Re: [PATCH 1/2] sched/fair: Fix how load gets propagated from cfs_rq to its sched_entity

From: Tejun Heo <tj@kernel.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Mike Galbraith <efault@gmx.de>, Paul Turner <pjt@google.com>,
	Chris Mason <clm@fb.com>,
	kernel-team@fb.com
Subject: Re: [PATCH 1/2] sched/fair: Fix how load gets propagated from cfs_rq to its sched_entity
Date: Fri, 28 Apr 2017 13:46:15 -0400	[thread overview]
Message-ID: <20170428174615.GB4628@htj.duckdns.org> (raw)
In-Reply-To: <CAKfTPtC4_e1MkN2GUVqzxdcD7xBNMrQR=ZZcgnvK9of62_Bz5w@mail.gmail.com>

Hello, Vincent.

On Thu, Apr 27, 2017 at 10:59:12AM +0200, Vincent Guittot wrote:
> > But the only difference there is that we lose accuracy in calculation;
> > otherwise, the end results are the same, no?
> 
> Yes the end result is the same, it was mainly to point out the range
> difference and explain why we need scale_load_down(shares) for the 2nd
> argument of min.
> This should also explain the warning issue you mentioned earlier

I'm not sure this makes sense.  Practically, we're doing more shifts
just to lose calculation accuracy.  Even conceptually, what we're
doing is

             C
	A * ---
             B

Where A is in a different scale while B and C are in the same.  What
you're suggesting is

             scale_down(C)
	A * ---------------
	     scale_down(B)

I can't see why this is better in any way.

Thanks.

-- 
tejun