linux-kernel - Re: [RFC PATCH 1/1] tracing: Fix poll() and select() do not work on per_cpu trace_pipe and trace_pipe

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Date:   Thu, 2 Feb 2023 16:18:31 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     <shiju.jose@...wei.com>
Cc:     <mhiramat@...nel.org>, <mchehab@...nel.org>,
        <linux-edac@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>, <linuxarm@...wei.com>,
        <jonathan.cameron@...wei.com>, <tanxiaofei@...wei.com>
Subject: Re: [RFC PATCH 1/1] tracing: Fix poll() and select() do not work on
 per_cpu trace_pipe and trace_pipe_raw

On Thu, 2 Feb 2023 18:23:09 +0000
<shiju.jose@...wei.com> wrote:

> From: Shiju Jose <shiju.jose@...wei.com>
> 
> poll() and select() on per_cpu trace_pipe and trace_pipe_raw do not work
> since kernel 6.1-rc6. This issue is seen after the commit
> 42fb0a1e84ff525ebe560e2baf9451ab69127e2b ("tracing/ring-buffer: Have
> polling block on watermark").
> 
> This issue is firstly detected and reported, when testing the CXL error
> events in the rasdaemon and also erified using the test application for poll()
> and select().
> 
> This issue occurs for the per_cpu case, when calling the ring_buffer_poll_wait(),
> in kernel/trace/ring_buffer.c, with the buffer_percent > 0 and then wait until the
> percentage of pages are available. The default value set for the buffer_percent is 50
> in the kernel/trace/trace.c.
> 
> As a fix, allow userspace application could set buffer_percent as 0 through
> the buffer_percent_fops, so that the task will wake up as soon as data is added
> to any of the specific cpu buffer.
> 
> Fixes: 42fb0a1e84ff5 ("tracing/ring-buffer: Have polling block on watermark")
> Signed-off-by: Shiju Jose <shiju.jose@...wei.com>
> ---

This makes sense to me. I'm going to run a bunch of tests on this and
see if anything else breaks.

Thanks!

-- Steve


>  kernel/trace/trace.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index a555a861b978..01164c78483a 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9148,9 +9148,6 @@ buffer_percent_write(struct file *filp, const char __user *ubuf,
>  	if (val > 100)
>  		return -EINVAL;
>  
> -	if (!val)
> -		val = 1;
> -
>  	tr->buffer_percent = val;
>  
>  	(*ppos)++;