linux-kernel - Re: [PATCH] ring-buffer: Update pages_touched to reflect persistent buffer content

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Message-Id: <20250216001755.b5e1f1ac00bef0b46dec74f4@kernel.org>
Date: Sun, 16 Feb 2025 00:17:55 +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>,
 Vincent Donnefort <vdonnefort@...gle.com>
Subject: Re: [PATCH] ring-buffer: Update pages_touched to reflect persistent
 buffer content

On Fri, 14 Feb 2025 12:35:12 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> The pages_touched field represents the number of subbuffers in the ring
> buffer that have content that can be read. This is used in accounting of
> "dirty_pages" and "buffer_percent" to allow the user to wait for the
> buffer to be filled to a certain amount before it reads the buffer in
> blocking mode.
> 
> The persistent buffer never updated this value so it was set to zero, and
> this accounting would take it as it had no content. This would cause user
> space to wait for content even though there's enough content in the ring
> buffer that satisfies the buffer_percent.

Looks good to me.

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

Thanks,

> 
> Cc: stable@...r.kernel.org
> Fixes: 5f3b6e839f3ce ("ring-buffer: Validate boot range memory events")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/ring_buffer.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 0419d41a2060..bb6089c2951e 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1850,6 +1850,11 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
>  				cpu_buffer->cpu);
>  			goto invalid;
>  		}
> +
> +		/* If the buffer has content, update pages_touched */
> +		if (ret)
> +			local_inc(&cpu_buffer->pages_touched);
> +
>  		entries += ret;
>  		entry_bytes += local_read(&head_page->page->commit);
>  		local_set(&cpu_buffer->head_page->entries, ret);
> -- 
> 2.47.2
> 


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