From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: A unresponsive file system can hang all I/O in the system
 on linux-2.6.23-rc6 (dirty_thresh problem?)
Date: Fri, 28 Sep 2007 12:16:42 -0700
Message-ID: <20070928121642.56a380ce.akpm@linux-foundation.org>
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	<20070927235034.ae7bd73d.akpm@linux-foundation.org>
	<1190998853.6702.17.camel@heimdal.trondhjem.org>
	<20070928114930.2c201324.akpm@linux-foundation.org>
	<1191005339.18147.89.camel@lappy>
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Cc: nfs@lists.sourceforge.net,
	linux-pm <linux-pm@lists.linux-foundation.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Trond Myklebust <trond.myklebust@fys.uio.no>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
In-Reply-To: <1191005339.18147.89.camel@lappy>
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On Fri, 28 Sep 2007 20:48:59 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> 
> On Fri, 2007-09-28 at 11:49 -0700, Andrew Morton wrote:
> 
> > Do you know where the stalls are occurring?  throttle_vm_writeout(), or via
> > direct calls to congestion_wait() from page_alloc.c and vmscan.c?  (running
> > sysrq-w five or ten times will probably be enough to determine this)
> 
> would it make sense to instrument congestion_wait() callsites with
> vmstats?

Better than nothing, but it isn't a great fit: we'd need one vmstat counter
per congestion_wait() callsite, and it's all rather specific to the
kernel-of-the-day.

taskstats delay accounting isn't useful either - it will aggregate all the
schedule() callsites.

profile=sleep is just about ideal for this, isn't it?  I suspect that most
people don't know it's there, or forgot about it.

It could be that profile=sleep just tells us "you're spending a lot of time
in io_schedule()" or congestion_wait(), so perhaps we need to teach it to
go for walk up the stack somehow.

But lockdep knows how to do that already so perhaps we (ie: you ;)) can
bolt sleep instrumentation onto lockdep as we (ie you ;)) did with the
lockstat stuff?

(Searches for the lockstat documentation)

Did we forget to do that?

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