linux-kernel - Re: [PATCH RFC] Change softlockup watchdog to ignore stolen time

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Date:	Thu, 22 Mar 2007 16:51:51 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Ingo Molnar <mingo@...e.hu>,
	Chris Wright <chrisw@...s-sol.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dan Hecht <dhecht@...are.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Akinobu Mita <mita@...aclelinux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Morris <jmorris@...ei.org>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [PATCH RFC] Change softlockup watchdog to ignore stolen time

Jeremy Fitzhardinge wrote:
> The softlockup watchdog is currently a nuisance in a virtual machine,
> since the whole system could have the CPU stolen from it for a long
> period of time.  While it would be unlikely for a guest domain to be
> denied timer interrupts for over 10s, it could happen and any softlockup
> message would be completely spurious.
>   

No, it is not unlikely.  4-way SMP VMs idling exhibit this behavior with 
NO_HZ or NO_IDLE_HZ because they get quiet enough to schedule nothing on 
the APs.

And that can happen on native hardware as well.

> Earlier I proposed that sched_clock() return time in unstolen
> nanoseconds, which is how Xen and VMI currently implement it.  If the
> softlockup watchdog uses sched_clock() to measure time, it would
> automatically ignore stolen time, and therefore only report when the
> guest itself locked up.  When running native, sched_clock() returns
> real-time nanoseconds, so the behaviour would be unchanged.
>
> Does this seem sound?
>
> Also, softlockup.c's use of jiffies seems archaic now.  Should it be
> converted to use timers?  Mightn't it report lockups just because there
> was no timer event?
>   

This looks good to me, as a first order approximation.  But on native 
hardware, with NO_HZ, this is just broken to begin with.  Perhaps we 
should make SOFTLOCKUP depend on !NO_HZ.

Zach
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