linux-kernel - Re: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring

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Date:   Mon, 31 Jul 2017 13:28:01 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org
Cc:     mark.rutland@....com, linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long

Will Deacon <will.deacon@....com> writes:

> The aux_head and aux_wakeup members of struct ring_buffer are defined
> using the local_t type, despite the fact that they are only accessed via
> the perf_aux_output_* functions, which cannot race with each other for a
> given ring buffer.
>
> This patch changes the type of the members to long, so we can avoid
> using the local_* API where it isn't needed.

Thanks for digging this up! Some minor nits below.

> @@ -434,12 +434,12 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
>  		handle->aux_flags |= PERF_AUX_FLAG_OVERWRITE;
>  
>  		aux_head = handle->head;
> -		local_set(&rb->aux_head, aux_head);
> +		rb->aux_head = aux_head;
>  	} else {
>  		handle->aux_flags &= ~PERF_AUX_FLAG_OVERWRITE;
>  
> -		aux_head = local_read(&rb->aux_head);
> -		local_add(size, &rb->aux_head);
> +		aux_head = rb->aux_head;
> +		rb->aux_head += size;
>  	}
>  
>  	if (size || handle->aux_flags) {
> @@ -451,11 +451,11 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
>  		                     handle->aux_flags);
>  	}
>  
> -	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> +	aux_head = rb->user_page->aux_head = rb->aux_head;
>  
> -	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> +	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {

Can't we just do away with aux_head here:

  rb->user_page->aux_head = rb->aux_head;
  if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) { ...

?

> @@ -485,14 +485,13 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size)
>  	if (size > handle->size)
>  		return -ENOSPC;
>  
> -	local_add(size, &rb->aux_head);
> +	rb->aux_head += size;
>  
> -	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> -	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> +	aux_head = rb->user_page->aux_head = rb->aux_head;
> +	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
>  		perf_output_wakeup(handle);
> -		local_add(rb->aux_watermark, &rb->aux_wakeup);
> -		handle->wakeup = local_read(&rb->aux_wakeup) +
> -				 rb->aux_watermark;
> +		rb->aux_wakeup += rb->aux_watermark;
> +		handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
>  	}
>  
>  	handle->head = aux_head;

And here I think we don't need aux_head at all.

Thanks,
--
Alex