[PATCH] smp: Micro-optimize smp_call_function_many_cond()

[Ingo Molnar]

* Peter Zijlstra <pet...@infradead.org> wrote:

> On Sat, Feb 20, 2021 at 03:17:04PM -0800, Nadav Amit wrote:
> > +           /*
> > +            * Choose the most efficient way to send an IPI. Note that the
> > +            * number of CPUs might be zero due to concurrent changes to the
> > +            * provided mask.
> > +            */
> > +           if (nr_cpus == 1)
> > +                   arch_send_call_function_single_ipi(last_cpu);
> > +           else if (likely(nr_cpus > 1))
> > +                   arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
> 
> I just ran into conflicts with another patch set, and noticed that the
> above should probably be:
> 
>               if (nr_cpus == 1)
>                       send_call_function_single_ipi(last_cpu);
>               else if (likely(nr_cpus > 1))
>                       arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
> 
> Which will avoid the IPI when @last_cpu is idle.

I turned this into the additional patch below, on top of Nadav's series.

Thanks,

        Ingo

===============>
From: Ingo Molnar <mi...@kernel.org>
Date: Tue, 2 Mar 2021 08:02:43 +0100
Subject: [PATCH] smp: Micro-optimize smp_call_function_many_cond()

Call the generic send_call_function_single_ipi() function, which
will avoid the IPI when @last_cpu is idle.

Signed-off-by: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 kernel/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index b6375d775e93..af0d51da84a2 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -694,7 +694,7 @@ static void smp_call_function_many_cond(const struct 
cpumask *mask,
                 * provided mask.
                 */
                if (nr_cpus == 1)
-                       arch_send_call_function_single_ipi(last_cpu);
+                       send_call_function_single_ipi(last_cpu);
                else if (likely(nr_cpus > 1))
                        arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
        }