linux-kernel - Re: High lock spin time for zone->lru

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Message-ID: <45A86291.8090408@yahoo.com.au>
Date:	Sat, 13 Jan 2007 15:39:45 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ravikiran G Thirumalai <kiran@...lex86.org>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andi Kleen <ak@...e.de>, Andrew Morton <akpm@...l.org>,
	"Shai Fultheim (Shai@...lex86.org)" <shai@...lex86.org>,
	pravin b shelar <pravin.shelar@...softinc.com>
Subject: Re: High lock spin time for zone->lru_lock under extreme conditions

Ravikiran G Thirumalai wrote:
> Hi,
> We noticed high interrupt hold off times while running some memory intensive
> tests on a Sun x4600 8 socket 16 core x86_64 box.  We noticed softlockups,

[...]

> We did not use any lock debugging options and used plain old rdtsc to
> measure cycles.  (We disable cpu freq scaling in the BIOS). All we did was
> this:
> 
> void __lockfunc _spin_lock_irq(spinlock_t *lock)
> {
>         local_irq_disable();
>         ------------------------> rdtsc(t1);
>         preempt_disable();
>         spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
>         _raw_spin_lock(lock);
>         ------------------------> rdtsc(t2);
>         if (lock->spin_time < (t2 - t1))
>                 lock->spin_time = t2 - t1;
> }
> 
> On some runs, we found that the zone->lru_lock spun for 33 seconds or more
> while the maximal CS time was 3 seconds or so.

What is the "CS time"?

It would be interesting to know how long the maximal lru_lock *hold* time is,
which could give us a better indication of whether it is a hardware problem.

For example, if the maximum hold time is 10ms, that it might indicate a
hardware fairness problem.

-- 
SUSE Labs, Novell Inc.
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