Re: [PATCH] tracing: Fix to avoid wakeup loop in splice read of per-cpu buffer

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 21 Aug 2023 23:19:18 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> ftrace user can set 0 or small number to the 'buffer_percent' for quick
> response for the ring buffer. In that case wait_on_pipe() will return
> before filling a page of the ring buffer. That is too soon for splice()
> because ring_buffer_read_page() will fail again.
> This leads unnecessary loop in tracing_buffers_splice_read().
> 
> Set a minimum percentage of the buffer which is enough to fill a page to
> wait_on_pipe() to avoid this situation.
> 
> Fixes: 03329f993978 ("tracing: Add tracefs file buffer_percentage")
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

So this is what I found when I played with virtio-trace.

I think now trace-cmd has richer feature, so it can be replaced with that,
maybe what we need is a selftest that ensures trace_pipe_raw supports
splice correctly.

BTW, even with this fix, blocking splice still causes a strange behavior.
If I set '400' to buffer_size_kb (so 100 pages) and '1' to buffer_percent,
splice always returns 8192 (2 pages) to read. But I expected that should
return 4096 (1 page). This means splice() waits longer than I thought.

I think the fullfilled percentage calculation will be a bit wrong.

Thank you,

> ---
>  kernel/trace/trace.c |   12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index b8870078ef58..88448e8d8214 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -8462,6 +8462,8 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
>  	/* did we read anything? */
>  	if (!spd.nr_pages) {
>  		long wait_index;
> +		size_t nr_pages;
> +		size_t full;
>  
>  		if (ret)
>  			goto out;
> @@ -8472,7 +8474,15 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
>  
>  		wait_index = READ_ONCE(iter->wait_index);
>  
> -		ret = wait_on_pipe(iter, iter->tr->buffer_percent);
> +		/* For splice, we have to ensure at least 1 page is filled */
> +		nr_pages = ring_buffer_nr_pages(iter->array_buffer->buffer, iter->cpu_file);
> +		if (nr_pages * iter->tr->buffer_percent < 100) {
> +			full = nr_pages + 99;
> +			do_div(full, nr_pages);
> +		} else
> +			full = iter->tr->buffer_percent;
> +
> +		ret = wait_on_pipe(iter, full);
>  		if (ret)
>  			goto out;
>  
> 


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