Re: filp usage when cpu busy

[Nick Piggin <nickpiggin@yahoo.com.au>]

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).
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