linux-kernel - Re: LTTng finds abnormally long APIC interrupt handler : 58.2 ms

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Message-Id: <1218266065.29098.36.camel@lappy.programming.kicks-ass.net>
Date:	Sat, 09 Aug 2008 09:14:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	ltt-dev@...ts.casi.polymtl.ca
Subject: Re: LTTng finds abnormally long APIC interrupt handler : 58.2 ms

On Fri, 2008-08-08 at 19:11 -0400, Mathieu Desnoyers wrote:

> The basic question that arises from this trace is : is it normal for
> find_usage_backwards to be called ~8500 times during a spinlock ?

Yes and no - its O(n^2) graph walking. That said, David Miller did a
cool patch that reduces the graph walk by not doing the same subgraph
over and over again. Look at this series:

  http://lkml.org/lkml/2008/8/4/175

Andrew picked all that up - so it should be in the next -mm.

It also contains a new scheduler run-queue annotation which should
greatly reduce the chain length, adding to even more benefit.

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