Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

[Dietmar Eggemann]

On 03/02/2021 14:12, Vincent Guittot wrote:
> On Wed, 3 Feb 2021 at 12:54, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>>
>> On 29/01/2021 18:27, Vincent Guittot wrote:
>>> Le vendredi 29 janv. 2021 � 11:33:00 (+0100), Vincent Guittot a �crit :
>>>> On Thu, 28 Jan 2021 at 16:09, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>>>>>
>>>>> Hi Vincent,
>>>>>
>>>>> On Thu, Jan 28, 2021 at 8:57 AM Vincent Guittot
>>>>> <vincent.guittot@xxxxxxxxxx> wrote:
>>>>>>> On Mon, Jan 25, 2021 at 03:42:41PM +0100, Vincent Guittot wrote:
>>>>>>>> On Fri, 22 Jan 2021 at 20:10, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>>>>>>>>> On Fri, Jan 22, 2021 at 05:56:22PM +0100, Vincent Guittot wrote:
>>>>>>>>>> On Fri, 22 Jan 2021 at 16:46, Joel Fernandes (Google)
>>>>>>>>>> <joel@xxxxxxxxxxxxxxxxx> wrote:
>>
>> [...]
>>
>>>> The only point that I agree with, is that running
>>>> update_blocked_averages with preempt and irq off is not a good thing
>>>> because we don't manage the number of csf_rq to update and I'm going
>>>> to provide a patchset for this
>>>
>>> The patch below moves the update of the blocked load of CPUs outside newidle_balance().
>>>
>>> Instead, the update is done with the usual idle load balance update. I'm working on an
>>> additonnal patch that will select this cpu that is about to become idle, instead of a
>>> random idle cpu but this 1st step fixe the problem of lot of update in newly idle.
>>
>> I'm trying to understand the need for this extra patch.
>>
>> The patch below moves away from doing update_blocked_averages() (1) for
>> all CPUs in sched groups of the sched domain:
>>
>> newidle_balance()->load_balance()->
>> find_busiest_group()->update_sd_lb_stats()->update_sg_lb_stats()
>>
>> to:
>>
>> calling (1) for CPUs in nohz.idle_cpus_mask in _nohz_idle_balance() via
>> update_nohz_stats() and for the ilb CPU.
>>
>> newidle_balance() calls (1) for newidle CPU already.
>>
>> What would be the benefit to choose newidle CPU as ilb CPU?
> 
> To prevent waking up another idle cpu to run the update whereas
> newidle cpu is already woken up and about to be idle so the best
> candidate.
> All the aim of the removed code was to prevent waking up an idle cpu
> for doing something that could be done by the newidle cpu before it
> enters idle state

Ah OK, makes sense. So the only diff would be that newidle CPU will call
(1) on nohz.idle_cpus_mask rather on on sd->span.

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