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Date:   Thu, 25 Apr 2019 20:59:12 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Ingo Molnar <mingo@kernel.org>
cc:     Mel Gorman <mgorman@techsingularity.net>,
        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>,
        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
In-Reply-To: <20190425185343.GA122353@gmail.com>
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On Thu, 25 Apr 2019, Ingo Molnar wrote:
> * Mel Gorman <mgorman@techsingularity.net> wrote:
> > I don't have the data in a format that can be present everything in a clear
> > format but here is an attempt anyway. This is long but the central point
> > that when when a machine is lightly loaded, HT Off generally performs
> > better than HT On and even when heavily utilised, it's still not a
> > guaranteed loss. I only suggest reading after this if you have coffee
> > and time. Ideally all this would be updated with a comparison to core
> > scheduling but I may not get it queued on my test grid before I leave
> > for LSF/MM and besides, the authors pushing this feature should be able
> > to provide supporting data justifying the complexity of the series.
> 
> BTW., a side note: I'd suggest introducing a runtime toggle 'nosmt' 
> facility, i.e. turn a system between SMT and non-SMT execution runtime, 
> with full reversability between these states and no restrictions.

It exists already: /sys/devices/system/cpu/smt/control

Setting it to off will offline all siblings, on will online them again.

Thanks,

	tglx