it's tricky. CPU time is an inherently
> per-cpu quantity. If cpu A is waiting for cpu B, and wants to do the
> timeout in cpu-seconds, then it has to be in *B*s cpu-seconds (and if A
> is waiting on B,C,D,E,F... it needs to measure separate timeouts with
> separate timebases for each other CPU). It also means that if B is
> unresponsive but also not consuming any time (blocked in IO,
> administratively paused, etc), then the timeout will never trigger.
> 
> So I think monotonic wallclock time actually makes the most sense here.

This is asking for trouble... a config option to disable this would be
nice. But as I don't know which problem this patch originally addresses
it might be that this is needed anyway. So lets see why we need it first.

> The other issue is whether cpu_relax() is the right thing to put in the
> busywait. We don't hook it in pvops, so it's just an x86 "pause"
> instruction, so from the hypervisor's perspective it just looks like a
> spinning CPU. We could either hook cpu_relax() into a hypervisor yield,
> or come up with a heavier-weight cpu_snooze() (cpu_relax() is often used
> in loops which are expected to have a short duration, where doing a
> hypercall+yield would be overkill).

cpu_relax() translates to a hypervisor yield on s390. Probably makes sense
if other architectures would do the same.
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