linux-kernel - Re: [PATCH v6 5/9] x86/sysctl: Add sysctl for ITMT scheduling feature

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Date:   Thu, 27 Oct 2016 12:32:19 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>, rjw@...ysocki.net,
        mingo@...hat.com, bp@...e.de, x86@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, jolsa@...hat.com,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v6 5/9] x86/sysctl: Add sysctl for ITMT scheduling
 feature

On Wed, 2016-10-26 at 20:09 +0200, Thomas Gleixner wrote:
> On Wed, 26 Oct 2016, Tim Chen wrote:
> > 
> > On Wed, 2016-10-26 at 13:24 +0200, Thomas Gleixner wrote:
> > > 
> > > > 
> > > > There were reservations on the multi-socket case of ITMT, maybe it would
> > > > help to spell those out in great detail here. That is, have the comment
> > > > explain the policy instead of simply stating what the code does (which
> > > > is always bad comment policy, you can read the code just fine).
> > > What is the objection for multi sockets? If it improves the behaviour then
> > > why would this be a bad thing for multi sockets?
> > For multi-socket (server system), it is much more likely that they will
> > have multiple cpus in a socket busy and not run in turbo mode. So the extra
> > work in migrating the workload to the one with extra headroom will
> > not make use of those headroom in that scenario.  I will update the comment
> > to reflect this policy.
> So on a single socket server system the extra work does not matter, right?
> Don't tell me that single socket server systems are irrelevant. Intel is
> actively promoting single socket CPUs, like XEON D, for high densitiy
> servers...
> 
> Instead of handwaving arguments I prefer a proper analysis of what the
> overhead is and why it is not a good thing for loaded servers in general.
> 
> Then instead of slapping half baken heuristics into the code, we should sit
> down and think a bit harder about it.
> 

The ITMT scheduling overhead should be small.  Mostly a small number of 
cycles initially spent to idle balance tasks towards an idled favored core, and cycles to refill
hot data in the mid level cache for the migrated task.  Those should be a very
small percentage of the cycles that the task spent running on the favored core.
So any extra boost in frequency should compensate so should be a good trade off.

After some internal discussions, we think we should enable the ITMT feature by 
default for all systems supporting ITMT.  I will remove the single socket
restriction.

Thanks.

Tim