Re: [PATCH] perf/aux: Only update aux_wakeup in non-overwrite mode

[Will Deacon]

Hi Alexander,

On Wed, Sep 06, 2017 at 07:08:11PM +0300, Alexander Shishkin wrote:
> Commit d9a50b0256 ("perf/aux: Ensure aux_wakeup represents most recent
> wakeup index") changed aux wakeup position calculation to rounddown(),
> which causes a division-by-zero in AUX overwrite mode (aka "snapshot
> mode").
> 
> The zero denominator results from the fact that perf record doesn't set
> aux_watermark to anything, in which case the kernel will set it to half
> the AUX buffer size, but only for non-overwrite mode. In the overwrite
> mode aux_watermark stays zero.
> 
> The good news is that, AUX overwrite mode, wakeups don't happen and
> related bookkeeping is not relevant, so we can simply forego the whole
> wakeup updates.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> ---
>  kernel/events/ring_buffer.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)

Damn, sorry about that. How did you spot the problem?
Anyway, I think the code is much better with this factored out:

Acked-by: Will Deacon <will.deacon@xxxxxxx>

Thanks,

Will

> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index f4ebe42879..4dae1da4c1 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -412,6 +412,19 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
>  	return NULL;
>  }
>  
> +static bool __always_inline rb_need_aux_wakeup(struct ring_buffer *rb)
> +{
> +	if (rb->aux_overwrite)
> +		return false;
> +
> +	if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) {
> +		rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark);
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
>  /*
>   * Commit the data written by hardware into the ring buffer by adjusting
>   * aux_head and posting a PERF_RECORD_AUX into the perf buffer. It is the
> @@ -451,10 +464,8 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
>  	}
>  
>  	rb->user_page->aux_head = rb->aux_head;
> -	if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) {
> +	if (rb_need_aux_wakeup(rb))
>  		wakeup = true;
> -		rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark);
> -	}
>  
>  	if (wakeup) {
>  		if (handle->aux_flags & PERF_AUX_FLAG_TRUNCATED)
> @@ -484,9 +495,8 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size)
>  	rb->aux_head += size;
>  
>  	rb->user_page->aux_head = rb->aux_head;
> -	if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) {
> +	if (rb_need_aux_wakeup(rb)) {
>  		perf_output_wakeup(handle);
> -		rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark);
>  		handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
>  	}
>  
> -- 
> 2.14.1
>