From:   Valentin Schneider <vschneid@redhat.com>
To:     hupu <921889738@qq.com>, peterz@infradead.org
Cc:     bristot@redhat.com, bsegall@google.com, dietmar.eggemann@arm.com,
        ionela.voinescu@arm.com, juri.lelli@redhat.com,
        len.brown@intel.com, linux-kernel@vger.kernel.org, mgorman@suse.de,
        rafael.j.wysocki@intel.com, ravi.v.shankar@intel.com,
        ricardo.neri-calderon@linux.intel.com, ricardo.neri@intel.com,
        rostedt@goodmis.org, srinivas.pandruvada@linux.intel.com,
        tim.c.chen@intel.com, tim.c.chen@linux.intel.com,
        vincent.guittot@linaro.org, x86@kernel.org, hupu@oppo.com
Subject: Re: [PATCH v3 06/10] sched/fair: Use the prefer_sibling flag of the
 current sched domain
In-Reply-To: <tencent_60A804570F09C0CFE0495D5B984941123A05@qq.com>
References: <Y+Z2b/OtZDk9cT53@hirez.programming.kicks-ass.net>
 <tencent_60A804570F09C0CFE0495D5B984941123A05@qq.com>
Date:   Mon, 20 Feb 2023 09:45:37 +0000
Message-ID: <xhsmh1qmkr7pa.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 20/02/23 15:22, hupu wrote:
> From: Peter Zijlstra <peterz@infradead.org>
>> Specifically, I'm thinking something in the degenerate area where it
>> looks if a given domain has equal depth children or so.
>
> By the way, in the last email you mentioned "the degenerate area". I can't understand what it means because of cultural differences. Does it refer to this scenario: sched_domain at the SMT layer is destroyed after an SMT thread goes offline, so mc_domain->child is NULL?
>

This references sched domain degeneration, cf. sd_parent_degenerate() and
sd_degenerate() in sched/topology.c

Sched domains are built following a layout described in
sched_domain_topology[], and depending on the actual CPU layout some
domains end up not being worth keeping (e.g. they only contain one CPU - we
need at least 2 to balance things), so we destroy (degenerate) them.

> hupu