Re: [PATCH RT] kernel/sched: Don't recompute cpumask weight in migrate_enable_update_cpus_allowed()

From: Scott Wood
Date: Sat Oct 12 2019 - 03:06:22 EST

On Fri, 2019-10-11 at 10:09 -0400, Waiman Long wrote:
> At each invocation of rt_spin_unlock(), cpumask_weight() is called
> via migrate_enable_update_cpus_allowed() to recompute the weight of
> cpus_mask which doesn't change that often.
> The following is a sample output of perf-record running the testpmd
> microbenchmark on an RT kernel:
> 34.77% 1.65% testpmd [kernel.kallsyms] [k] rt_spin_unlock
> 34.32% 2.52% testpmd [kernel.kallsyms] [k] migrate_enable
> 21.76% 21.76% testpmd [kernel.kallsyms] [k] __bitmap_weight
> By adding an extra variable to keep track of the weight of cpus_mask,
> we could eliminate the frequent call to cpumask_weight() and replace
> it with simple assignment.

Can you try this with my migrate disable patchset (which makes
amigrate_enable_update_cpus_allowed() be called much less often) and see if
caching nr_cpus_allowed still makes a difference?