linux-kernel - Re: [PATCH RFC 00/11] lock monitor: Separate features related to lock

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Date:	Wed, 17 Mar 2010 16:39:03 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, h.mitake@...il.com,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH RFC 00/11] lock monitor: Separate features related to
	lock

On Wed, Mar 17, 2010 at 04:30:53PM +0900, Hitoshi Mitake wrote:
> On 03/17/10 10:32, Frederic Weisbecker wrote:
> > On Sun, Mar 14, 2010 at 07:13:55PM +0100, Peter Zijlstra wrote:
> >> On Sun, 2010-03-14 at 19:38 +0900, Hitoshi Mitake wrote:
> >>> Current lockdep is too complicated because,
> >>>   * dependency validation
> >>>   * statistics
> >>>   * event tracing
> >>> are all implemented by it.
> >>> This cause problem of overhead.
> >>> If user enables one of them, overhead of rests part is not avoidable.
> >>> (tracing is exception. If user enables validation or stat,
> >>> overhead of tracing doesn't occur.)
> >>>
> >>> So I suggest new subsystem "lock monitor".
> >>> This is a general purpose lock event hooking mechanism.
> >>>
> >>> lock monitor will be enable easy implementing and running
> >>> these features related to lock.
> >>>
> >>> And I'm hoping that lock monitor will reduce overhead of perf lock.
> >>> Because lock monitor separates dependency validation and event  
> tracing clearly,
> >>> so calling of functions of lockdep (e.g. lock_acquire()) only for  
> validation
> >>> will not occur lock events.
> >>>
> >>> I implemented it on the branch perf/inject of Frederic's  
> random-tracing tree.
> >>> Because the branch is hottest place of lock and tracing :)
> >>
> >> OK, so I really don't like this much..
> >>
> >> Building a lockstat kernel (PROVE_LOCKING=n) should not have much more
> >> overhead than the proposed solution, if the simple lock acquistion
> >> tracking bothers you, you can do a patch to weaken that.
> >>
> >> I really really dislike how you add a monitor variable between
> >> everything for no reason what so ever.
> >>
> >> You use a new rwlock_t, which is an instant fail, those things are worse
> >> than useless.
> >>
> >> You add chained indirect calls into all lock ops, that's got to hurt.
> >
> >
> > Well, the idea was not bad at the first glance. It was separating
> > lockdep and lock events codes.
> >
> > But indeed, the indirect calls plus the locking are not good
> > for such a fast path.
> >
> > There is something else, it would be nice to keep the
> > lockdep_map ->  lockdep_class mapping so that we can
> > do lock profiling based on classes too. So we actually
> > need the lockdep code. What we don't need is the prove
> > locking or the lock stats. So I guess we can have a new
> > config to enable lock events and get rid of the prove
> > locking / lock stat code if we don't need it.
> >
> >
>
> Thanks for your comments, Peter and Frederic.
>
> My main motivation of writing this patch series was that
> some kernel codes uses lockdep functions (e.g. lock_acquire()) directly,
> so perf lock gets a lot of trace events without actual locks (e.g.  
> might_lock_read()).
> I think that these are confusable things for users.
>
> But I noticed that these events can be reduced by
> turning off CONFIG_PROVE_LOCKING. Yeah, my patch series was pointless... :)
>
> Should perf lock warn not to use with CONFIG_PROVE_LOCKING?


Ah I see.

might_lock_read() uses might_fault(), rcu, workqueues and probably
yet some others use sequences of lock_acquire/lock_release to prove
locking while there is actually no real lock operation involved, but
this is to detect dependency/balance mistakes.

I think that these cases are easily detectable in that they never have
any lock_acquired in their scenario. So may be we can just ignore
scenarios without lock_acquired and indeed advise users not to use
PROVE_LOCKING.

Thanks.

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