Re: [Intel-gfx] [PATCH] drm/i915: Make the GEM reclaim workqueue high priority

["Tang, CQ" <cq.tang@intel.com>]

i915_gem_free_object() is called by multiple threads/processes, they all add objects onto the same free_list. The free_list processing worker thread becomes bottle-neck. I see that the worker is mostly a single thread (with particular thread ID), but sometimes multiple threads are launched to process the 'free_list' work concurrently. But the processing speed is still slower than the multiple process's feeding speed, and 'free_list' is holding more and more memory.

The worker launching time is delayed a lot, we call queue_work() when we add the first object onto the empty 'free_list', but when the worker is launched, the 'free_list' has sometimes accumulated 1M objects. Maybe it is because of waiting currently running worker to finish?

This happens with direct call to __i915_gem_free_object_rcu() and no cond_resched().

--CQ

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces@lists.freedesktop.org> On Behalf Of
> Tang, CQ
> Sent: Tuesday, October 13, 2020 9:41 AM
> To: Chris Wilson <chris@chris-wilson.co.uk>; intel-gfx@lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH] drm/i915: Make the GEM reclaim workqueue
> high priority
> 
> 
> 
> > -----Original Message-----
> > From: Chris Wilson <chris@chris-wilson.co.uk>
> > Sent: Tuesday, October 13, 2020 9:25 AM
> > To: Tang, CQ <cq.tang@intel.com>; intel-gfx@lists.freedesktop.org
> > Subject: Re: [Intel-gfx] [PATCH] drm/i915: Make the GEM reclaim
> > workqueue high priority
> >
> > Quoting Tang, CQ (2020-10-13 17:19:27)
> > > Chris,
> > >     I tested this patch. It is still not enough, I keep catch running out of
> lmem.
> > Every worker invocation takes larger and larger freeing object count.
> > >
> >
> > Was that with the immediate call (not via call_rcu) to
> > __i915_gem_free_object_rcu?
> >
> > If this brings the freelist under control, the next item is judicious
> > use of cond_synchronize_rcu(). We just have to make sure we penalize
> > the right hog.
> >
> > Otherwise, we have to shotgun apply i915_gem_flush_free_objects() and
> > still find somewhere to put the rcu sync.
> 
> This is with call_rcu().
> 
> Then I removed cond_resched(), it does not help, and then I call
> __i915_gem_free_object_rcu() directly, still the same error, However, I
> noticed that sometimes 'queue_work()' return false, which means the work
> is already queued, how? The worker had been called so 'free_list' is empty:
> 
> [  117.381888] queue_work: 107967, 107930; 1 [  119.180230] queue_work:
> 125531, 125513; 1 [  121.349308] queue_work: 155017, 154996; 1 [  124.214885]
> queue_work: 193918, 193873; 1 [  127.967260] queue_work: 256838, 256776;
> 1 [  133.281045] queue_work: 345753, 345734; 1 [  141.457995] queue_work:
> 516943, 516859; 1 [  156.264420] queue_work: 863622, 863516; 1 [  156.322619]
> queue_work: 865849, 3163; 0 [  156.448551] queue_work: 865578, 7141; 0
> [  156.882985] queue_work: 866984, 24138; 0 [  157.952163] queue_work:
> 862902, 53365; 0 [  159.838412] queue_work: 842522, 95504; 0 [  174.321508]
> queue_work: 937179, 657323; 0
> 
> --CQ
> 
> > -Chris
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