[Intel-gfx] [PATCH 1/3] drm/i915: Use pagecache write to prepopulate shmemfs from pwrite-ioctl

[Chris Wilson]

On Mon, Mar 06, 2017 at 04:32:45PM +0000, Tvrtko Ursulin wrote:
> 
> On 06/03/2017 14:14, Chris Wilson wrote:
> >Remember when I said that nobody would touch pages without using them,
> >(and so could defer the update for the shrinker until we had the
> >struct_mutex) and certainly not 16GiB of written-but-unused pages on a
> >small box? libva happened.
> 
> Oh dear.. Ok, going back to the previous reply..
> 
> I can see the benefit of avoiding the shrinker and struct mutex but
> haven't found that other benefit.
> 
> I've been rummaging in the shmem.c & co but so far haven't found
> anything to explain me the possibility of it avoiding
> clearing/swapping-in the pages. It looks like both our normal page
> allocation and this new one boil down to the same shmem_getpage.
> 
> Could you explain what I am missing?

Normally we use shmem_getpages(SGP_CACHE). write_begin uses SGP_WRITE.
That has the clear avoidance, but alas I can't see it taking advantage
of avoiding a swapin - probably because SGP_WRITE doesn't differentiate
between a full or partial page write at that point, though it has the
information to do so. (swapin avoidance is then just a pipe dream.) For
bonus points would be handling high-order pages...

> Also, would we have any IGT coverage for this new path? And keep a
> solid amount of coverage for the old paths as well.

Virtually every test has some form of
gem_pwrite(gem_create(4096), 0, &bbe, sizeof(bbe));
and this path is extensively tested by gem_concurrent_blit, gem_pwrite
which should both exercise initial pwrites plus pwrites following
shrinking, as well as the ordinary pwrite with obj->mm.pages.

In real workloads though, while pwrite is fairly widespread in mesa for
uploading the batch buffer (on !llc at least), the userspace bo cache
means that the number of times we can expect to hit this path are rare.
Which just leaves the confusing case of libva.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre