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

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Date:   Fri, 22 Mar 2019 17:06:18 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, tim.c.chen@...ux.intel.com,
        torvalds@...ux-foundation.org
Cc:     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 3/21/19 2:20 PM, Julien Desfossez wrote:
> On Tue, Mar 19, 2019 at 10:31 PM Subhra Mazumdar <subhra.mazumdar@...cle.com>
> wrote:
>> On 3/18/19 8:41 AM, Julien Desfossez wrote:
>>
> On further investigation, we could see that the contention is mostly in the
> way rq locks are taken. With this patchset, we lock the whole core if
> cpu.tag is set for at least one cgroup. Due to this, __schedule() is more or
> less serialized for the core and that attributes to the performance loss
> that we are seeing. We also saw that newidle_balance() takes considerably
> long time in load_balance() due to the rq spinlock contention. Do you think
> it would help if the core-wide locking was only performed when absolutely
> needed ?
>
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.