linux-kernel - Re: [PATCH] ftrace: Don't use tracing_record

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Date:	Tue, 10 Mar 2009 13:11:25 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: Don't use tracing_record_cmdline() in
	workqueue tracer


* KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> > 
> > * KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> > 
> > > Currently, /sys/kernel/debug/tracing/trace_stat/workqueues can 
> > > display wrong and strang thread name.
> > > 
> > > Why?
> > > 
> > > Currently, ftrace has 
> > > tracing_record_cmdline()/trace_find_cmdline() convinience 
> > > function. they implement task->comm string cache. it can avoid 
> > > unnecessary memcpy overhead. and workqueue tracer use it.
> > > 
> > > However, in general, any trace stastics feature shouldn't use 
> > > tracing_record_cmdline(). A trace stastics can display very 
> > > old process. then comm cache can return wrong string because 
> > > recent process override the cache.
> > > 
> > > Fortunately, workqueue trace gerantee to live displayed 
> > > process. Then, we can search comm string from pid at display 
> > > time.
> > 
> > Applied, thanks!
> > 
> > We might need to improve the comm-cache - i've seen frequent 
> > artifacts due to it. Displaying <...> is _far_ better than an 
> > outright misleading string displayed.
> > 
> > I.e. the cache should be improved to be properly coherent with 
> > reality.
> > 
> > 	Ingo
> 
> Doh!
> My last mail's cc list didn't include lkml. that's unintensional silly 
> my mistake. very sorry.
> 
> and Yes, memcpy(buf, task->comm, 16) mean two movq 
> instruction. that is worthless for caching. I think we can 
> remove this cache completely.

ok, agreed - mind sending a patch for that?

> and, I find my last patch has one race window. fixing below.

Could you please send a delta patch against tip:master? Thanks,

	Ingo
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