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

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Date: Tue, 9 Apr 2024 14:29:09 -0500
From: Kyle Meyer <kyle.meyer@....com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, yury.norov@...il.com,
        linux@...musvillemoes.dk, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        russ.anderson@....com, dimitri.sivanich@....com, steve.wahl@....com
Subject: Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()

On Tue, Apr 09, 2024 at 07:25:06PM +0300, Andy Shevchenko wrote:
> On Tue, Apr 09, 2024 at 10:52:50AM -0500, Kyle Meyer wrote:
> > Optimize topology_span_sane() by removing duplicate comparisons.
> > 
> > The total number of comparisons is reduced from N * (N - 1) to
> > N * (N - 1) / 2 (per non-NUMA scheduling domain level).
> 
> ...
> 
> > -	for_each_cpu(i, cpu_map) {
> > -		if (i == cpu)
> > -			continue;
> > +	for_each_cpu_from(i, cpu_map) {
> 
> Hmm... I'm not familiar with the for_each_cpu*(), but from the above
> it seems only a single comparison? Or i.o.w. can i ever repeat the value?

for_each_cpu() is a macro around for_each_set_bit() which iterates over each set
bit in a bitmap starting at zero.

for_each_cpu_from() is a macro around for_each_set_bit_from() which iterates
over each set bit in a bitmap starting at the specified bit.

The above (topology_span_sane()) currently does a "single comparison" for each
CPU in cpu_map, but it's called for each CPU in cpu_map and for each scheduling
domain level (please see build_sched_domains() in kernel/sched/topology.c).

> And what about i < cpu cases?

Those values have already been passed to topology_span_sane(). This patch uses
for_each_cpu_from() starting at cpu + 1 to prevent those duplicate comparisons.

Thanks,
Kyle Meyer