[PATCH 2/2] perf/aux: Ensure aux_wakeup represents most recent wakeup index

[Will Deacon]

Hi Alexander,

Thanks for having a look at these.

On Mon, Jul 31, 2017 at 01:02:16PM +0300, Alexander Shishkin wrote:
> Will Deacon <will.deacon at arm.com> writes:
> 
> > The aux_watermark member of struct ring_buffer represents the period (in
> > terms of bytes) at which wakeup events should be generated when data is
> > written to the aux buffer in non-snapshot mode. On hardware that cannot
> > generate an interrupt when the aux_head reaches an arbitrary wakeup index
> 
> Curious: how do you support non-snapshot trace collection on such
> hardware?

The watermark is constrained to lie on a page boundary, so as long as the
buffer is at least a page (which it is!), we end up rounding up to the next
page boundary, with lots of fun and games to avoid going past the head.

> > (such as ARM SPE), the aux_head sampled from handle->head in
> > perf_aux_output_{skip,end} may in fact be past the wakeup index. This
> 
> I think this is also true of hw where the interrupt is not
> precise. Thanks for looking at this.

Yes, it all looks like "skid" to userspace.

> > can lead to wakeup slowly falling behind the head. For example, consider
> > the case where hardware can only generate an interrupt on a page-boundary
> > and the aux buffer is initialised as follows:
> >
> >   // Buffer size is 2 * PAGE_SIZE
> >   rb->aux_head = rb->aux_wakeup = 0
> >   rb->aux_watermark = PAGE_SIZE / 2
> >
> > following the first perf_aux_output_begin call, the handle is
> > initialised with:
> >
> >   handle->head = 0
> >   handle->size = 2 * PAGE_SIZE
> >   handle->wakeup = PAGE_SIZE / 2
> >
> > and the hardware will be programmed to generate an interrupt at
> > PAGE_SIZE.
> >
> > When the interrupt is raised, the hardware head will be at PAGE_SIZE,
> > so calling perf_aux_output_end(handle, PAGE_SIZE) puts the ring buffer
> > into the following state:
> >
> >   rb->aux_head = PAGE_SIZE
> >   rb->aux_wakeup = PAGE_SIZE / 2
> >   rb->aux_watermark = PAGE_SIZE / 2
> >
> > and then the next call to perf_aux_output_begin will result in:
> >
> >   handle->head = handle->wakeup = PAGE_SIZE
> >
> > for which the semantics are unclear and, for a smaller aux_watermark
> > (e.g. PAGE_SIZE / 4), then the wakeup would in fact be behind head at
> > this point.
> >
> > This patch fixes the problem by rounding down the aux_head (as sampled
> > from the handle) to the nearest aux_watermark boundary when updating
> > rb->aux_wakeup, therefore taking into account any overruns by the
> > hardware.
> 
> Let's add a small comment to the @aux_wakeup field definition? Other
> than that,

Good thinking; I'll do that.

> Acked-by: Alexander Shishkin <alexander.shishkin at linux.intel.com>

Thanks!

Will