Re: filp usage when cpu busy

[bc Wong <bcwong@cisco.com>]

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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