Re: [PATCH v2] drm/i915: Remove __GFP_NORETRY from our buffer allocator

[Michal Hocko <mhocko@suse.com>]

On Mon 05-06-17 16:04:55, Chris Wilson wrote:
> Quoting Michal Hocko (2017-06-05 14:08:10)
> > On Mon 05-06-17 13:49:38, Chris Wilson wrote:
> > > Quoting Michal Hocko (2017-06-05 13:26:30)
> > > > On Mon 05-06-17 11:35:12, Chris Wilson wrote:
> > > > > I tried __GFP_NORETRY in the belief that __GFP_RECLAIM was effective. It
> > > > > struggles with handling reclaim via kswapd (through inconsistency within
> > > > > throttle_direct_reclaim() and even then the race between multiple
> > > > > allocators makes the two step of reclaim then allocate fragile), and as
> > > > > our buffers are always dirty (with very few exceptions), we required
> > > > > kswapd to perform pageout on them. The only effective means of waiting
> > > > > on kswapd is to retry the allocations (i.e. not set __GFP_NORETRY). That
> > > > > leaves us with the dilemma of invoking the oomkiller instead of
> > > > > propagating the allocation failure back to userspace where it can be
> > > > > handled more gracefully (one hopes).  In the future we may have
> > > > > __GFP_MAYFAIL to allow repeats up until we genuinely run out of memory
> > > > > and the oomkiller would have been invoked. Until then, let the oomkiller
> > > > > wreck havoc.
> > > > > 
> > > > > v2: Stop playing with side-effects of gfp flags and await __GFP_MAYFAIL
> > > > > 
> > > > > Fixes: 24f8e00a8a2e ("drm/i915: Prefer to report ENOMEM rather than incur the oom for gfx allocations")
> > > > > Testcase: igt/gem_tiled_swapping
> > > > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > > > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > > > Cc: Michal Hocko <mhocko@suse.com>
> > > > > ---
> > > > >  drivers/gpu/drm/i915/i915_gem.c | 15 ++++++++++++++-
> > > > >  1 file changed, 14 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > > > > index 7286f5dd3e64..845df6067e90 100644
> > > > > --- a/drivers/gpu/drm/i915/i915_gem.c
> > > > > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > > > > @@ -2406,7 +2406,20 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
> > > > >                       if (!*s) {
> > > > >                               /* reclaim and warn, but no oom */
> > > > >                               gfp = mapping_gfp_mask(mapping);
> > > > > -                             gfp |= __GFP_NORETRY;
> > > > > +
> > > > > +                             /* Our bo are always dirty and so we require
> > > > > +                              * kswapd to reclaim our pages (direct reclaim
> > > > > +                              * performs no swapping on its own). However,
> > > > 
> > > > Not sure whether this is exactly what you mean. The only pageout the
> > > > direct reclaim is allowed to the swap partition (so anonymous and
> > > > shmem). So the above is not 100% correct.
> > > 
> > > Hmm, I didn't see anything that allows direct reclaim to perform
> > > writeback into swap.
> > 
> > shrink_page_list
> >   add_to_swap
> >     add_to_swap_cache (gains mapping see page_mapping)
> >   pageout
> >     mapping->a_ops->writepage = shmem_writepage
> >       swap_writepage
> 
> Fwiw, sticking a few counters into writepage, i915 objects are only ever
> called for shmem_writepage from kswapd.
> 
> Ideas? I'll continue on with a few more printks to try and work out why
> I never see direct reclaim doing pageout.

We have some tracepoints for vmscan, I would try looking at them. You
should be able to see from which LRUs pages have been isolated and
reclaimed that way.

-- 
Michal Hocko
SUSE Labs
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