linux-kernel - Re: [PATCH 3/7] lockdep: lock contention tracking

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Message-ID: <Pine.LNX.4.64.0705231312340.22023@dhcp83-20.boston.redhat.com>
Date:	Wed, 23 May 2007 13:24:18 -0400 (EDT)
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Bill Huey <billh@...ppy.monkey.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 3/7] lockdep: lock contention tracking


On Wed, 23 May 2007, Peter Zijlstra wrote:

> On Wed, 2007-05-23 at 12:11 -0400, Jason Baron wrote:
> > On Wed, 23 May 2007, Peter Zijlstra wrote:
> > 
> > > On Wed, 2007-05-23 at 10:40 -0400, Jason Baron wrote:
> > > > On Wed, 23 May 2007, Peter Zijlstra wrote:
> > > > 
> > > > > Count lock contention events per lock class. Additionally track the first four
> > > > > callsites that resulted in the contention.
> > > > > 
> > > > 
> > > > I think that we need the total number of locking calls, not just the total 
> > > > number of *contended* locking calls, in order to make the data 
> > > > significant. Same for waittime. Yes, this pollutes the fastpath. In the 
> > > > contention case, its one extra addition, and for the waittime, its a call 
> > > > the sched_clock(). I don't think that's too much to pay for much more 
> > > > meaningful data.
> > > 
> > > The holdtime statistics add these numbers.
> > > 
> > 
> > ok, i see what you are saying...however, the 'waittime' statistic as 
> > implemented, is only recorded when we don't get the lock immediately. 
> > Thus, it's really measuring the waittime when there is lock contention. I 
> > understand that in the non-contended case we are only talking a a few 
> > cycles, but the fact that the non-contended case is not counted as another 
> > waittime of even zero length (so no measurement is required), might skew 
> > the stats a bit. For examples, if there was a lock that was almost never 
> > contended but one time happened to be contended for a long time, its 
> > average wait time would look really high.
> 
> I'm not seeing how or why this is a problem. The number of contentions
> is reported on the same line, so it should be obvious.
> 
> Your definition of wait-time is also obtainable from the numbers, one
> would just divide waittime-total by acquisitions, instead of
> contentions.
> 

agreed, I just want to point out that under my definition of waitime, I 
would have to make the assumption that the waittime is 0 for a lock that 
is acquired without fallback to the slowpath. For a spinlock acquisition, 
this is likely a fair assumption, however the trylock path for semaphores 
can be longer.

Acked-by: Jason Baron <jbaron@...hat.com>


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