linux-kernel - Re: PATCH: softlockup: Fix hung_task_check

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Date:	Fri, 27 Nov 2009 05:22:47 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	Anton Blanchard <anton@...ba.org>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, msb@...gle.com
Subject: Re: PATCH: softlockup: Fix hung_task_check_count sysctl

On Fri, Nov 27, 2009 at 11:02:13AM +0800, Américo Wang wrote:
> On Fri, Nov 27, 2009 at 10:55 AM, Frederic Weisbecker
> <fweisbec@...il.com> wrote:
> > On Fri, Nov 27, 2009 at 10:46:53AM +0800, Américo Wang wrote:
> >> On Fri, Nov 27, 2009 at 10:28 AM, Anton Blanchard <anton@...ba.org> wrote:
> >> >
> >> > I'm seeing spikes of up to 0.5ms in khungtaskd on a large machine. To reduce
> >> > this source of jitter I tried setting hung_task_check_count to 0:
> >> >
> >> > # echo 0 > /proc/sys/kernel/hung_task_check_count
> >> >
> >> > which didn't have the intended response. Change to a post increment of
> >> > max_count, so a value of 0 means check 0 tasks.
> >> >
> >> > Signed-off-by: Anton Blanchard <anton@...ba.org>
> >>
> >>
> >> Ack.
> >>
> >> I would also suggest to make 'max_count' as unsigned long,
> >> since sysctl_hung_task_check_count is.
> >>
> >> Thanks.
> >
> >
> > Also, the batch_count thing should be dropped I think.
> > This is a hardcoded, not overridable pause after 1024
> > threads checks to avoid latencies caused by rcu_read_lock.
> > But now we have PREEMPT_RCU so people can enable it if
> > they care about latency. We should remove it as it adds
> > unnecessary complexity.
> 
> This sounds OK for me.
> 
> >
> > I'm preparing a patch for that, on top of Anton patch.
> >
> 
> Great!
> 
> Thanks.


Actually it looks like this check is not only there to take
care of latency but also to avoid too much waiting before
a grace period.

Hm, well, I'm not sure what to do.

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