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Subject[PATCH] smp: Micro-optimize smp_call_function_many_cond()

* Peter Zijlstra <peterz@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 <mingo@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 <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@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);
}
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