linux-kernel - Re: [RFC][PATCH] blktrace: fix original blktrace

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Message-ID: <20090325130138.GO27476@kernel.dk>
Date:	Wed, 25 Mar 2009 14:01:39 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] blktrace: fix original blktrace

On Wed, Mar 25 2009, Ingo Molnar wrote:
> 
> * Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > On Wed, Mar 25 2009, Li Zefan wrote:
> > > Jens Axboe wrote:
> > > > On Wed, Mar 25 2009, Li Zefan wrote:
> > > >> I'm wondering what we are going to do with the original blktrace which
> > > >> is using relay and is used via ioctl.
> > > >>
> > > >> The problem is currently it's totally broken. You can use ftrace to see
> > > >> the output of blktrace, but user-space blktrace is unusable.
> > > >>
> > > >> With this patch, both ioctl and ftrace can be used, but of course you
> > > >> can't use both of them at the same time.
> > > > 
> > > > Even if ftrace was as fast as storing huge amounts of data as blktrace,
> > > > it's still of utmost importance that nothing is broken there. There are
> > > 
> > > agreed
> > > 
> > > > people actually USING this tracing to do real work, it's not a
> > > > playground.
> > > > 
> > > 
> > > and we are using blktrace to test the cgroup-based io controller
> > > 
> > > > I appreciate the efforts to unify and improve our tracing, but we must
> > > > not be breaking blktrace along the way. Otherwise the whole thing goes
> > > 
> > > this patch fixes it.
> > > 
> > > it's broken by "blktrace: add ftrace plugin"
> > > (c71a896154119f4ca9e89d6078f5f63ad60ef199)
> > > 
> > > @@ -131,13 +162,14 @@ static void __blk_add_trace()
> > > - 	if (unlikely(bt->trace_state != Blktrace_running))
> > > + 	if (unlikely(bt->trace_state != Blktrace_running || !blk_tracer_enabled))
> > >  		return;
> > 
> > Good, as long as it still works with blktrace, that's all I care 
> > about (for now, at least).
> 
> Sorry about that. I've applied Li's fixes and double checked 
> blktrace+blktrace functionality and pushed out a new tracing
> tree to linux-next.
> 
> Btw., blktrace could be updated in the future to make use the
> new per CPU buffering and sys_splice() code available in ftrace
> plugin. [ splice is cool! =B-) ]
> 
> Plus blktrace could make use of built-in event filtering 
> capabilities - for example to only trace events in a specific
> sector range on the disk. Or to trace all IO of a given PID only.
> 
> But that is a different project that needs changes both on the 
> kernel side and on the user-space side (.31-ish for sure) and the 
> relayfs+ioctl method must work fine in any case.

Don't get me wrong, I completely agree with the unification idea, it'll
be awesome to have a single stream of info for several event types. My
worry was just if it was going too fast, I just don't want a broken
blktrace before the dust has even settled. And moving blktrace from
relay to per-cpu buffering and splice definitely sounds like a good
longer term direction.

-- 
Jens Axboe

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