Re: [PATCH v2 3/7] sched: Teach arch_asym_cpu_priority() the idle state of SMT siblings

From: Valentin Schneider <vschneid@redhat.com>
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: "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>,
	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 3/7] sched: Teach arch_asym_cpu_priority() the idle state of SMT siblings
Date: Thu, 22 Dec 2022 16:56:10 +0000	[thread overview]
Message-ID: <xhsmhtu1ne5sl.mognet@vschneid.remote.csb> (raw)
In-Reply-To: <20221222045505.GB407@ranerica-svr.sc.intel.com>

On 21/12/22 20:55, Ricardo Neri wrote:
> On Wed, Dec 21, 2022 at 06:12:52PM +0100, Dietmar Eggemann wrote:
>> Imagine an arch w/ SD_ASYM_PACKING on SMT & MC overwriting
>> arch_asym_cpu_priority(). `bool check_smt` wouldn't be sufficient to
>> know whether a call to something like sched_smt_siblings_idle()
>> (is_core_idle()) which iterates over cpu_smt_mask(cpu) would make sense.
>
> Agreed. I was hoping I could get away with this. x86 would not have the
> the SD_ASYM_PACKING flag at the SMT level and Power7 would ignore
> `check_smt`. :)
>
> Callers of sched_asym_prefer() could use the flags of the sched domain to
> check if we are at the SMT level.
>
> I rescanned the code again and it looks like the needed sched domain flags
> are available in all the places sched_asym_prefer() is called. The only
> exception is asym_smt_can_pull_tasks(), but we already know that we don't
> need such check. (We are looking for the sched group priority, regardless
> of the idle state of the SMT siblings).
>

Given this is fed back to arch code, another option here would be to feed
the topology level this is happening at. You get it via sd->level and it
maps back into the arch's sched_domain_topology_level array.

Though the SD flags are probably the more generic solution.