linux-kernel - Re: Yet more softlockups.

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Date:	Fri, 12 Jul 2013 11:45:21 -0400
From:	Dave Jones <davej@...hat.com>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Peter Anvin <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: Yet more softlockups.

On Fri, Jul 12, 2013 at 08:38:52AM -0700, Dave Hansen wrote:
 
 > The warning comes from calling perf_sample_event_took(), which is only
 > called from one place: perf_event_nmi_handler().
 > 
 > So we can be pretty sure that the perf NMI is firing, or at least that
 > this handler code is running.
 > 
 > nmi_handle() says:
 >         /*
 >          * NMIs are edge-triggered, which means if you have enough
 >          * of them concurrently, you can lose some because only one
 >          * can be latched at any given time.  Walk the whole list
 >          * to handle those situations.
 >          */
 > 
 > perf_event_nmi_handler() probably gets _called_ when the watchdog NMI
 > goes off.  But, it should hit this check:
 > 
 >         if (!atomic_read(&active_events))
 >                 return NMI_DONE;
 > 
 > and return quickly. This is before it has a chance to call
 > perf_sample_event_took().
 > 
 > Dave, for your case, my suspicion would be that it got turned on
 > inadvertently, or that we somehow have a bug which bumped up
 > perf_event.c's 'active_events' and we're running some perf code that we
 > don't have to.
 
What do you 'inadvertantly' ? I see this during bootup every time.
Unless systemd or something has started playing with perf, (which afaik it isn't)

 > But, I'm suspicious.  I was having all kinds of issues with perf and
 > NMIs taking hundreds of milliseconds.  I never isolated it to having a
 > real, single, cause.  I attributed it to my large NUMA system just being
 > slow.  Your description makes me wonder what I missed, though.

Here's a fun trick:

trinity -c perf_event_open -C4 -q -l off

Within about a minute, that brings any of my boxes to its knees.
The softlockup detector starts going nuts, and then the box wedges solid.

(You may need to bump -C depending on your CPU count. I've never seen it happen
 with a single process, but -C2 seems to be a minimum)

That *is* using perf though, so I kind of expect bad shit to happen when there are bugs.
The "during bootup" case is still a head-scratcher.

	Dave

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