Re: [PATCH v2 4/7] sched/fair: Introduce sched_smt_siblings_idle()

From: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To: Chen Yu <yu.c.chen@intel.com>
Cc: Valentin Schneider <vschneid@redhat.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Ricardo Neri <ricardo.neri@intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Len Brown <len.brown@intel.com>, Mel Gorman <mgorman@suse.de>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	"Tim C . Chen" <tim.c.chen@intel.com>
Subject: Re: [PATCH v2 4/7] sched/fair: Introduce sched_smt_siblings_idle()
Date: Thu, 29 Dec 2022 16:17:23 -0800	[thread overview]
Message-ID: <20221230001723.GA15109@ranerica-svr.sc.intel.com> (raw)
In-Reply-To: <Y6xg8Ir9hQ+H/+uX@chenyu5-mobl1>

On Wed, Dec 28, 2022 at 11:29:52PM +0800, Chen Yu wrote:
> On 2022-12-23 at 21:28:50 -0800, Ricardo Neri wrote:
> > On Thu, Dec 22, 2022 at 04:56:22PM +0000, Valentin Schneider wrote:
> > > On 22/11/22 12:35, Ricardo Neri wrote:
> > > > Architectures that implement arch_asym_cpu_priority() may need to know the
> > > > idle state of the SMT siblings of a CPU. The scheduler has this information
> > > > and functionality. Expose it.
> > > >
> > > > Move the existing functionality outside of the NUMA code.
> > > >
> > > 
> > > test_idle_cores() does something similar without an iteration, did you
> > > consider using that instead?
> > 
> > IIUC, test_idle_cores() returns true if there is at least one idle core in
> > the package. In my case, I need to know the idle state of only the SMT
> > siblings of a specific CPU. Am I missing something?
> >
> I guess a similar one is select_idle_core(), but it also consider the CPU with
> SCHED_IDLE task as idle. Is CPU with SCHED_IDLE task a candidate in your
> scenario?

However, we are not looking for an idle CPU. We want to know the idle state of
the siblings of a CPU. I see that select_idle_core() uses available_idle_cpu(),
which in turn uses idle_cpu(). is_core_idle() also uses it.

As per 943d355d7fee ("sched/core: Distinguish between idle_cpu() calls based on
desired effect, introduce available_idle_cpu()") the load balancer can just
call idle_cpu().

Thanks and BR,
Ricardo