Hi Alexander,

Thanks for having a look at these.

On Mon, Jul 31, 2017 at 01:02:16PM +0300, Alexander Shishkin wrote:

Will Deacon [off-list ref] writes:

quoted

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.

quoted

(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.

quoted

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@linux.intel.com>

Thanks!

Will