linux-kernel - Re: [PATCH 1/2] membarrier: allow cpu

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Message-ID: <4db5be54-7c27-4aed-b70f-3722a890904a@efficios.com>
Date: Thu, 26 Jun 2025 12:07:07 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Dylan Yudaken <dyudaken@...il.com>, paulmck@...nel.org
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
 vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org,
 bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, shuah@...nel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] membarrier: allow cpu_id to be set on more commands

On 2025-06-26 11:52, Dylan Yudaken wrote:
> No reason to not allow MEMBARRIER_CMD_FLAG_CPU on
> MEMBARRIER_CMD_PRIVATE_EXPEDITED or
> MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE.
> 
> If it is known specifically what cpu you want to interrupt then there
> is a decent efficiency saving in not interrupting all the other ones.
> 
> Also - the code already works as is for them.

Can you elaborate on a concrete use-case justifying adding this ?

Thanks,

Mathieu

> 
> Signed-off-by: Dylan Yudaken <dyudaken@...il.com>
> ---
>   kernel/sched/membarrier.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 809194cd779f..def6d4094ad6 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -595,7 +595,9 @@ static int membarrier_get_registrations(void)
>    *          contains the CPU on which to interrupt (= restart)
>    *          the RSEQ critical section.
>    * @cpu_id: if @flags == MEMBARRIER_CMD_FLAG_CPU, indicates the cpu on which
> - *          RSEQ CS should be interrupted (@cmd must be
> + *          RSEQ CS should be interrupted (@cmd must be one of
> + *          MEMBARRIER_CMD_PRIVATE_EXPEDITED,
> + *          MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE,
>    *          MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ).
>    *
>    * If this system call is not implemented, -ENOSYS is returned. If the
> @@ -625,6 +627,8 @@ static int membarrier_get_registrations(void)
>   SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id)
>   {
>   	switch (cmd) {
> +	case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> +	case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE:
>   	case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
>   		if (unlikely(flags && flags != MEMBARRIER_CMD_FLAG_CPU))
>   			return -EINVAL;


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com