Date:   Thu, 16 Feb 2023 13:16:42 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <vschneid@redhat.com>,
        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>,
        Ionela Voinescu <ionela.voinescu@arm.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, "Tim C . Chen" <tim.c.chen@intel.com>
Subject: Re: [PATCH v3 06/10] sched/fair: Use the prefer_sibling flag of the
 current sched domain
Message-ID: <Y+4eqmq22fDuFZAJ@hirez.programming.kicks-ass.net>
References: <20230207045838.11243-1-ricardo.neri-calderon@linux.intel.com>
 <20230207045838.11243-7-ricardo.neri-calderon@linux.intel.com>
 <Y+YXrk5NRuWaSOGR@hirez.programming.kicks-ass.net>
 <xhsmhmt5lr2nz.mognet@vschneid.remote.csb>
 <Y+Z2b/OtZDk9cT53@hirez.programming.kicks-ass.net>
 <xhsmhk00pqwap.mognet@vschneid.remote.csb>
 <20230210183155.GA11997@ranerica-svr.sc.intel.com>
 <8300f288-7157-5e2d-3bb3-badcffd15d34@arm.com>
 <20230214064328.GA11859@ranerica-svr.sc.intel.com>
 <20230216052105.GA20785@ranerica-svr.sc.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230216052105.GA20785@ranerica-svr.sc.intel.com>
Precedence: bulk

On Wed, Feb 15, 2023 at 09:21:05PM -0800, Ricardo Neri wrote:

> I tweaked the solution that Dietmar proposed:
> 
> -	sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
> +	if (sds->busiest)
> +		sds->prefer_sibling = sds->busiest->flags & SD_PREFER_SIBLING;
> 
> This comes from the observation that the prefer_sibling setting acts on
> busiest group. It then depends on whether the busiest group, not the local
> group, has child sched sched domains. Today it works because in most cases
> both the local and the busiest groups have child domains with the SD_
> PREFER_SIBLING flag.
> 
> This would also satisfy sched domains with the SD_ASYM_CPUCAPACITY flag as
> prefer_sibling would not be set in that case.
> 
> It would also conserve the current behavior at the NUMA level. We would
> not need to implement SD_SPREAD_TASKS.
> 
> This would both fix the SMT vs non-SMT bug and be less invasive.
> 
> Thoughts?

That does look nice. Be sure to put in a nice comment too.