linux-kernel - Re: [RFC v2 6/7] perf: add api for pmus to write to AUX space

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Date:	Wed, 25 Jun 2014 14:12:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Robert Richter <rric@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [RFC v2 6/7] perf: add api for pmus to write to AUX space

On Wed, Jun 25, 2014 at 02:24:51PM +0300, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > On Wed, Jun 11, 2014 at 06:41:49PM +0300, Alexander Shishkin wrote:
> >> +void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size,
> >> +			 bool truncated)
> >> +{
> >> +	struct ring_buffer *rb = handle->rb;
> >> +	unsigned long aux_head;
> >> +
> >> +	aux_head = local_read(&rb->aux_head);
> >> +	local_add(size, &rb->aux_head);
> >> +
> >> +	rb->user_page->aux_head = local_read(&rb->aux_head);
> >> +	smp_wmb();
> >> +
> >> +	perf_output_wakeup(handle);
> >> +	handle->event = NULL;
> >> +
> >> +	ring_buffer_put(rb);
> >> +}
> >
> > This thing is distinctly less complex than perf_output_put_handle().. ?
> 
> This one doesn't support nested writers, but there shouldn't be any
> either. But I notice now that the barrier is misplaced. Wakeup watermark
> is not yet taken care of, but will be. Anything else I'm missing?

Maybe put a comment in about explicitly not supporting nesting etc.
Maybe also a debug check to make sure we catch anybody doing it anyway.
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