Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 2 Apr 2019 09:42:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Subhra Mazumdar <subhra.mazumdar@oracle.com>
Cc:     Julien Desfossez <jdesfossez@digitalocean.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, tim.c.chen@linux.intel.com,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
        Vineeth Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>
Subject: Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access
Message-ID: <20190402074235.GI12232@hirez.programming.kicks-ass.net>
References: <fcad329b-e421-624a-0d1a-a98466eaf2ed@oracle.com>
 <1553866527-18879-1-git-send-email-jdesfossez@digitalocean.com>
 <6e8e6fa0-8976-5e97-d90c-af0b4a6fc8b2@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <6e8e6fa0-8976-5e97-d90c-af0b4a6fc8b2@oracle.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Mar 29, 2019 at 03:23:14PM -0700, Subhra Mazumdar wrote:
> 
> On 3/29/19 6:35 AM, Julien Desfossez wrote:
> > On Fri, Mar 22, 2019 at 8:09 PM Subhra Mazumdar <subhra.mazumdar@oracle.com>
> > wrote:
> > > Is the core wide lock primarily responsible for the regression? I ran
> > > upto patch
> > > 12 which also has the core wide lock for tagged cgroups and also calls
> > > newidle_balance() from pick_next_task(). I don't see any regression.  Of
> > > course
> > > the core sched version of pick_next_task() may be doing more but
> > > comparing with
> > > the __pick_next_task() it doesn't look too horrible.
> > On further testing and investigation, we also agree that spinlock contention
> > is not the major cause for the regression, but we feel that it should be one
> > of the major contributing factors to this performance loss.
> > 
> > 
> I finally did some code bisection and found the following lines are
> basically responsible for the regression. Commenting them out I don't see
> the regressions. Can you confirm? I am yet to figure if this is needed for
> the correctness of core scheduling and if so can we do this better?

It was meant to be an optimization; specifically, when no cookie was
set, don't bother to schedule the sibling(s).

> -------->8-------------
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index fe3918c..3b3388a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3741,8 +3741,8 @@ pick_next_task(struct rq *rq, struct task_struct
> *prev, struct rq_flags *rf)
> ???????????????????????????????? * If there weren't no cookies; we don't
> need
> ???????????????????????????????? * to bother with the other siblings.
> */
> -?????????????????????????????? if (i == cpu && !rq->core->core_cookie)
> -?????????????????????????????????????? goto next_class;
> +?????????????????????????????? //if (i == cpu && !rq->core->core_cookie)
> +?????????????????????????????????????? //goto next_class;
> 
> continue;
> ??????????????????????? }