linux-kernel - Re: [PATCH 4/4] Implement semaphore latency tracer

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Date:	Wed, 22 Oct 2008 17:28:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Török Edwin <edwintorok@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, sandmann@...mi.au.dk,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 4/4] Implement semaphore latency tracer


* Török Edwin <edwintorok@...il.com> wrote:

> On 2008-10-12 22:13, Peter Zijlstra wrote:
> > On Sun, 2008-10-12 at 16:12 +0300, Török Edwin wrote:
> >   
> >> Each time a down_read or down_write fails, a unique latency id is generated.
> >> Later when someone releases the semaphore, it is blamed for the latency of all
> >> tasks on the wait_list of the semaphore.
> >> If you would group the output from latency_trace by the latency_id you get all those
> >> who were contending on a lock, and the tasks that were holding the lock.
> >> An entry in latency_trace has the format:
> >> (latency_id) [semaphore_id] read|write
> >> stacktrace <= stacktrace
> >>     
> >
> > What can this tracer do that latencytop cannot already do?
> 
> Latencytop can show latencies in down_read or down_write (and is very
> useful at doing that), but it cannot show who else was holding the
> semaphore,
> i.e. the true cause of the latency.
> 
> Consider:
> process A holds a semaphore for reading, process B tries to acquire it
> for writing and fails. Latencytop shows the latency in process B, but
> doesn't
> show anything about process A.
> 
> The semlat tracer is doing something more similar to lockstat, but
> doesn't need lockdep, and it adds tracepoints on the slowpath only (lock
> failed, wakeup).

hm, but the most common synchronization primitive are mutexes - and 
those are not covered by your patchset.

	Ingo
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