linux-kernel - Re: [BUG] soft lockup while booting machine with more than 700 cores

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Date:	Thu, 10 Feb 2011 13:03:25 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	steiner@....com
Cc:	mingo@...e.hu, raz@...lemp.com, linux-kernel@...r.kernel.org,
	mingo@...hat.com, a.p.zijlstra@...llo.nl, efault@....de,
	cpw@....com, travis@....com, tglx@...utronix.de, hpa@...or.com
Subject: Re: [BUG] soft lockup while booting machine with more than 700
 cores

From: Jack Steiner <steiner@....com>
Date: Thu, 10 Feb 2011 14:56:48 -0600

> We also noticed that the rebalance_domains() code references many per-cpu
> run queue structures. All of the structures have identical offsets relative
> to the size of a cache leaf. The result is that all index into the same lines in the
> L3 caches. That causes many evictions. We tried an experimental to
> stride the run queues at 128 byte offsets. That helped in some cases but the
> results were mixed.  We are still experimenting with the patch.

I think chasing after cache alignment issues misses the point entirely.

The core issue is that rebalance_domains() is insanely expensive, by
design.  It's complexity is N factorial for the idle non-HZ cpu that is
selected to balance every single domain.

A statistic datastructure that is approximately 128 bytes in size is
repopulated N! times each time this global rebalance thing runs.

I've been seeing rebalance_domains() in my perf top output on 128 cpu
machines for several years now.  Even on an otherwise idle machine,
the system churns in thus code path endlessly.

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