Re: [PATCH v2 00/25] AMDKFD kernel driver

[Jerome Glisse]

On Thu, Jul 24, 2014 at 09:57:16PM +0300, Oded Gabbay wrote:
> On 24/07/14 21:47, Jerome Glisse wrote:
> > On Thu, Jul 24, 2014 at 01:35:53PM -0400, Alex Deucher wrote:
> >> On Thu, Jul 24, 2014 at 11:44 AM, Jerome Glisse <j.glisse@xxxxxxxxx> wrote:
> >>> On Thu, Jul 24, 2014 at 01:01:41AM +0300, Oded Gabbay wrote:
> >>>> On 24/07/14 00:46, Bridgman, John wrote:
> >>>>>
> >>>>>> -----Original Message----- From: dri-devel
> >>>>>> [mailto:dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of Jesse
> >>>>>> Barnes Sent: Wednesday, July 23, 2014 5:00 PM To:
> >>>>>> dri-devel@xxxxxxxxxxxxxxxxxxxxx Subject: Re: [PATCH v2 00/25]
> >>>>>> AMDKFD kernel driver
> >>>>>>
> >>>>>> On Mon, 21 Jul 2014 19:05:46 +0200 daniel at ffwll.ch (Daniel
> >>>>>> Vetter) wrote:
> >>>>>>
> >>>>>>> On Mon, Jul 21, 2014 at 11:58:52AM -0400, Jerome Glisse wrote:
> >>>>>>>> On Mon, Jul 21, 2014 at 05:25:11PM +0200, Daniel Vetter wrote:
> >>>>>>>>> On Mon, Jul 21, 2014 at 03:39:09PM +0200, Christian K?nig
> >>>>>>>>> wrote:
> >>>>>>>>>> Am 21.07.2014 14:36, schrieb Oded Gabbay:
> >>>>>>>>>>> On 20/07/14 20:46, Jerome Glisse wrote:
> >>>>>>
> >>>>>> [snip!!]
> >>>>> My BlackBerry thumb thanks you ;)
> >>>>>>
> >>>>>>>>>>
> >>>>>>>>>> The main questions here are if it's avoid able to pin down
> >>>>>>>>>> the memory and if the memory is pinned down at driver load,
> >>>>>>>>>> by request from userspace or by anything else.
> >>>>>>>>>>
> >>>>>>>>>> As far as I can see only the "mqd per userspace queue"
> >>>>>>>>>> might be a bit questionable, everything else sounds
> >>>>>>>>>> reasonable.
> >>>>>>>>>
> >>>>>>>>> Aside, i915 perspective again (i.e. how we solved this):
> >>>>>>>>> When scheduling away from contexts we unpin them and put them
> >>>>>>>>> into the lru. And in the shrinker we have a last-ditch
> >>>>>>>>> callback to switch to a default context (since you can't ever
> >>>>>>>>> have no context once you've started) which means we can evict
> >>>>>>>>> any context object if it's
> >>>>>> getting in the way.
> >>>>>>>>
> >>>>>>>> So Intel hardware report through some interrupt or some channel
> >>>>>>>> when it is not using a context ? ie kernel side get
> >>>>>>>> notification when some user context is done executing ?
> >>>>>>>
> >>>>>>> Yes, as long as we do the scheduling with the cpu we get
> >>>>>>> interrupts for context switches. The mechanic is already
> >>>>>>> published in the execlist patches currently floating around. We
> >>>>>>> get a special context switch interrupt.
> >>>>>>>
> >>>>>>> But we have this unpin logic already on the current code where
> >>>>>>> we switch contexts through in-line cs commands from the kernel.
> >>>>>>> There we obviously use the normal batch completion events.
> >>>>>>
> >>>>>> Yeah and we can continue that going forward.  And of course if your
> >>>>>> hw can do page faulting, you don't need to pin the normal data
> >>>>>> buffers.
> >>>>>>
> >>>>>> Usually there are some special buffers that need to be pinned for
> >>>>>> longer periods though, anytime the context could be active.  Sounds
> >>>>>> like in this case the userland queues, which makes some sense.  But
> >>>>>> maybe for smaller systems the size limit could be clamped to
> >>>>>> something smaller than 128M.  Or tie it into the rlimit somehow,
> >>>>>> just like we do for mlock() stuff.
> >>>>>>
> >>>>> Yeah, even the queues are in pageable memory, it's just a ~256 byte
> >>>>> structure per queue (the Memory Queue Descriptor) that describes the
> >>>>> queue to hardware, plus a couple of pages for each process using HSA
> >>>>> to hold things like doorbells. Current thinking is to limit #
> >>>>> processes using HSA to ~256 and #queues per process to ~1024 by
> >>>>> default in the initial code, although my guess is that we could take
> >>>>> the #queues per process default limit even lower.
> >>>>>
> >>>>
> >>>> So my mistake. struct cik_mqd is actually 604 bytes, and it is allocated
> >>>> on 256 boundary.
> >>>> I had in mind to reserve 64MB of gart by default, which translates to
> >>>> 512 queues per process, with 128 processes. Add 2 kernel module
> >>>> parameters, # of max-queues-per-process and # of max-processes (default
> >>>> is, as I said, 512 and 128) for better control of system admin.
> >>>>
> >>>
> >>> So as i said somewhere else in this thread, this should not be reserved
> >>> but use a special allocation. Any HSA GPU use virtual address space for
> >>> userspace so only issue is for kernel side GTT.
> >>>
> >>> What i would like is seeing radeon pinned GTT allocation at bottom of
> >>> GTT space (ie all ring buffer and the ib pool buffer). Then have an
> >>> allocator that allocate new queue from top of GTT address space and
> >>> grow to the bottom.
> >>>
> >>> It should not staticly reserved 64M or anything. When doing allocation
> >>> it should move any ttm buffer that are in the region it want to allocate
> >>> to a different location.
> >>>
> >>>
> >>> As this needs some work, i am not against reserving some small amount
> >>> (couple MB) as a first stage but anything more would need a proper solution
> >>> like the one i just described.
> >>
> >> It's still a trade off.  Even if we reserve a couple of megs it'll be
> >> wasted if we are not running HSA apps. And even today if we run a
> >> compute job using the current interfaces we could end up in the same
> >> case.  So while I think it's definitely a good goal to come up with
> >> some solution for fragmentation, I don't think it should be a
> >> show-stopper right now.
> >>
> > 
> > Seems i am having a hard time to express myself. I am not saying it is a
> > showstopper i am saying until proper solution is implemented KFD should
> > limit its number of queue to consume at most couple MB ie not 64MB or more
> > but 2MB, 4MB something in that water.
> So we thought internally about limiting ourselves through two kernel
> module parameters, # of queues per process and # of processes. Default
> values will be 128 queues per process and 32 processes. mqd takes 768
> bytes at most, so that gives us a maximum of 3MB.
> 
> For absolute maximum, I think using H/W limits which are 1024 queues per
> process and 512 processes. That gives us 384MB.
> 
> Would that be acceptable ?

Yes and no, yes as _temporary_ solution ie a proper solution must be
implemented.

Moreover i sincerely hope that CZ will allow for easy way to unpin
any buffer and move them.

Cheers,
Jérôme

> > 
> >> A better solution to deal with fragmentation of GTT and provide a
> >> better way to allocate larger buffers in vram would be to break up
> >> vram <-> system pool transfers into multiple transfers depending on
> >> the available GTT size.  Or use GPUVM dynamically for  vram <-> system
> >> transfers.
> > 
> > Isn't the UVD engine still using the main GTT ? I have not look much at
> > UVD in a while.
> > 
> > Yes there is way to fix buffer migration but i would also like to see
> > address space fragmentation to a minimum which is the main reason i
> > uterly hate any design that forbid kernel to take over and do its thing.
> > 
> > Buffer pining should really be only for front buffer and thing like ring
> > ie buffer that have a lifetime bound to the driver lifetime.
> > 
> > Cheers,
> > Jérôme
> > 
> >>
> >> Alex
> 
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