On 11 April 2018 at 18:00, Peter Zijlstra <pet...@infradead.org> wrote:
> On Wed, Apr 11, 2018 at 05:41:24PM +0200, Vincent Guittot wrote:
>> Yes. and to be honest I don't have any clues of the root cause :-(
>> Heiner mentioned that it's much better in latest linux-next but I
>> haven't seen any changes related to the code of those patches
> Yeah, it's a bit of a puzzle. Now you touch nohz, and the patches in
> next that are most likely to have affected this are rjw's
> cpuidle-vs-nohz patches. The common demoninator being nohz.
> Now I think rjw's patches will ensure we enter nohz _less_, they avoid
> stopping the tick when we expect to go idle for a short period only.
> So if your patch makes nohz go wobbly, going nohz less will make that
> better.
> Of course, I've no actual clue as to what that patch (it's the last one
> in the series, right?:
>   31e77c93e432 ("sched/fair: Update blocked load when newly idle")
> ) does that is so offensive to that one machine. You never did manage to
> reproduce, right?


> Could is be that for some reason the nohz balancer now takes a very long
> time to run?

Heiner mentions that is was a relatively slow celeron and he uses
ondemand governor. So I was about to ask him to use performance
governor to see if it can be because cpu runs slow and takes too muche
time to enter idle

> Could something like the following happen (and this is really flaky
> thinking here):
> last CPU goes idle, we enter idle_balance(), that kicks ilb, ilb runs,
> which somehow again triggers idle_balance and around we go?
> I'm not immediately seeing how that could happen, but if we do something
> daft like that we can tie up the CPU for a while, mostly with IRQs
> disabled, and that would be visible as that latency he sees.

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