[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <37d12346-4249-d699-70b7-fc24a5774fb3@arm.com>
Date: Thu, 19 Sep 2019 19:07:09 +0100
From: Valentin Schneider <valentin.schneider@....com>
To: Parth Shah <parth@...ux.ibm.com>,
Patrick Bellasi <patrick.bellasi@....com>
Cc: linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
subhra mazumdar <subhra.mazumdar@...cle.com>,
tim.c.chen@...ux.intel.com, mingo@...hat.com,
morten.rasmussen@....com, dietmar.eggemann@....com, pjt@...gle.com,
vincent.guittot@...aro.org, quentin.perret@....com,
dhaval.giani@...cle.com, daniel.lezcano@...aro.org, tj@...nel.org,
rafael.j.wysocki@...el.com, qais.yousef@....com,
Patrick Bellasi <patrick.bellasi@...bug.net>
Subject: Re: Usecases for the per-task latency-nice attribute
On 19/09/2019 17:41, Parth Shah wrote:
> So jotting down separately, in case if we think to have "latency-nice"
> terminology, then we might need to select one of the 2 interpretation:
>
> 1).
>> -20 (least nice to latency, i.e. sacrifice latency for throughput)
>> +19 (most nice to latency, i.e. sacrifice throughput for latency)
>>
>
> 2).
> -20 (least nice to other task in terms of sacrificing latency, i.e.
> latency-sensitive)
> +19 (most nice to other tasks in terms of sacrificing latency, i.e.
> latency-forgoing)
>
>
I'd vote for 1 (duh) but won't fight for it, if it comes to it I'd be
happy with a random draw :D
>> Aren't we missing the point about tweaking the sched domain scans (which
>> AFAIR was the original point for latency-nice)?
>>
>> Something like default value is current behaviour and
>> - Being less latency-sensitive means increasing the scans (e.g. trending
>> towards only going through the slow wakeup-path at the extreme setting)
>> - Being more latency-sensitive means reducing the scans (e.g. trending
>> towards a fraction of the domain scanned in the fast-path at the extreme
>> setting).
>>
>
> Correct. But I was pondering upon the values required for this case.
> Is having just a range from [-20,19] even for larger system sufficient enough?
>
As I said in the original thread by Subhra, this range should be plenty
enough IMO. You get ~5% deltas in each direction after all.
>>>
>>
>> $> Load balance tuning
>> ======================
>>
>> Already mentioned these in [4]:
>>
>> - Increase (reduce) nr_balance_failed threshold when trying to active
>> balance a latency-sensitive (non-latency-sensitive) task.
>>
>> - Increase (decrease) sched_migration_cost factor in task_hot() for
>> latency-sensitive (non-latency-sensitive) tasks.
>>
>
> Thanks for listing down your ideas.
>
> These are pretty useful optimization in general. But one may wonder if we
> reduce the search scans for idle-core in wake-up path and by-chance selects
> the busy core, then one would expect load balancer to move the task to idle
> core.
>
> If I got it correct, the in such cases, the sched_migration_cost should be
> carefully increased, right?
>
IIUC you're describing a scenario where we fail to find an idle core due to
a wakee being latency-sensitive (thus shorter scan), and place it on a rq
that already has runnable tasks (despite idle rqs being available).
In this case yes, we could potentially have a balance attempt trying to pull
from that rq. We'd try to pull the non-running tasks first, and if a
latency-sensitive task happens to be one of them we should be careful with
what we do - a migration could lead to unwanted latency.
It might be a bit more clear when you're balancing between busy cores -
overall I think you should try to migrate the non-latency-sensitive
tasks first. Playing with task_hot() could be one of the ways to do that, but
it's just a suggestion at this time.
>
>>>> References:
>>>> ===========
>>>> [1]. https://lkml.org/lkml/2019/8/30/829
>>>> [2]. https://lkml.org/lkml/2019/7/25/296
>>>
>>> [3]. Message-ID: <20190905114709.GM2349@...ez.programming.kicks-ass.net>
>>> https://lore.kernel.org/lkml/20190905114709.GM2349@hirez.programming.kicks-ass.net/
>>>
>>
>> [4]: https://lkml.kernel.org/r/3d3306e4-3a78-5322-df69-7665cf01cc43@arm.com
>>
>>>
>>> Best,
>>> Patrick
>>>
>
> Thanks,
> Parth
>
Powered by blists - more mailing lists