linux-kernel - Re: [PATCH] sched: fix incorrect PELT values on SMT

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Date:   Mon, 22 Aug 2016 10:31:40 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Morten Rasmussen <morten.rasmussen@....com>
Cc:     Steve Muckle <steve.muckle@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Steve Muckle <smuckle@...aro.org>
Subject: Re: [PATCH] sched: fix incorrect PELT values on SMT

2016-08-19 23:30 GMT+08:00 Morten Rasmussen <morten.rasmussen@....com>:
> Hi Steve,
>
> On Thu, Aug 18, 2016 at 06:55:41PM -0700, Steve Muckle wrote:
>> PELT scales its util_sum and util_avg values via
>> arch_scale_cpu_capacity(). If that function is passed the CPU's sched
>> domain then it will reduce the scaling capacity if SD_SHARE_CPUCAPACITY
>> is set. PELT does not pass in the sd however. The other caller of
>> arch_scale_cpu_capacity, update_cpu_capacity(), does. This means
>> util_sum and util_avg scale beyond the CPU capacity on SMT.
>>
>> On an Intel i7-3630QM for example rq->cpu_capacity_orig is 589 but
>> util_avg scales up to 1024.
>
> I can't convince myself whether this is the right thing to do. SMT is a
> bit 'special' and it depends on how you model SMT capacity.
>
> I'm no SMT expert, but the way I understand the current SMT capacity
> model is that capacity_orig represents the capacity of the SMT-thread
> when all its thread-siblings are busy. The true capacity of an
> SMT-thread where all thread-siblings are idle is actually 1024, but we
> don't model this (it would be nightmare to track when the capacity
> should change). The capacity of a core with two or more SMT-threads is
> chosen to be 1024 + smt_gain, where smt_gain is supposed represent the
> additional throughput we gain for the additional SMT-threads. The reason
> why we don't have 1024 per thread is that we would prefer to have only
> one task per core if possible.

Agreed, maybe the capacity of an SMP-thread where all thread-siblings
are idle can be 1024 + smt_gain after latest IA technology.
http://www.intel.com/content/www/us/en/architecture-and-technology/turbo-boost/turbo-boost-max-technology.html

Regards,
Wanpeng Li