linux-kernel - Re: [PATCH v3 06/10] sched/fair: Use the prefer

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Message-ID: <8300f288-7157-5e2d-3bb3-badcffd15d34@arm.com>
Date:   Mon, 13 Feb 2023 13:17:09 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v3 06/10] sched/fair: Use the prefer_sibling flag of the
 current sched domain

On 10/02/2023 19:31, Ricardo Neri wrote:
> On Fri, Feb 10, 2023 at 05:12:30PM +0000, Valentin Schneider wrote:
>> On 10/02/23 17:53, Peter Zijlstra wrote:
>>> On Fri, Feb 10, 2023 at 02:54:56PM +0000, Valentin Schneider wrote:
>>>
>>>> So something like have SD_PREFER_SIBLING affect the SD it's on (and not
>>>> its parent), but remove it from the lowest non-degenerated topology level?
>>>
>>> So I was rather confused about the whole moving it between levels things
>>> this morning -- conceptually, prefer siblings says you want to try
>>> sibling domains before filling up your current domain. Now, balancing
>>> between siblings happens one level up, hence looking at child->flags
>>> makes perfect sense.
>>>
>>> But looking at the current domain and still calling it prefer sibling
>>> makes absolutely no sense what so ever.
>>>
>>
>> True :-)
>>
>>> In that confusion I think I also got the polarity wrong, I thought you
>>> wanted to kill prefer_sibling for the assymetric SMT cases, instead you
>>> want to force enable it as long as there is one SMT child around.
> 
> Exactly.
> 
>>>
>>> Whichever way around it we do it, I'm thinking perhaps some renaming
>>> might be in order to clarify things.
>>>
>>> How about adding a flag SD_SPREAD_TASKS, which is the effective toggle
>>> of the behaviour, but have it be set by children with SD_PREFER_SIBLING
>>> or something.
>>>
>>
>> Or entirely bin SD_PREFER_SIBLING and stick with SD_SPREAD_TASKS, but yeah
>> something along those lines.
> 
> I sense a consesus towards SD_SPREAD_TASKS.

Can you not detect the E-core dst_cpu case on MC with:

+       if (child)
+               sds->prefer_sibling = child->flags & SD_PREFER_SIBLING;
+       else if (sds->busiest)
+               sds->prefer_sibling = sds->busiest->group_weight > 1;
+

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