linux-kernel - Re: [PATCH] revert: timers: fix itimer/many thread hang

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Date:	Thu, 6 Nov 2008 22:44:34 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frank Mayhar <fmayhar@...gle.com>,
	Doug Chapman <doug.chapman@...com>, roland@...hat.com,
	adobriyan@...il.com, akpm@...ux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH] revert: timers: fix itimer/many thread hang


* Peter Zijlstra <peterz@...radead.org> wrote:

> This patch reverts all the itimer/many thread patches:
> 
> 7086efe1c1536f6bc160e7d60a9bfd645b91f279
> bb34d92f643086d546b49cef680f6f305ed84414
> 5ce73a4a5a4893a1aa4cdeed1b1a5a6de42c43b6
> 0a8eaa4f9b58759595a1bfe13a1295fdc25ba026
> f06febc96ba8e0af80bcc3eaec0a109e88275fac
> 
> Because I think the per-cpu accounting approach is wrong and makes 
> things worse for people with a machine that has more than a 
> hand-full of CPUs.

hm, the revert is rather large but i guess the best way forward. 
Unless we can avoid this loop:

-       for_each_possible_cpu(i) {
-               tot = per_cpu_ptr(tsk->signal->cputime.totals, i);
-               times->utime = cputime_add(times->utime, tot->utime);
-               times->stime = cputime_add(times->stime, tot->stime);
-               times->sum_exec_runtime += tot->sum_exec_runtime;
-       }

we have to revert it.

That loop could _perhaps_ be avoided by maintaining the sums from the 
scheduler tick: by just adding the latest delta values for the current 
task to times->*time. Plus at exit time cleaning up the remaining 
delta. That would be a far smaller patch.

	Ingo
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