linux-kernel - Re: [PATCH 2/2] sched/topology: optimize topology_span

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Message-ID: <xhsmhv80cmoc1.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 07 Aug 2024 15:53:18 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Yury Norov <yury.norov@...il.com>
Cc: linux-kernel@...r.kernel.org, Christophe JAILLET
 <christophe.jaillet@...adoo.fr>, Leonardo Bras <leobras@...hat.com>, Ingo
 Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri
 Lelli <juri.lelli@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel
 Gorman <mgorman@...e.de>
Subject: Re: [PATCH 2/2] sched/topology: optimize topology_span_sane()

On 06/08/24 11:00, Yury Norov wrote:
> On Tue, Aug 06, 2024 at 05:50:23PM +0200, Valentin Schneider wrote:
>> On 02/08/24 10:57, Yury Norov wrote:
>> > The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
>> > even when cpu != i.
>>
>> For which architecture have you observed this? AFAIA all implementations of
>> tl->sched_domain_mask_f are built on a per-CPU cpumask.
>
> x86_64, qemu emulating 16 CPUs in 4 nodes, Linux 6.10, approximately
> defconfig.

For the default_topology:
cpu_smt_mask() # SMT
  (per_cpu(cpu_sibling_map, cpu))

cpu_clustergroup_mask() # CLS
  per_cpu(cpu_l2c_shared_map, cpu);

cpu_coregroup_mask() # MC
  per_cpu(cpu_llc_shared_map, cpu);

cpu_cpu_mask() # PKG
  cpumask_of_node(cpu_to_node(cpu));

Ok so PKG can potentially hit that condition, and so can any
sched_domain_mask_f that relies on the node masks...

I'm thinking ideally we should have checks in place to ensure all
node_to_cpumask_map[] masks are disjoint, then we could entirely skip the levels
that use these masks in topology_span_sane(), but there's unfortunately no
nice way to flag them... Also there would cases where there's no real
difference between PKG and NODE other than NODE is still based on a per-cpu
cpumask and PKG isn't, so I don't see a nicer way to go about this.

Please add something like the following to the changelog, and with that:
Reviewed-by: Valentin Schneider <vschneid@...hat.com>

"""
Some topology levels (e.g. PKG in default_topology[]) have a
sched_domain_mask_f implementation that reuses the same mask for several
CPUs (in PKG's case, one mask for all CPUs in the same NUMA node).

For such topology levels, repeating cpumask_equal() checks is wasteful -
check that the tl->mask(i) pointers aren't the same first.
"""