linux-kernel - Re: [PATCH v2 2/2] ring-buffer: Reuse rb_watermark

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Date: Wed, 13 Mar 2024 00:38:42 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/2] ring-buffer: Reuse rb_watermark_hit() for the
 poll logic

On Tue, 12 Mar 2024 09:19:21 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> The check for knowing if the poll should wait or not is basically the
> exact same logic as rb_watermark_hit(). The only difference is that
> rb_watermark_hit() also handles the !full case. But for the full case, the
> logic is the same. Just call that instead of duplicating the code in
> ring_buffer_poll_wait().
> 

This changes a bit (e.g. adding pagebusy check) but basically that should
be there. And new version appears to be consistent between ring_buffer_wait()
and ring_buffer_poll_wait(). So looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/ring_buffer.c | 21 +++++++--------------
>  1 file changed, 7 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index adfe603a769b..857803e8cf07 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -959,25 +959,18 @@ __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
>  	}
>  
>  	if (full) {
> -		unsigned long flags;
> -
>  		poll_wait(filp, &rbwork->full_waiters, poll_table);
>  
> -		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> -		if (!cpu_buffer->shortest_full ||
> -		    cpu_buffer->shortest_full > full)
> -			cpu_buffer->shortest_full = full;
> -		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> -		if (full_hit(buffer, cpu, full))
> +		if (rb_watermark_hit(buffer, cpu, full))
>  			return EPOLLIN | EPOLLRDNORM;
>  		/*
>  		 * Only allow full_waiters_pending update to be seen after
> -		 * the shortest_full is set. If the writer sees the
> -		 * full_waiters_pending flag set, it will compare the
> -		 * amount in the ring buffer to shortest_full. If the amount
> -		 * in the ring buffer is greater than the shortest_full
> -		 * percent, it will call the irq_work handler to wake up
> -		 * this list. The irq_handler will reset shortest_full
> +		 * the shortest_full is set (in rb_watermark_hit). If the
> +		 * writer sees the full_waiters_pending flag set, it will
> +		 * compare the amount in the ring buffer to shortest_full.
> +		 * If the amount in the ring buffer is greater than the
> +		 * shortest_full percent, it will call the irq_work handler
> +		 * to wake up this list. The irq_handler will reset shortest_full
>  		 * back to zero. That's done under the reader_lock, but
>  		 * the below smp_mb() makes sure that the update to
>  		 * full_waiters_pending doesn't leak up into the above.
> -- 
> 2.43.0
> 
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>