linux-kernel - Re: [PATCH v4 10/10] cpufreq: intel_pstate: Use CPPC to get max performance

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Date:   Thu, 22 Sep 2016 14:41:14 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, jolsa@...hat.com
Subject: Re: [PATCH v4 10/10] cpufreq: intel_pstate: Use CPPC to get max
 performance

On Thu, 2016-09-22 at 22:58 +0200, Rafael J. Wysocki wrote:
> 
> > > so what if there are two CPU packages
> > > and there are highest_perf differences in both, and we first enumerate
> > > the first package entirely before getting to the second one?
> > > 
> > > In that case we'll schedule the work item after enumerating the first
> > > package and it may rebuild the sched domains before all priorities are
> > > set for the second package, may it not?
> > That is not a problem.  For the second package, all the cpu priorities
> > are initialized to the same value.  So even if we start to do
> > asym_packing in the scheduler for the whole system,
> > on the second package, all the cpus are treated equally by the scheduler.
> > We will operate as if there is no favored core till we update the
> > priorities of the cpu on the second package.
> OK
> 
> But updating those priorities after we have set the "ITMT capable"
> flag is not a problem?  Nobody is going to be confused and so on?
> 

Not a problem.  The worst thing that could happen is we schedule a job
to a cpu with a lesser max turbo freq first while the priorities update are in
progress.

> > 
> > That said, we don't enable ITMT automatically for 2 package system.
> > So the explicit sysctl command to enable ITMT and cause the sched domain
> > rebuild for 2 package system is most likely to come after
> > we have discovered and set all the cpu priorities.
> Right, but if that behavior is relied on, there should be a comment
> about that in the code (and relying on it would be kind of fragile for
> that matter).

No, we don't rely on this behavior of not enabling ITMT automatically
for 2 package system.  We could enable ITMT for 2
package system by default if we want to.  Then asym_packing will just
consider the second package's cpus to be equal priorities if they haven't
been set.  

> 
> > 
> > > 
> > > 
> > > This seems to require some more consideration.
> > > 
> > > > 
> > > > 
> > > > +               /*
> > > > +                * Since this function is in the hotcpu notifier callback
> > > > +                * path, submit a task to workqueue to call
> > > > +                * sched_set_itmt_support().
> > > > +                */
> > > > +               schedule_work(&sched_itmt_work);
> > > It doesn't make sense to do this more than once IMO and what if we
> > > attempt to schedule the work item again when it has been scheduled
> > > once already?  Don't we need any protection here?
> > It is not a problem for sched_set_itmt_support to be called more than
> > once.
> While it is not incorrect, it also is not particularly useful to
> schedule a work item just to find out later that it had nothing to do
> to begin with.

Setting ITMT capability is done per socket during system boot.  So there is no
performance impact at all so it should not be an issue.

Tim