Subject: Re: regression introduced by - timers: fix itimer/many thread hang
From: Peter Zijlstra <peterz@infradead.org>
To: Frank Mayhar <fmayhar@google.com>
Cc: Christoph Lameter <cl@linux-foundation.org>,
       Doug Chapman <doug.chapman@hp.com>, mingo@elte.hu, roland@redhat.com,
       adobriyan@gmail.com, akpm@linux-foundation.org,
       linux-kernel <linux-kernel@vger.kernel.org>
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Date: Fri, 07 Nov 2008 21:26:14 +0100
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OK, so how about going about this differently.

How about we group the threads per cpu, and appoint one the cpu-leader
and collect the per cpu sum in there.

Then, on tick time we sum the cpu-leaders to obtain the total.

This would require finding the cpu-leader for any particular cpu, which
for example, we could do by maintaining a rb-tree ordered on cpu. If you
find an entry, its the leader and you store a pointer to it, if its
empty, insert yourself.

We'd have to update this whenever a task migrates.

When the cpu-leader migrates it can hand off its sum to the cpu-leader
of the target cpu (assuming there is one, otherwise it will again be
cpu-leader).

The advantage is that the memory foot-print scales with nr_tasks and the
runtime cost is min(nr_tasks, nr_cpus) where nr_cpus is limited to the
cpus the process actually runs on, so this takes full advantage of
things like cpusets.


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