[Intel-gfx] [RFC] drm/i915 : Reduce the shmem page allocation time by using blitter engines for clearing pages.

[Chris Wilson]

On Tue, May 06, 2014 at 12:59:37PM +0000, Gupta, Sourab wrote:
> On Tue, 2014-05-06 at 11:34 +0000, Chris Wilson wrote:
> > On Tue, May 06, 2014 at 04:40:58PM +0530, sourab.gupta at intel.com wrote:
> > > From: Sourab Gupta <sourab.gupta at intel.com>
> > > 
> > > This patch is in continuation of and is dependent on earlier patch
> > > series to 'reduce the time for which device mutex is kept locked'.
> > > (http://lists.freedesktop.org/archives/intel-gfx/2014-May/044596.html)
> > > 
> > > This patch aims to reduce the allocation time of pages from shmem
> > > by using blitter engines for clearing the pages freshly alloced.
> > > This is utilized in case of fresh pages allocated in shmem_preallocate
> > > routines in execbuffer path and page_fault path only.
> > > 
> > > Even though the CPU memset routine is optimized, but still sometimes
> > > the time consumed in clearing the pages of a large buffer comes in
> > > the order of milliseconds.
> > > We intend to make this operation asynchronous by using blitter engine,
> > > so irrespective of the size of buffer to be cleared, the execbuffer
> > > ioctl processing time will not be affected. Use of blitter engine will
> > > make the overall execution time of 'execbuffer' ioctl lesser for
> > > large buffers.
> > > 
> > > There may be power implications here on using blitter engines, and
> > > we have to evaluate this. As a next step, we can selectively enable
> > > this HW based memset only for large buffers, where the overhead of
> > > adding commands in a blitter ring(which will otherwise be idle),
> > > cross ring synchronization, will be negligible compared to using the
> > > CPU to clear out the buffer.
> > 
> > You leave a lot of holes by which you leak the uncleared pages to
> > userspace.
> > -Chris
> > 
> Hi Chris,
> 
> Are you ok with the overall design as such, and the
> shmem_read_mapping_page_gfp_noclear interface?
> Is the leak of uncleared pages happening due to implementation issues?
> If so, we'll try to mitigate them.

Actually, along similar lines there is an even more fundamental issue.
You should only clear the objects if the pages have not been
prepopulated.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre