Hi Alexander, Thanks for having a look at these.
On Mon, Jul 31, 2017 at 01:02:16PM +0300, Alexander Shishkin wrote: > Will Deacon <[email protected]> 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 <[email protected]> Thanks! Will
