linux-kernel - Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

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Date:   Fri, 29 Mar 2019 15:23:14 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Julien Desfossez <jdesfossez@...italocean.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, tim.c.chen@...ux.intel.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Vineeth Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>
Subject: Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access


On 3/29/19 6:35 AM, Julien Desfossez wrote:
> On Fri, Mar 22, 2019 at 8:09 PM Subhra Mazumdar <subhra.mazumdar@...cle.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?

-------->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;
                         }