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;
MIME-Version: 1.0
References: <cover.1583332764.git.vpillai@digitalocean.com>
 <855831b59e1b3774b11c3e33050eac4cc4639f06.1583332765.git.vpillai@digitalocean.com>
 <20200414113639.GS20730@hirez.programming.kicks-ass.net>
In-Reply-To: <20200414113639.GS20730@hirez.programming.kicks-ass.net>
From:   Vineeth Remanan Pillai <vpillai@digitalocean.com>
Date:   Tue, 14 Apr 2020 17:35:07 -0400
Message-ID: <CANaguZDwK6bXvJabt5gS=kLMVsGNf97u50+1-v_t8bJxCrJ6sQ@mail.gmail.com>
Subject: Re: [RFC PATCH 03/13] sched: Core-wide rq->lock
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     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>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.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>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

> Aside from the fact that it's probably much saner to write this as:
>
>         rq->core_enabled = static_key_enabled(&__sched_core_enabled);
>
> I'm fairly sure I didn't write this part. And while I do somewhat see
> the point of disabling core scheduling for a core that has only a single
> thread on, I wonder why we care.
>
I think this change was to fix some crashes which happened due to
uninitialized rq->core if a sibling was offline during boot and is
onlined after coresched was enabled.

https://lwn.net/ml/linux-kernel/20190424111913.1386-1-vpillai@digitalocean.com/

I tried to fix it by initializing coresched members during a cpu online
and tearing it down on a cpu offline. This was back in v3 and do not
remember the exact details. I shall revisit this and see if there is a
better way to fix the race condition above.

Thanks,
Vineeth