linux-kernel - Re: Negative values in /proc/latency

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Date:	Mon, 2 Feb 2009 20:55:45 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: Negative values in /proc/latency_stats

On Sat, 31 Jan 2009 14:42:20 +0100 Corrado Zoccolo <czoccolo@...il.com> wrote:

> Hello,
> I found negative numbers sometimes appear in /proc/latency_stats
> (vanilla kernel 2.6.29-rc3, on x86 cpu, configuration attached)
> 
> [corrado@...alhost ~]$ while sleep 1; do grep -- - /proc/latency_stats
> >> neg_stats; done
> ^Z
> [1]+  Stopped                 sleep 1
> [corrado@...alhost ~]$ cat neg_stats
> 1 -486373534 -486373534 sys_rt_sigsuspend sysenter_do_call
> 1 -486373534 -486373534 sys_rt_sigsuspend sysenter_do_call
> 1 -486373534 -486373534 sys_rt_sigsuspend sysenter_do_call
> 
> 
> I suspect this can be the cause for
> https://bugs.launchpad.net/ubuntu/+source/latencytop/+bug/297776 , as
> I saw it happening on my machine during a kernel recompilation.
> 

<discovers kernel/latencytop.c>

- It implements a kernel/userspace interface yet it has zero documentation.

- It was committed with a 2-line changelog which tells us practically
  nothing.

- It implements an up-to-1536-loops loop followed by an
  up-to-384-loops loop on a scheduler hotpath.

  All under spin_lock_irqsave()!

- store_stacktrace() unnecessarily initalises trace.skip.

- account_scheduler_latency() should be an inline:

	if (unlikely(latencytop_enabled))
		__account_scheduler_latency(...);

- ditto clear_all_latency_tracing()

- it's schizophrenic in its placement of spaces around semicolons in
  `for' statements.

- it seems to only be implemented if CONFIG_FAIR_GROUP_SCHED=y.

- lstats_fops should be const.


And it emits negative numbers too ;)
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