linux-kernel - Re: regression introduced by - timers: fix itimer/many thread hang

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Date:	Fri, 7 Nov 2008 09:35:55 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frank Mayhar <fmayhar@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Doug Chapman <doug.chapman@...com>, roland@...hat.com,
	adobriyan@...il.com, akpm@...ux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: regression introduced by - timers: fix itimer/many thread hang


* Frank Mayhar <fmayhar@...gle.com> wrote:

> On Thu, 2008-11-06 at 16:08 +0100, Peter Zijlstra wrote: 
> > On Thu, 2008-11-06 at 09:03 -0600, Christoph Lameter wrote:
> > > On Thu, 6 Nov 2008, Peter Zijlstra wrote:
> > > 
> > > > Also, you just introduced per-cpu allocations for each thread-group,
> > > > while Christoph is reworking the per-cpu allocator, with one unfortunate
> > > > side-effect - its going to have a limited size pool. Therefore this will
> > > > limit the number of thread-groups we can have.
> > > 
> > > Patches exist that implement a dynamically growable percpu pool (using
> > > virtual mappings though). If the cost of the additional complexity /
> > > overhead is justifiable then we can make the percpu pool dynamically
> > > extendable.
> > 
> > Right, but I don't think the patch under consideration will fly anyway,
> > doing a for_each_possible_cpu() loop on every tick on all cpus isn't
> > really healthy, even for moderate sized machines.
> 
> I personally think that you're overstating this.  First, the current 
> implementation walks all threads for each tick, which is simply not 
> scalable and results in soft lockups with large numbers of threads. 
> This patch fixes a real bug.  Second, this only happens "on every 
> tick" for processes that have more than one thread _and_ that use 
> posix interval timers.  Roland and I went to some effort to keep 
> loops like the on you're referring to out of the common paths.
> 
> In any event, while this particular implementation may not be 
> optimal, at least it's _right_.  Whatever happened to "make it 
> right, then make it fast?"

Well, you pushed the lockup to another place: previously we locked up 
with enough threads added, now we'll lock up with enough CPUs added.

So ... please get rid of the for-each-cpu loop for good? (Also, the 
task-exit race needs to be fixed first i guess, before we worry about 
loops.)

	Ingo
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