Re: [PATCH v3 03/12] smp: Remove get_cpu from smp_call_function_any

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-03-18 10:32:37 [-0400], Steven Rostedt wrote:
> On Wed, 18 Mar 2026 12:56:29 +0800
> "Chuyi Zhou" <zhouchuyi@bytedance.com> wrote:
> 
> > Now smp_call_function_single() would enable preemption before
> > csd_lock_wait() to reduce the critical section. To allow callers of
> > smp_call_function_any() to also benefit from this optimization, remove
> > get_cpu()/put_cpu() from smp_call_function_any().
> > 
> > Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> > Reviewed-by: Muchun Song <muchun.song@linux.dev>
> > ---
> >  kernel/smp.c | 16 +++++++++++++---
> >  1 file changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index b603d4229f95..80daf9dd4a25 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -761,16 +761,26 @@ EXPORT_SYMBOL_GPL(smp_call_function_single_async);
> >  int smp_call_function_any(const struct cpumask *mask,
> >  			  smp_call_func_t func, void *info, int wait)
> >  {
> > +	bool local = true;
> >  	unsigned int cpu;
> >  	int ret;
> >  
> > -	/* Try for same CPU (cheapest) */
> > +	/*
> > +	 * Prevent migration to another CPU after selecting the current CPU
> > +	 * as the target.
> > +	 */
> >  	cpu = get_cpu();
> > -	if (!cpumask_test_cpu(cpu, mask))
> > +
> > +	/* Try for same CPU (cheapest) */
> > +	if (!cpumask_test_cpu(cpu, mask)) {
> >  		cpu = sched_numa_find_nth_cpu(mask, 0, cpu_to_node(cpu));
> 
> Hmm, isn't this looking for another CPU that is closest to the current CPU?
> 
> By allowing migration, it is possible that the task will migrate to another
> CPU where this will pick one that is much farther.
> 
> I'm not sure if that's really an issue or not, as I believe it's mostly for
> performance reasons. Then again, why even keep preemption disabled for the
> current CPU case? Isn't disabling preemption more for performance than
> correctness?
> 
> Perhaps migrate_disable() is all that is needed?

You want the local-CPU so it is cheap as in direct invocation. But if
you migrate away between the get_cpu() and the check in
generic_exec_single() you do trigger an interrupt.
A migration here would not be wrong, it would just not be as cheap as
intended. Not sure if migrate_disable() counters the cheap part again.

An alternative would be to have the local execution in
__generic_exec_local() and have it run with disabled preemption and a
fallback with enabled preemption where it is enqueue and probably waited
for.

> -- Steve

Sebastian