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

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Message-Id: <1253023725.20020.103.camel@gandalf.stny.rr.com>
Date:	Tue, 15 Sep 2009 10:08:45 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
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)

On Tue, 2009-09-15 at 16:43 +0300, Pekka Enberg wrote:
> 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_?

No that's what LOCKSTAT is for ;-)

-- Steve


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