Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 8 May 2020 11:09:25 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Aaron Lu <aaron.lwe@gmail.com>
Cc:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Aaron Lu <aaron.lu@linux.alibaba.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        "Li, Aubrey" <aubrey.li@linux.intel.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Joel Fernandes <joelaf@google.com>,
        Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH updated v2] sched/fair: core wide cfs task priority
 comparison
Message-ID: <20200508090925.GV5298@hirez.programming.kicks-ass.net>
References: <20200415033408.GA168322@ziqianlu-desktop.localdomain>
 <20200415040741.GA169001@ziqianlu-desktop.localdomain>
 <CANaguZAXsjD=X-bB7dvQZ3FSqLioiZU=YREHH_7JqiFsZCTxHA@mail.gmail.com>
 <20200417094045.GA197704@ziqianlu-desktop.localdomain>
 <20200420080759.GA224731@ziqianlu-desktop.localdomain>
 <CANaguZDP3nyBdjLeeCZGoUoYk6Lf0HUv19N2Qxo4CpZfrjPZHA@mail.gmail.com>
 <20200421025131.GA227300@aaronlu-desktop>
 <20200424142443.GA263207@aaronlu-desktop>
 <20200506143506.GH5298@hirez.programming.kicks-ass.net>
 <20200508084419.GA120223@aaronlu-desktop>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200508084419.GA120223@aaronlu-desktop>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, May 08, 2020 at 04:44:19PM +0800, Aaron Lu wrote:
> On Wed, May 06, 2020 at 04:35:06PM +0200, Peter Zijlstra wrote:

> > Aside from this being way to complicated for what it does -- you
> > could've saved the min_vruntime for each rq and compared them with
> > subtraction -- it is also terminally broken afaict.
> >
> > Consider any infeasible weight scenario. Take for instance two tasks,
> > each bound to their respective sibling, one with weight 1 and one with
> > weight 2. Then the lower weight task will run ahead of the higher weight
> > task without bound.
> 
> I don't follow how this could happen. Even the lower weight task runs
> first, after some time, the higher weight task will get its turn and
> from then on, the higher weight task will get more chance to run(due to
> its higher weight and thus, slower accumulation of vruntime).

That seems to assume they're mutually exclusive. In that case, as I
argued, we only have a single runqueue and then yes it works. But if
they're not exclusive, and can run concurrently, it comes apart.