linux-kernel - Re: [PATCH] ring-buffer: Do not set shortest

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Date: Wed, 13 Mar 2024 01:13:55 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] ring-buffer: Do not set shortest_full when full target
 is hit

On Tue, 12 Mar 2024 11:56:41 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> The rb_watermark_hit() checks if the amount of data in the ring buffer is
> above the percentage level passed in by the "full" variable. If it is, it
> returns true.
> 
> But it also sets the "shortest_full" field of the cpu_buffer that informs
> writers that it needs to call the irq_work if the amount of data on the
> ring buffer is above the requested amount.
> 
> The rb_watermark_hit() always sets the shortest_full even if the amount in
> the ring buffer is what it wants. As it is not going to wait, because it
> has what it wants, there's no reason to set shortest_full.
> 

Yeah, it should avoid setting if !ret. Looks good to me.

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

Thank you,

> Cc: stable@...r.kernel.org
> Fixes: 42fb0a1e84ff5 ("tracing/ring-buffer: Have polling block on watermark")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/ring_buffer.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 9b887d44b8d9..350607cce869 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -834,9 +834,10 @@ static bool rb_watermark_hit(struct trace_buffer *buffer, int cpu, int full)
>  		pagebusy = cpu_buffer->reader_page == cpu_buffer->commit_page;
>  		ret = !pagebusy && full_hit(buffer, cpu, full);
>  
> -		if (!cpu_buffer->shortest_full ||
> -		    cpu_buffer->shortest_full > full)
> -			cpu_buffer->shortest_full = full;
> +		if (!ret && (!cpu_buffer->shortest_full ||
> +			     cpu_buffer->shortest_full > full)) {
> +		    cpu_buffer->shortest_full = full;
> +		}
>  		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  	}
>  	return ret;
> -- 
> 2.43.0
> 


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