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

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Message-ID: <20170804145835.GB6780@arm.com>
Date:   Fri, 4 Aug 2017 15:58:35 +0100
From:   Will Deacon <will.deacon@....com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, mark.rutland@....com,
        linux-arm-kernel@...ts.infradead.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members
 long

On Mon, Jul 31, 2017 at 01:28:01PM +0300, Alexander Shishkin wrote:
> 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.

Agreed on both counts. Will fix for v2.

Will