linux-kernel - Re: [PATCH v2 2/4] relayfs: introduce dump of relayfs statistics function

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Message-ID: <d5032331-7fbc-4c7b-9d1e-845121664872@kernel.dk>
Date: Thu, 15 May 2025 12:19:08 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Jason Xing <kerneljasonxing@...il.com>, rostedt@...dmis.org,
 mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
 akpm@...ux-foundation.org
Cc: 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 v2 2/4] relayfs: introduce dump of relayfs statistics
 function

On 5/15/25 12:16 AM, Jason Xing wrote:
> From: Jason Xing <kernelxing@...cent.com>
> 
> In this version, only support dumping the counter for buffer full and
> implement the framework of how it works.
> 
> Users can pass certain flag to fetch what field/statistics they expect
> to know. Each time it only returns one result. So do not pass multiple
> flags.
> 
> Reviewed-by: Yushan Zhou <katrinzhou@...cent.com>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> v2
> 1. refactor relay_dump() and make it only return a pure size_t result
> of the value that users specifies.
> 2. revise the commit log.
> ---
>  include/linux/relay.h |  7 +++++++
>  kernel/relay.c        | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/include/linux/relay.h b/include/linux/relay.h
> index ce7a1b396872..3fb285716e34 100644
> --- a/include/linux/relay.h
> +++ b/include/linux/relay.h
> @@ -31,6 +31,12 @@
>  /*
>   * Relay buffer statistics dump
>   */
> +enum {
> +	RELAY_DUMP_BUF_FULL = (1 << 0),
> +
> +	RELAY_DUMP_LAST = RELAY_DUMP_BUF_FULL,
> +};
> +
>  struct rchan_buf_stats
>  {
>  	unsigned int full_count;	/* counter for buffer full */
> @@ -167,6 +173,7 @@ struct rchan *relay_open(const char *base_filename,
>  			 void *private_data);
>  extern void relay_close(struct rchan *chan);
>  extern void relay_flush(struct rchan *chan);
> +extern size_t relay_dump(struct rchan *chan, int flags);
>  extern void relay_subbufs_consumed(struct rchan *chan,
>  				   unsigned int cpu,
>  				   size_t consumed);
> diff --git a/kernel/relay.c b/kernel/relay.c
> index eb3f630f3896..f47fc750e559 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -701,6 +701,37 @@ void relay_flush(struct rchan *chan)
>  }
>  EXPORT_SYMBOL_GPL(relay_flush);
>  
> +/**
> + *	relay_dump - dump channel buffer statistics
> + *	@chan: the channel
> + *	@flags: select particular information to dump
> + *
> + *	Returns the count of certain field that caller specifies.
> + */
> +size_t relay_dump(struct rchan *chan, int flags)
> +{
> +	unsigned int i, count = 0;
> +	struct rchan_buf *rbuf;
> +
> +	if (!chan || flags > RELAY_DUMP_LAST)
> +		return 0;
> +
> +	if (chan->is_global) {
> +		rbuf = *per_cpu_ptr(chan->buf, 0);
> +		if (flags & RELAY_DUMP_BUF_FULL)
> +			count = rbuf->stats.full_count;
> +	} else {
> +		for_each_online_cpu(i) {
> +			if ((rbuf = *per_cpu_ptr(chan->buf, i)))
> +				if (flags & RELAY_DUMP_BUF_FULL)
> +					count += rbuf->stats.full_count;

Kernel tends to avoid the rolled-into-one assignment and check, it's
easy to misread. This:

	rbuf = *per_cpu_ptr(chan->buf, i);
	if (rbuf && flags & RELAY_DUMP_BUF_FULL)
		count += rbuf->stats.full_count;

reads much easier. IMHO.

-- 
Jens Axboe