linux-kernel - Re: [PATCH 2/4] virtual block device driver (ramzswap)

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Message-Id: <1253022207.4754.1.camel@penberg-laptop>
Date:	Tue, 15 Sep 2009 16:43:27 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	rostedt@...dmis.org
Cc:	Nitin Gupta <ngupta@...are.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Ed Tomlinson <edt@....ca>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-mm-cc@...top.org,
	Ingo Molnar <mingo@...e.hu>,
	"Frédéric Weisbecker" <fweisbec@...il.com>
Subject: Re: [PATCH 2/4] virtual block device driver (ramzswap)

Hi Steve,

On Tue, 2009-09-15 at 09:14 -0400, Steven Rostedt wrote:
> > >>> +
> > >>> +       trace_mark(ramzswap_lock_wait, "ramzswap_lock_wait");
> > >>> +       mutex_lock(&rzs->lock);
> > >>> +       trace_mark(ramzswap_lock_acquired, "ramzswap_lock_acquired");
> > >>
> > >> Hmm? What's this? I don't think you should be doing ad hoc
> > >> trace_mark() in driver code.
> > >
> > > This is not ad hoc. It is to see contention over this lock which I believe is a
> > > major bottleneck even on dual-cores. I need to keep this to measure improvements
> > > as I gradually make this locking more fine grained (using per-cpu buffer etc).
> > 
> > It is ad hoc. Talk to the ftrace folks how to do it properly. I'd keep
> > those bits out-of-tree until the issue is resolved, really.
> 
> Yes, trace_mark is deprecated. You want to use TRACE_EVENT. See how gfs2
> does it in:
> 
>   fs/gfs2/gfs2_trace.h
> 
> and it is well documented in
> samples/trace_events/trace-events-samples.[ch]

Does it really make sense to add special-case tracing in driver code to
profile lock contention for a _single mutex_?

			Pekka

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