Re: [PATCH 2/2] drm/i915: Shrink the GEM kmem_caches upon idling

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 24/01/2018 10:37, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-01-24 10:32:16)
> > On 19/01/2018 15:23, Chris Wilson wrote:
> > > When we finally decide the gpu is idle, that is a good time to shrink
> > > our kmem_caches.
> > >
> > > v3: Defer until an rcu grace period after we idle.
> > >
> > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/i915/i915_gem.c | 65 +++++++++++++++++++++++++++++++++++++++++
> > >    1 file changed, 65 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > > index 7f0684ccc724..6a8fbcae835b 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > > @@ -3341,12 +3341,59 @@ new_requests_since_last_retire(const struct drm_i915_private *i915)
> > >                work_pending(&i915->gt.idle_work.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);
> > > +}
> > > +
> > > +struct sleep_rcu_work {
> > > +     struct drm_i915_private *i915;
> > > +     struct rcu_head rcu;
> > > +     struct work_struct work;
> > > +     u32 epoch;
> > > +};
> > > +
> > > +static void __sleep_work(struct work_struct *work)
> > > +{
> > > +     struct sleep_rcu_work *s = container_of(work, typeof(*s), work);
> > > +     struct drm_i915_private *i915 = s->i915;
> > > +     u32 epoch = s->epoch;
> > > +
> > > +     kfree(s);
> > > +     if (epoch == READ_ONCE(i915->gt.epoch))
> > > +             shrink_caches(i915);
> > > +}
> > > +
> > > +static void __sleep_rcu(struct rcu_head *rcu)
> > > +{
> > > +     struct sleep_rcu_work *s = container_of(rcu, typeof(*s), rcu);
> > > +     struct drm_i915_private *i915 = s->i915;
> > > +
> > > +     if (s->epoch == READ_ONCE(i915->gt.epoch)) {
> > > +             INIT_WORK(&s->work, __sleep_work);
> > > +             queue_work(i915->wq, &s->work);
> > > +     } else {
> > > +             kfree(s);
> > > +     }
> > > +}
> > > +
> > >    static void
> > >    i915_gem_idle_work_handler(struct work_struct *work)
> > >    {
> > >        struct drm_i915_private *dev_priv =
> > >                container_of(work, typeof(*dev_priv), gt.idle_work.work);
> > >        bool rearm_hangcheck;
> > > +     u32 epoch = 0;
> > >        ktime_t end;
> > >    
> > >        if (!READ_ONCE(dev_priv->gt.awake))
> > > @@ -3406,6 +3453,7 @@ i915_gem_idle_work_handler(struct work_struct *work)
> > >        GEM_BUG_ON(!dev_priv->gt.awake);
> > >        dev_priv->gt.awake = false;
> > >        rearm_hangcheck = false;
> > > +     epoch = dev_priv->gt.epoch;
> > >    
> > >        if (INTEL_GEN(dev_priv) >= 6)
> > >                gen6_rps_idle(dev_priv);
> > > @@ -3421,6 +3469,23 @@ i915_gem_idle_work_handler(struct work_struct *work)
> > >                GEM_BUG_ON(!dev_priv->gt.awake);
> > >                i915_queue_hangcheck(dev_priv);
> > >        }
> > > +
> > > +     /*
> > > +      * When we are idle, it is an opportune time to reap our caches.
> > > +      * However, we have many objects that utilise RCU and the ordered
> > > +      * i915->wq that this work is executing on. To try and flush any
> > > +      * pending frees now we are idle, we first wait for an RCU grace
> > > +      * period, and then queue a task (that will run last on the wq) to
> > > +      * shrink and re-optimize the caches.
> > > +      */
> > > +     if (epoch == READ_ONCE(dev_priv->gt.epoch)) {
> >
> > Theoretically this can be true on epoch wrap-around, when trylock
> > failed. It's one in four billion busy transitions but it could be just
> > worth handling it explicitly. Simplest probably to ensure gt.epoch is
> > never zero when incrementing?
>
> I was working on the principle that a u32 wraparound takes at least
> 100ms * 2^32, or about 326 years. :)

Ah forgot there is a time delay.. okay then. :)

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

To be kept if/when you change to unsigned int.

Regards,

Tvrtko


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