linux-kernel - Re: filp usage when cpu busy

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Message-ID: <472A1222.70103@cisco.com>
Date:	Thu, 01 Nov 2007 10:51:30 -0700
From:	bc Wong <bcwong@...co.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	linux-kernel@...r.kernel.org
Subject: Re: filp usage when cpu busy

Nick Piggin wrote:
> On Thursday 01 November 2007 12:56, bc Wong (chimwong) wrote:
>> Hi,
>>
>> With 2.6.16 x86_64 on a 4 core machine, I noticed
>> that the filp usage (according to /proc/slabinfo)
>> shoots up and keeps on increasing sharply when one
>> of the CPUs is (1) locked up, or (2) very busy
>> doing a lot of printk()'s with KERN_EMERG.
>>
>> In the case of (1), it's permanent until it runs
>> out of memory eventually. For (2), it's temporary;
>> filp count comes back down when the printk()'s are
>> done.
>>
>> I can't think of any relationship between a busy/
>> locked-up CPU and filp count. The system is still
>> functional. New short-lived processes kept being
>> created, but the overall number of processes is
>> stable.
>>
>> Does anyone know why filp count would go up like
>> that?
> 
> Yeah, it's probably because filp structures are freed by
> RCU, and if you have a locked up CPU then it can't go
> through a quiescent state so RCU stops freeing your filps.
> 
> If you add some cond_resched()s to your code, you should
> find that RCU will force a reschedule and things will work
> (actually, for 2.6.16, I'm not sure if RCU had the code to
> force a reschedule... it's force_quiescent_state() in
> kernel/rcupdate.c upstream).

Thanks! You're absolutely right. Btw, 2.6.16 does
have force_quiescent_state().

Cheers,
bc
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