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Date:   Fri, 26 Apr 2019 10:42:22 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Aubrey Li <aubrey.intel@gmail.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        Fr?d?ric Weisbecker <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jiri Kosina <jkosina@suse.cz>
Subject: Re: [RFC PATCH v2 00/17] Core scheduling v2
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* Mel Gorman <mgorman@techsingularity.net> wrote:

> > > Same -- performance is better until the machine gets saturated and
> > > disabling HT hits scaling limits earlier.
> > 
> > Interesting. This strongly suggests sub-optimal SMT-scheduling in the 
> > non-saturated HT case, i.e. a scheduler balancing bug.
> > 
> 
> Yeah, it does but mpstat didn't appear to indicate that SMT siblings are
> being used prematurely so it's a bit of a curiousity.
> 
> > As long as loads are clearly below the physical cores count (which they 
> > are in the early phases of your table) the scheduler should spread tasks 
> > without overlapping two tasks on the same core.
> > 
> 
> It should, but it's not perfect. For example, wake_affine_idle does not
> take sibling activity into account even though select_idle_sibling *may*
> take it into account. Even select_idle_sibling in its fast path may use
> an SMT sibling instead of searching.
> 
> There are also potential side-effects with cpuidle. Some workloads
> migration around the socket as they are communicating because of how the
> search for an idle CPU works. With SMT on, there is potentially a longer
> opportunity for a core to reach a deep c-state and incur a bigger wakeup
> latency. This is a very weak theory but I've seen cases where latency
> sensitive workloads with only two communicating tasks are affected by
> CPUs reaching low c-states due to migrations.
> 
> > Clearly it doesn't.
> > 
> 
> It's more that it's best effort to wakeup quickly instead of being perfect
> by using an expensive search every time.

Yeah, but your numbers suggest that for *most* not heavily interacting 
under-utilized CPU bound workloads we hurt in the 5-10% range compared to 
no-SMT - more in some cases.

So we avoid a maybe 0.1% scheduler placement overhead but inflict 5-10% 
harm on the workload, and also blow up stddev by randomly co-scheduling 
two tasks on the same physical core? Not a good trade-off.

I really think we should implement a relatively strict physical core 
placement policy in the under-utilized case, and resist any attempts to 
weaken this for special workloads that ping-pong quickly and benefit from 
sharing the same physical core.

I.e. as long as load is kept below ~50% the SMT and !SMT benchmark 
results and stddev numbers should match up. (With a bit of a leewy if the 
workload gets near to 50% or occasionally goes above it.)

There's absolutely no excluse for these numbers at 30-40% load levels I 
think.

Thanks,

	Ingo