Re: [patch] sched: fix SMT scheduler regression in find_busiest_queue()

From: Suresh Siddha <suresh.b.siddha@intel.com>
To: "svaidy@linux.vnet.ibm.com" <svaidy@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"Ma, Ling" <ling.ma@intel.com>,
	"Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
	"ego@in.ibm.com" <ego@in.ibm.com>
Subject: Re: [patch] sched: fix SMT scheduler regression in find_busiest_queue()
Date: Sat, 13 Feb 2010 10:39:36 -0800	[thread overview]
Message-ID: <1266086376.2677.45.camel@sbs-t61> (raw)
In-Reply-To: <20100213182748.GB5882@dirshya.in.ibm.com>

On Sat, 2010-02-13 at 11:27 -0700, Vaidyanathan Srinivasan wrote:
> The fix that you have posted will solve the problem described.

Thanks. This SMT scheduler regression is critical for performance and
would like Ingo/Peterz to push this to Linus as soon as possible. We can
fix other known issues when we have patches ready and acceptable to
everyone. Agree?

> However we need to make sched_smt_powersavings also work by increasing
> the group capacity and allowing two tasks to run in a core.

I don't think you saying that this patch breaks sched_smt_powersavings?
If so, We need to address power-saving aspect differently. Atleast this
is not as critical, as we don't have any customer who is using the
smt/mc powersavings tunables.

> As Peter mentioned, SD_PREFER_SIBLING flag is meant to spread the work
> across group at any sched domain so that the solution will work for
> pre-Nehalem quad cores also.  But it still needs some work to get it
> right.

Agree.

> The solution you have posted will not work for non-HT quad cores where
> we want the tasks to be spread across cache domains for best
> performance though not a severe performance regression as in the case
> of Nehalem.

This is completely different issue from this patch and I started another
thread for this.

thanks
suresh