Date:   Tue, 1 Sep 2020 17:23:00 -0400
From:   Vineeth Pillai <viremana@...ux.microsoft.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     peterz@...radead.org,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Chris Hyser <chris.hyser@...cle.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        mingo@...nel.org, tglx@...utronix.de, pjt@...gle.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        rostedt@...dmis.org, derkling@...gle.com, benbjiang@...cent.com,
        Aaron Lu <aaron.lu@...ux.alibaba.com>
Subject: Re: [RFC PATCH v7 08/23] sched: Add core wide task selection and
 scheduling.

Hi Joel,

On 9/1/20 1:30 PM, Joel Fernandes wrote:
>> I think we can come here when hotplug thread is scheduled during online, but
>> mask is not yet updated. Probably can add it with this comment as well.
>>
> I don't see how that is possible. Because the cpuhp threads run during the
> CPU onlining process, the boot thread for the CPU coming online would have
> already updated the mask.
Sorry my mistake. I got confused with the online state ordering.

>> Another unrelated, but related note :-)
>> Besides this, I think we need to retain on more change from the previous
>> patch. We would need to make core_pick_seq per sibling instead of per
>> core. Having it per core might lead to unfairness. For eg: When a cpu
>> sees that its sibling's core_pick is the one which is already running, it
>> will not send IPI. but core_pick remains set and core->core_pick_seq is
>> incremented. Now if the sibling is preempted due to a high priority task
> Then don't keep the core_pick set then. If you don't send it IPI and if
> core_pick is already running, then NULL it already. I don't know why we add
> to more corner cases by making assumptions. We have enough open issues that
> are not hotplug related. Here's my suggestion :
>
> 1.  Keep the ideas consistent, forget about the exact code currently written
> and just understand the pick_seq is for siblings knowing that something was
> picked for the whole core.  So if their pick_seq != sched_seq, then they have
> to pick what was selected.
I was trying to keep the ideas consistent. The requirement of core_pick
was to let the scheduled cpu know that a pick has been made. And
initial idea was to have the counter core wide. But I found this gap
that pick is not always core wide and assuming it to be core wide can
cause fairness issues. So I was proposing the idea of changing it from
core wide to per sibling. In other words, I was trying to make sure 
core_pick,
along with task_seq and sched_seq is trying to serve its purpose of letting
a sibling know that a new task pick has been made for it. I cannot think of
a reason, why core_pick should be core wide. I might be missing something.

> 2. If core_pick should be NULL, then NULL it in some path. If you keep some
> core_pick and you increment pick_seq, then you are automatically asking the
> sibling to pick that task up then next time it enters schedule(). See if [1]
> will work?
>
> Note that, we have added logic in this patch that does a full selection if
> rq->core_pick == NULL.

I agree, setting rq->core_pick = NULL is another way to solve this 
issue, but
still I feel its semantically incorrect to think that a pick is core 
wide when it
could actually be to only a subset of siblings in the core. If there is 
a valid
reasoning for having core_pick to be core wide, I completely agree with the
fix of resetting core_pick.

>> or its time slice expired, it enters schedule. But it goes to fast path and
>> selects the running task there by starving the high priority task. Having
>> the core_pick_seq per sibling will avoid this. It might also help in some
>> hotplug corner cases as well.
> That can be a separate patch IMHO. It has nothing to do with
> stability/crashing of concurrent and rather infrequent CPU hotplug
> operations.
Agree. Sorry for the confusion, my intention was to not have the logic in
this patch.

> Also, Peter said pick_seq is for core-wide picking. If you want to add
> another semantic, then maybe add another counter which has a separate
> meaning and justify why you are adding it.
I think just one counter is enough. Unless, there is a need to keep the 
counter
to track core wide pick, I feel it is worth to change the design and 
make the
counter serve its purpose. Will think through this and send it as a separate
patch if needed.

Thanks,
Vineeth

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