Re: [PATCH v2] drm/i915: Shrink the GEM kmem_caches upon idling

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH v2] drm/i915: Shrink the GEM kmem_caches upon idling
Date: Tue, 16 Jan 2018 15:21:57 +0000	[thread overview]
Message-ID: <151611611787.21947.7115673200318591861@mail.alporthouse.com> (raw)
In-Reply-To: <4ce412fe-13d0-b06b-4e4d-7fe14bbb7ef4@linux.intel.com>

Quoting Tvrtko Ursulin (2018-01-16 15:12:43)
> 
> On 16/01/2018 13:05, Chris Wilson wrote:
> > When we finally decide the gpu is idle, that is a good time to shrink
> > our kmem_caches.
> > 
> > v2: Comment upon the random sprinkling of rcu_barrier() inside the idle
> > worker.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_gem.c | 30 ++++++++++++++++++++++++++++++
> >   1 file changed, 30 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 335731c93b4a..61b13fdfaa71 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -4716,6 +4716,21 @@ i915_gem_retire_work_handler(struct work_struct *work)
> >       }
> >   }
> >   
> > +static void shrink_caches(struct drm_i915_private *i915)
> > +{
> > +     /*
> > +      * kmem_cache_shrink() discards empty slabs and reorders partially
> > +      * filled slabs to prioritise allocating from the mostly full slabs,
> > +      * with the aim of reducing fragmentation.
> > +      */
> > +     kmem_cache_shrink(i915->priorities);
> > +     kmem_cache_shrink(i915->dependencies);
> > +     kmem_cache_shrink(i915->requests);
> > +     kmem_cache_shrink(i915->luts);
> > +     kmem_cache_shrink(i915->vmas);
> > +     kmem_cache_shrink(i915->objects);
> > +}
> > +
> >   static inline bool
> >   new_requests_since_last_retire(const struct drm_i915_private *i915)
> >   {
> > @@ -4803,6 +4818,21 @@ i915_gem_idle_work_handler(struct work_struct *work)
> >               GEM_BUG_ON(!dev_priv->gt.awake);
> >               i915_queue_hangcheck(dev_priv);
> >       }
> > +
> > +     /*
> > +      * We use magical TYPESAFE_BY_RCU kmem_caches whose pages are not
> > +      * returned to the system imediately but only after an RCU grace
> > +      * period. We want to encourage such pages to be returned and so
> > +      * incorporate a RCU barrier here to provide some rate limiting
> > +      * of the driver and flush the old pages before we free a new batch
> > +      * from the next round of shrinking.
> > +      */
> > +     rcu_barrier();
> 
> Should this go into the conditional below? I don't think it makes a 
> difference effectively, but may be more logical.

My thinking was to have the check after the sleep as the state is
subject to change. I'm not concerned about the random unnecessary pauses
on this wq, since it is subject to struct_mutex delays, so was quite
happy to think of this as being "we shall only do one idle pass per RCU
grace period".

> > +
> > +     if (!new_requests_since_last_retire(dev_priv)) {
> > +             __i915_gem_free_work(&dev_priv->mm.free_work);
> 
> I thought for a bit if re-using the worker from here is completely fine 
> but I think it is. We expect only one pass when called from here so 
> need_resched will be correctly neutralized/not-relevant from this path. 

At present, I was only thinking about the single path. This was meant to
resemble i915_gem_drain_objects(), without the recursion :)

> Hm, unless if we consider mmap_gtt users.. so we could still have new 
> objects appearing on the free_list after the 1st pass. And then 
> need_resched might kick us out. What do you think?

Not just mmap_gtt, any user freeing objects (coupled with RCU grace
periods). I don't think it matters if we happen to loop until the
timeslice is consumed as we are doing work that we would be doing
anyway on this i915->wq.
-Chris
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