linux-kernel - Re: [PATCH 04/11] sched,x86: Enable Turbo Boost Max Technology

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Date:   Wed, 24 Aug 2016 12:18:53 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        rjw@...ysocki.net, peterz@...radead.org, x86@...nel.org,
        bp@...e.de, sudeep.holla@....com, ak@...ux.intel.com,
        linux-acpi@...r.kernel.org, linux-pm@...r.kernel.org,
        alexey.klimov@....com, viresh.kumar@...aro.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        lenb@...nel.org, tim.c.chen@...ux.intel.com,
        paul.gortmaker@...driver.com, jpoimboe@...hat.com,
        mcgrof@...nel.org, jgross@...e.com, robert.moore@...el.com,
        dvyukov@...gle.com, jeyu@...hat.com,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 04/11] sched,x86: Enable Turbo Boost Max Technology


* Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com> wrote:

> From: Tim Chen <tim.c.chen@...ux.intel.com>
> 
> On some Intel cores, they can boosted to a higher turbo frequency than
> the other cores on the same die.  So we prefer processes to be run on
> them vs other lower frequency ones for extra performance.
> 
> We extend the asym packing feature in the scheduler to support packing
> task to the higher frequency core at the core sched domain level.
> 
> We set up a core priority metric to abstract the core preferences based
> on the maximum boost frequency.  The priority is instantiated such that
> the core with a higher priority is favored over the core with lower
> priority when making scheduling decision using ASYM_PACKING.  The smt
> threads that are of higher number are discounted in their priority so
> we will not try to pack tasks onto all the threads of a favored core
> before using other cpu cores.  The cpu that's of the highese priority
> in a sched_group is recorded in sched_group->asym_prefer_cpu during
> initialization to save lookup during load balancing.
> 
> A sysctl variable /proc/sys/kernel/sched_itmt_enabled is provided so
> the scheduling based on favored core can be turned on or off at run time.

> +/*
> + * Boolean to control whether we want to move processes to cpu capable
> + * of higher turbo frequency for cpus supporting Intel Turbo Boost Max
> + * Technology 3.0.
> + *
> + * It can be set via /proc/sys/kernel/sched_itmt_enabled
> + */
> +unsigned int __read_mostly sysctl_sched_itmt_enabled = 0;

Ugh, no.

We don't add features to the scheduler in the hope that they might or might not 
help. We either enable a new feature by default (and make damn sure it helps!),
or don't add the feature at all.

Thanks,

	Ingo