On Mon, 24 Mar 2014 13:20:25 +0100
Nicholas Mc Guire <der.herr@xxxxxxx> wrote:
> HI !
>
>
> Minor cleanup: drop unnecessary migrate_disable in _cpu_down
>
> in kernel/cpu.c:_cpu_down
> ...
> cpumask_andnot(cpumask, cpu_online_mask, cpumask_of(cpu));
> set_cpus_allowed_ptr(current, cpumask);
> ...
> migrate_disable();
> mycpu = smp_processor_id();
> if (mycpu == cpu) {
> printk(KERN_ERR "Yuck! Still on unplug CPU\n!");
> migrate_enable();
> err = -EBUSY;
> goto restore_cpus;
> }
> migrate_enable();
>
> If we got migrated off "cpu" then we can not get migrate
> back as the cpus_allowed was updated. so if smp_processor_id(); returns
> anything != cpu we are fine even if migrated off to a third cpu between
> the mycpu = smp_processor_id() and the if (mycpu == cpu). No need for
> migration protection here.
>
> Further the migrate disable is effectively only needed for the condition
> check in the if statement - so I think it could be replaced by:
>
> if (cpu==smp_processor_id()) {
> pr_err("Yuck! Still on unplug CPU\n!");
> err = -EBUSY;
> goto restore_cpus;
> }
>
> even in the case that the cpus_allowed had not been updated.
>
> Patch is against 3.12.13-rt21
This is checking for an anomaly, and it's in a very slow path. I feel
more comfortable with keeping the code as is, than to change it for a
micro optimization.
-- Steve
>
> Signed-off-by: Nicholas Mc Guire <der.herr@xxxxxxx>
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