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

[Oded Gabbay]

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.

	Oded

>>>> The issue with radeon hardware AFAICT is that the hardware do
>>>> not report any thing about the userspace context running ie you
>>>> do not get notification when a context is not use. Well AFAICT.
>>>> Maybe hardware
>> do provide that.
>>> 
>>> I'm not sure whether we can do the same trick with the hw
>>> scheduler. But then unpinning hw contexts will drain the pipeline
>>> anyway, so I guess we can just stop feeding the hw scheduler
>>> until it runs dry. And then unpin and evict.
>> 
>> Yeah we should have an idea which contexts have been fed to the
>> scheduler, at least with kernel based submission.  With userspace
>> submission we'll be in a tougher spot...  but as you say we can
>> always idle things and unpin everything under pressure.  That's a
>> really big hammer to apply though.
>> 
>>>> Like the VMID is a limited resources so you have to dynamicly
>>>> bind them so maybe we can only allocate pinned buffer for each
>>>> VMID and then when binding a PASID to a VMID it also copy back
>>>> pinned buffer to
>> pasid unpinned copy.
>>> 
>>> Yeah, pasid assignment will be fun. Not sure whether Jesse's
>>> patches will do this already. We _do_ already have fun with ctx
>>> id assigments though since we move them around (and the hw id is
>>> the ggtt address afaik). So we need to remap them already. Not
>>> sure on the details for pasid mapping, iirc it's a separate field
>>> somewhere in the context struct. Jesse knows the details.
>> 
>> The PASID space is a bit bigger, 20 bits iirc.  So we probably
>> won't run out quickly or often.  But when we do I thought we could
>> apply the same trick Linux uses for ASID management on SPARC and
>> ia64 (iirc on sparc anyway, maybe MIPS too): "allocate" a PASID
>> everytime you need one, but don't tie it to the process at all,
>> just use it as a counter that lets you know when you need to do a
>> full TLB flush, then start the allocation process over.  This lets
>> you minimize TLB flushing and gracefully handles oversubscription.
> 
> IIRC we have a 9-bit limit for PASID on current hardware, although
> that will go up in future.
>> 
>> My current code doesn't bother though; context creation will fail
>> if we run out of PASIDs on a given device.
>> 
>> -- Jesse Barnes, Intel Open Source Technology Center 
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