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:   Tue, 24 Nov 2020 10:35:08 +1100
From:   Balbir Singh <bsingharora@gmail.com>
To:     "Li, Aubrey" <aubrey.li@linux.intel.com>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, mingo@kernel.org,
        torvalds@linux-foundation.org, fweisbec@gmail.com,
        keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.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>, vineeth@bitbyteword.org,
        Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Agata Gruza <agata.gruza@intel.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>,
        graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com,
        pjt@google.com, rostedt@goodmis.org, derkling@google.com,
        benbjiang@tencent.com,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        James.Bottomley@hansenpartnership.com, OWeisse@umich.edu,
        Dhaval Giani <dhaval.giani@oracle.com>,
        Junaid Shahid <junaids@google.com>, jsbarnes@google.com,
        chris.hyser@oracle.com, Ben Segall <bsegall@google.com>,
        Josh Don <joshdon@google.com>, Hao Luo <haoluo@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Tim Chen <tim.c.chen@intel.com>
Subject: Re: [PATCH -tip 13/32] sched: Trivial forced-newidle balancer
Message-ID: <20201123233508.GC8893@balbir-desktop>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-14-joel@joelfernandes.org>
 <20201123043836.GG110669@balbir-desktop>
 <1de89fd3-fb5f-5aaa-8ea6-7a12d3307fa4@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1de89fd3-fb5f-5aaa-8ea6-7a12d3307fa4@linux.intel.com>
Precedence: bulk

On Mon, Nov 23, 2020 at 11:07:27PM +0800, Li, Aubrey wrote:
> On 2020/11/23 12:38, Balbir Singh wrote:
> > On Tue, Nov 17, 2020 at 06:19:43PM -0500, Joel Fernandes (Google) wrote:
> >> From: Peter Zijlstra <peterz@infradead.org>
> >>
> >> When a sibling is forced-idle to match the core-cookie; search for
> >> matching tasks to fill the core.
> >>
> >> rcu_read_unlock() can incur an infrequent deadlock in
> >> sched_core_balance(). Fix this by using the RCU-sched flavor instead.
> >>
> > ...
> >> +
> >> +		if (p->core_occupation > dst->idle->core_occupation)
> >> +			goto next;
> >> +
> > 
> > I am unable to understand this check, a comment or clarification in the
> > changelog will help. I presume we are looking at either one or two cpus
> > to define the core_occupation and we expect to match it against the
> > destination CPU.
> 
> IIUC, this check prevents a task from keeping jumping among the cores forever.
> 
> For example, on a SMT2 platform:
> - core0 runs taskA and taskB, core_occupation is 2
> - core1 runs taskC, core_occupation is 1
> 
> Without this check, taskB could ping-pong between core0 and core1 by core load
> balance.

But the comparison is p->core_occuption (as in tasks core occuptation,
not sure what that means, can a task have a core_occupation of > 1?)

Balbir Singh.