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Date: Tue, 9 Apr 2024 14:29:09 -0500
From: Kyle Meyer <kyle.meyer@hpe.com>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, yury.norov@gmail.com,
        linux@rasmusvillemoes.dk, mingo@redhat.com, peterz@infradead.org,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
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        russ.anderson@hpe.com, dimitri.sivanich@hpe.com, steve.wahl@hpe.com
Subject: Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()
References: <20240409155250.3660517-1-kyle.meyer@hpe.com>
 <20240409155250.3660517-3-kyle.meyer@hpe.com>
 <ZhVr4i5F1uWyrJ15@smile.fi.intel.com>
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In-Reply-To: <ZhVr4i5F1uWyrJ15@smile.fi.intel.com>

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