linux-kernel - Re: [PATCH v1 5/5] relayfs: uniformally use possible cpu iteration

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Message-Id: <20250512175204.8faa5fd646da7247137db14b@linux-foundation.org>
Date: Mon, 12 May 2025 17:52:04 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: axboe@...nel.dk, rostedt@...dmis.org, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Jason Xing
 <kernelxing@...cent.com>, Yushan Zhou <katrinzhou@...cent.com>
Subject: Re: [PATCH v1 5/5] relayfs: uniformally use possible cpu iteration

On Mon, 12 May 2025 10:49:35 +0800 Jason Xing <kerneljasonxing@...il.com> wrote:

> From: Jason Xing <kernelxing@...cent.com>
> 
> Use for_each_possible_cpu to create per-cpu relayfs file to avoid later
> hotplug cpu which doesn't have its own file.

I don't understand this.  Exactly what problem are we trying to solve?

> Reviewed-by: Yushan Zhou <katrinzhou@...cent.com>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
>  kernel/relay.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/relay.c b/kernel/relay.c
> index 27f7e701724f..dcb099859e83 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -519,7 +519,7 @@ struct rchan *relay_open(const char *base_filename,
>  	kref_init(&chan->kref);
>  
>  	mutex_lock(&relay_channels_mutex);
> -	for_each_online_cpu(i) {
> +	for_each_possible_cpu(i) {

num_possible_cpus() can sometimes greatly exceed num_online_cpus(), so
this is an unfortunate change.  It would be better to implement the
hotplug notifier?

>  		buf = relay_open_buf(chan, i);
>  		if (!buf)
>  			goto free_bufs;
> @@ -615,7 +615,7 @@ int relay_late_setup_files(struct rchan *chan,
>  	 * no files associated. So it's safe to call relay_setup_buf_file()
>  	 * on all currently online CPUs.
>  	 */
> -	for_each_online_cpu(i) {
> +	for_each_possible_cpu(i) {
>  		buf = *per_cpu_ptr(chan->buf, i);
>  		if (unlikely(!buf)) {
>  			WARN_ONCE(1, KERN_ERR "CPU has no buffer!\n");
> -- 
> 2.43.5