linux-kernel - Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and add cluster scheduler

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Date:   Tue, 26 Jan 2021 12:02:21 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     "valentin.schneider@....com" <valentin.schneider@....com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "sudeep.holla@....com" <sudeep.holla@....com>,
        "aubrey.li@...ux.intel.com" <aubrey.li@...ux.intel.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>,
        "xuwei (O)" <xuwei5@...wei.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        "tiantao (H)" <tiantao6@...ilicon.com>
Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and
 add cluster scheduler

On 25/01/2021 11:50, Song Bao Hua (Barry Song) wrote:
> 
>> -----Original Message-----
>> From: Dietmar Eggemann [mailto:dietmar.eggemann@....com]
>> Sent: Wednesday, January 13, 2021 12:00 AM
>> To: Morten Rasmussen <morten.rasmussen@....com>; Tim Chen
>> <tim.c.chen@...ux.intel.com>
>> Cc: Song Bao Hua (Barry Song) <song.bao.hua@...ilicon.com>;
>> valentin.schneider@....com; catalin.marinas@....com; will@...nel.org;
>> rjw@...ysocki.net; vincent.guittot@...aro.org; lenb@...nel.org;
>> gregkh@...uxfoundation.org; Jonathan Cameron <jonathan.cameron@...wei.com>;
>> mingo@...hat.com; peterz@...radead.org; juri.lelli@...hat.com;
>> rostedt@...dmis.org; bsegall@...gle.com; mgorman@...e.de;
>> mark.rutland@....com; sudeep.holla@....com; aubrey.li@...ux.intel.com;
>> linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org;
>> linux-acpi@...r.kernel.org; linuxarm@...neuler.org; xuwei (O)
>> <xuwei5@...wei.com>; Zengtao (B) <prime.zeng@...ilicon.com>; tiantao (H)
>> <tiantao6@...ilicon.com>
>> Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and
>> add cluster scheduler
>>
>> On 11/01/2021 10:28, Morten Rasmussen wrote:
>>> On Fri, Jan 08, 2021 at 12:22:41PM -0800, Tim Chen wrote:
>>>>
>>>>
>>>> On 1/8/21 7:12 AM, Morten Rasmussen wrote:
>>>>> On Thu, Jan 07, 2021 at 03:16:47PM -0800, Tim Chen wrote:
>>>>>> On 1/6/21 12:30 AM, Barry Song wrote:

[...]

>> wake_wide() switches between packing (select_idle_sibling(), llc_size
>> CPUs) and spreading (find_idlest_cpu(), all CPUs).
>>
>> AFAICS, since none of the sched domains set SD_BALANCE_WAKE, currently
>> all wakeups are (llc-)packed.
> 
> Sorry for late response. I was struggling with some other topology
> issues recently.
> 
> For "all wakeups are (llc-)packed",
> it seems you mean current want_affine is only affecting the new_cpu,
> and for wake-up path, we will always go to select_idle_sibling() rather
> than find_idlest_cpu() since nobody sets SD_WAKE_BALANCE in any
> sched_domain ?
> 
>>
>>  select_task_rq_fair()
>>
>>    for_each_domain(cpu, tmp)
>>
>>      if (tmp->flags & sd_flag)
>>        sd = tmp;
>>
>>
>> In case we would like to further distinguish between llc-packing and
>> even narrower (cluster or MC-L2)-packing, we would introduce a 2. level
>> packing vs. spreading heuristic further down in sis().
> 
> I didn't get your point on "2 level packing". Would you like
> to describe more? It seems you mean we need to have separate
> calculation for avg_scan_cost and sched_feat(SIS_) for cluster
> (or MC-L2) since cluster and llc are not in the same level
> physically?

By '1. level packing' I meant going sis() (i.e. sd=per_cpu(sd_llc,
target)) instead of routing WF_TTWU through find_idlest_cpu() which uses
a broader sd span (in case all sd's (or at least up to an sd > llc)
would have SD_BALANCE_WAKE set).
wake_wide() (wakee/waker flip heuristic) is currently used to make this
decision. But since no sd sets SD_BALANCE_WAKE we always go sis() for
WF_TTWU.

'2. level packing' would be the decision between cluster- and
llc-packing. The question was which heuristic could be used here.

>> IMHO, Barry's current implementation doesn't do this right now. Instead
>> he's trying to pack on cluster first and if not successful look further
>> among the remaining llc CPUs for an idle CPU.
> 
> Yes. That is exactly what the current patch is doing.

And this will be favoring cluster- over llc-packing for each task instead.