Re: [PATCH] ring-buffer: Only update pages_touched when a new page is touched

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

On Tue, 9 Apr 2024 15:13:09 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> The "buffer_percent" logic that is used by the ring buffer splice code to
> only wake up the tasks when there's no data after the buffer is filled to
> the percentage of the "buffer_percent" file is dependent on three
> variables that determine the amount of data that is in the ring buffer:
> 
>  1) pages_read - incremented whenever a new sub-buffer is consumed
>  2) pages_lost - incremented every time a writer overwrites a sub-buffer
>  3) pages_touched - incremented when a write goes to a new sub-buffer
> 
> The percentage is the calculation of:
> 
>   (pages_touched - (pages_lost + pages_read)) / nr_pages
> 
> Basically, the amount of data is the total number of sub-bufs that have been
> touched, minus the number of sub-bufs lost and sub-bufs consumed. This is
> divided by the total count to give the buffer percentage. When the
> percentage is greater than the value in the "buffer_percent" file, it
> wakes up splice readers waiting for that amount.
> 
> It was observed that over time, the amount read from the splice was
> constantly decreasing the longer the trace was running. That is, if one
> asked for 60%, it would read over 60% when it first starts tracing, but
> then it would be woken up at under 60% and would slowly decrease the
> amount of data read after being woken up, where the amount becomes much
> less than the buffer percent.
> 
> This was due to an accounting of the pages_touched incrementation. This
> value is incremented whenever a writer transfers to a new sub-buffer. But
> the place where it was incremented was incorrect. If a writer overflowed
> the current sub-buffer it would go to the next one. If it gets preempted
> by an interrupt at that time, and the interrupt performs a trace, it too
> will end up going to the next sub-buffer. But only one should increment
> the counter. Unfortunately, that was not the case.
> 
> Change the cmpxchg() that does the real switch of the tail-page into a
> try_cmpxchg(), and on success, perform the increment of pages_touched. This
> will only increment the counter once for when the writer moves to a new
> sub-buffer, and not when there's a race and is incremented for when a
> writer and its preempting writer both move to the same new sub-buffer.
> 

Looks good to me.

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

BTW, isn't this a real bugfix, because the page_touched can be
bigger than nr_pages without this fix?

Thank you,

> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/ring_buffer.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 25476ead681b..6511dc3a00da 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1393,7 +1393,6 @@ static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
>  	old_write = local_add_return(RB_WRITE_INTCNT, &next_page->write);
>  	old_entries = local_add_return(RB_WRITE_INTCNT, &next_page->entries);
>  
> -	local_inc(&cpu_buffer->pages_touched);
>  	/*
>  	 * Just make sure we have seen our old_write and synchronize
>  	 * with any interrupts that come in.
> @@ -1430,8 +1429,9 @@ static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
>  		 */
>  		local_set(&next_page->page->commit, 0);
>  
> -		/* Again, either we update tail_page or an interrupt does */
> -		(void)cmpxchg(&cpu_buffer->tail_page, tail_page, next_page);
> +		/* Either we update tail_page or an interrupt does */
> +		if (try_cmpxchg(&cpu_buffer->tail_page, &tail_page, next_page))
> +			local_inc(&cpu_buffer->pages_touched);
>  	}
>  }
>  
> -- 
> 2.43.0
> 


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