wake_wide mechanism clarification

From: Joel Fernandes <joelaf@google.com>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Juri Lelli <Juri.Lelli@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Brendan Jackman <brendan.jackman@arm.com>,
	Chris Redpath <Chris.Redpath@arm.com>,
	Josef Bacik <jbacik@fb.com>
Subject: wake_wide mechanism clarification
Date: Thu, 29 Jun 2017 17:19:14 -0700	[thread overview]
Message-ID: <CAJWu+oo0_zAWLjObRZp-r3VH5kKKb4aFVnRevtbYh+uH_tCNag@mail.gmail.com> (raw)

Dear Mike,

I wanted your kind help to understand your patch "sched: beef up
wake_wide()"[1] which is a modification to the original patch from
Michael Wang [2].

In particular, I didn't following the following comment:
" to shared cache, we look for a minimum 'flip' frequency of llc_size
in one partner, and a factor of lls_size higher frequency in the
other."

Why are wanting the master's flip frequency to be higher than the
slaves by the factor?

The code here is written as:

if (slave < factor || master < slave * factor)
   return 0;

However I think we should just do (with my current and probably wrong
understanding):

if (slave < factor || master < factor)
    return 0;

Basically, I didn't follow why we multiply the slave's flips with
llc_size. That makes it sound like the master has to have way more
flips than the slave to return 0 from wake_wide. Could you maybe give
an example to clarify? Thanks a lot for your help,

I am also CC'ing Peter and some ARM folks for the discussion (and also
Jocef who was discuss it with Mike on the mailing list few years ago).

Thanks,
Joel

[1] https://patchwork.kernel.org/patch/6787941/
[2] https://lkml.org/lkml/2013/7/4/20