From mboxrd@z Thu Jan  1 00:00:00 1970
From: benh@kernel.crashing.org (Benjamin Herrenschmidt)
Date: Sun, 23 Mar 2014 14:12:06 +1100
Subject: [PATCH v3 6/6] sched: powerpc: Add SD_SHARE_POWERDOMAIN for SMT
 level
In-Reply-To: <532E3DB4.9060908@linux.vnet.ibm.com>
References: <1395246165-31150-1-git-send-email-vincent.guittot@linaro.org>
 <1395246165-31150-7-git-send-email-vincent.guittot@linaro.org>
 <532E3DB4.9060908@linux.vnet.ibm.com>
Message-ID: <1395544326.3460.98.camel@pasglop>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Sun, 2014-03-23 at 07:19 +0530, Preeti U Murthy wrote:
> We were discussing the impact of this consolidation and we are not too
> sure if it will yield us good power efficiency. So we would want to
> experiment with the power aware scheduler to find the "sweet spot" for
> the number of threads to consolidate to and more importantly if there
> is
> one such number at all. Else we would not want to go this way at all.
> Hence it looks best if this patch is dropped until we validate it. We
> don't want the code getting in and then out if we find out later there
> are no benefits to it.
> 
> I am sorry that I suggested this patch a bit pre-mature in the
> experimentation and validation stage. When you release the load
> balancing patchset for power aware scheduler I shall validate this
> patch. But until then its best if it does not get merged.

It's quite possible that we never find a correct "sweet spot" for all
workloads.

Ideally, the "target" number of used threads per core should be a
tunable so that the user / distro can "tune" based on a given workload
whether to pack cores and how much to pack them, vs. spreading the
workload. Akin to scheduling for performance vs. power in a way (though
lower perf usually means higher power due to longer running jobs of
course).

In any case, we need to experiment.

Cheers,
Ben.