Re: [PATCH -tip 13/32] sched: Trivial forced-newidle balancer

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,
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	fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
	Phil Auld <pauld@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
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	Tom Lendacky <thomas.lendacky@amd.com>,
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	Tim Chen <tim.c.chen@intel.com>
Subject: Re: [PATCH -tip 13/32] sched: Trivial forced-newidle balancer
Date: Thu, 26 Nov 2020 08:28:28 +1100	[thread overview]
Message-ID: <20201125212828.GA163610@balbir-desktop> (raw)
In-Reply-To: <2277bfa9-7f67-6b66-b2db-a2130993de53@linux.intel.com>

On Tue, Nov 24, 2020 at 08:32:01AM +0800, Li, Aubrey wrote:
> On 2020/11/24 7:35, Balbir Singh wrote:
> > 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?)
> >
> 
> p->core_occupation is assigned to the core occupation in the last pick_next_task.
> (so yes, it can have a > 1 core_occupation).
>

Hmm.. I find that hard to interpret that. But I am happy to re-read the
code again.

Balbir Singh.