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

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

> -----Original Message-----
> From: Chris Wilson <chris@chris-wilson.co.uk>
> Sent: Thursday, October 15, 2020 1:33 PM
> 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
> > > We can also prune the free_list immediately, if we know we are
> > > outside of any critical section. (We do this before create ioctls,
> > > and I thought upon close(device), but I see that's just contexts.)
> > >
> > > > 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?
> > >
> > > 1M is a lot more than is comfortable, and that's even with a
> > > high-priority worker.  The problem with objects being freed from any
> > > context is that we can't simply put a flush_work around there. (Not
> > > without ridding ourselves of a few mutexes at least.) We could try
> > > more than worker, but it's no more more effort to starve 2 cpus than it is
> to starve 1.
> > >
> > > No, with that much pressure the only option is to apply the
> > > backpressure at the point of allocation ala create_ioctl. i.e. find
> > > the hog, and look to see if there's a convenient spot before/after
> > > to call i915_gem_flush_free_objects(). Since you highlight the
> > > vma-stash as the likely culprit, and the free_pt_stash is unlikely
> > > to be inside any critical section, might as well try flushing from there for
> starters.
> >
> > I have not yet tested, but I guess calling i915_gem_flush_free_objects()
> inside free_pt_stash() will solve the problem that gem_exec_gttfill has,
> because it will give some back pressure on the system traffic.
> 
> Still I'm slightly concerned that so many PD objects are being created; it's not
> something that shows up in the smem ppgtt tests (or at least it's been
> dwarfed by other bottlenecks), and the set of vma (and so the
> PD) are meant to reach a steady state. You would need to be using a
> constant set of objects and recycling the vma, not to hit the create_ioctl flush.
> However, it points back to the pressure point being around the vma bind.
> 
> > But this is only for the page table 4K lmem objects allocated/freed by vma-
> stash. We might encounter the same situation with user space allocated
> objects.
> 
> See gem_exec_create, it's mission is to cause memory starvation by creating
> as many new objects as it can and releasing them after a nop batch. That's
> why we have the freelist flush from create_ioctl.
> 
> Now I need to add a pass that tries to create as many vma from a few objects
> as is possible.
> 
> (And similarly why we try to free requests as they are created.)
> 
> One problem is that they will catch the client after the hog, not necessarily
> the hog themselves.
> 
> I'm optimistic we can make freeing the object atomic, even if that means
> pushing the pages onto some reclaim list. (Which is currently a really nasty
> drawback of the free worker, a trick lost with the removal of
> struct_mutex.)

On a DG1 system with 4GB lmem, and 7.7GB system memory, I can easily catch system OOM when running "gem_exec_gttfill --r all".

When I call i915_gem_flush_free_objects() inside i915_vm_free_pt_stash(), the test code passes without any problem.

With simple test 'gem_exec_gttfill --r all", I also concerned that why driver create/free so many vma-stash objects, when the system OOM happened, there are total 1.5M stash objects freed.

--CQ

> -Chris
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