Hi guys,
I was wondering why the heck Felix wants to add another lock for over an
hour before I realized that I accidentally forget to send out patch #4
and #5.
And yes what you describe is exactly what my patch #4 is doing and patch
#5 is then a prove of concept to use the new interface for evictions of
shared BOs.
Regards,
Christian.
Am 05.12.19 um 02:27 schrieb Felix Kuehling:
[adding the mailing list back]
Christian sent a 3-patch series that I just reviewed and commented on.
That removes the need to lock the page directory reservation for
updating page tables. That solves a part of the problem, but not all
of it.
We still need to implement that driver lock for HMM, and add a
vm_resident flag in the vm structure that could be checked in the new
amdgpu_vm_evictable function Christian added. That function still runs
in atomic context (under the ttm_bo_glob.lru_lock spinlock). So we
can't lock a mutex there. But we can use trylock and return "not
evictable" if we fail to take the mutex.
If we take the mutex successfully, we can update the vm_resident flag
to false and allow the eviction.
The rest should work just the way I proposed earlier.
Regards,
Felix
On 2019-12-04 8:01 p.m., Sierra Guiza, Alejandro (Alex) wrote:
[AMD Official Use Only - Internal Distribution Only]
Hi Christian,
I wonder if you had have time to check on this implementation?
Please let me know if there's something that I could help you with
Regards,
Alex
-----Original Message-----
From: Christian König <ckoenig.leichtzumerken@xxxxxxxxx>
Sent: Friday, November 29, 2019 1:41 AM
To: Kuehling, Felix <Felix.Kuehling@xxxxxxx>; Koenig, Christian
<Christian.Koenig@xxxxxxx>; Sierra Guiza, Alejandro (Alex)
<Alex.Sierra@xxxxxxx>
Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx
Subject: Re: Deadlock on PTEs update for HMM
[CAUTION: External Email]
Hi Felix,
yes that is exactly my thinking as well. Problem is that getting this
to work was much harder than I thought.
We can't use a mutex cause TTM is calling the eviction callback in
atomic context. A spinlock doesn't looked like a good idea either
because we potentially need to wait for the hardware with a fixed IB
pool.
Because of this I've started to rewrite the TTM handling to not call
the driver in an atomic context any more, but that took me way longer
than expected as well.
I'm currently experimenting with using a trylock driver mutex, that
at least that should work for now until we got something better.
Regards,
Christian.
Am 28.11.19 um 21:30 schrieb Felix Kuehling:
Hi Christian,
I'm thinking about this problem, trying to come up with a solution.
The fundamental problem is that we need low-overhead access to the
page table in the MMU notifier, without much memory management or
locking.
There is one "driver lock" that we're supposed to take in the MMU
notifier as well as when we update page tables that is prescribed by
the HMM documentation (Documentation/vm/hmm.rst). I don't currently
see such a lock in amdgpu. We'll probably need to add that anyway,
with all the usual precautions about lock dependencies around MMU
notifiers. Then we could use that lock to protect page table residency
state, in addition to the reservation of the top-level page directory.
We don't want to block eviction of page tables unconditionally, so the
MMU notifier must be able to deal with the situation that page tables
are not resident at the moment. But the lock can delay page tables
from being evicted while an MMU notifier is in progress and protect us
from race conditions between MMU notifiers invalidating PTEs, and page
tables getting evicted.
amdgpu_vm_bo_invalidate could detect when a page table is being
evicted, and update a new "vm_resident" flag inside the amdgpu_vm
while holding the "HMM driver lock". If an MMU notifier is in
progress, trying to take the "HMM driver lock" will delay the eviction
long enough for any pending PTE invalidation to complete.
In the case that page tables are not resident (vm_resident flag is
false), it means the GPU is currently not accessing any memory in that
amdgpu_vm address space. So we don't need to invalidate the PTEs right
away. I think we could implement a deferred invalidation mechanism for
this case, that delays the invalidation until the next time the page
tables are made resident. amdgpu_amdkfd_gpuvm_restore_process_bos
would replay any deferred PTE invalidations after validating the page
tables but before restarting the user mode queues for the process. If
graphics ever implements page-fault-based memory management, you'd
need to do something similar in amdgpu_cs.
Once all that is in place, we should be able to update PTEs in MMU
notifiers without reserving the page tables.
If we use SDMA for updating page tables, we may need a pre-allocated
IB for use in MMU notifiers. And there is problably other details to
be worked out about exactly how we implement the PTE invalidation in
MMU notifiers and reflecting that in the state of the amdgpu_vm and
amdgpu_bo_va_mapping.
Does this idea sound reasonable to you? Can you think of a simpler
solution?
Thanks,
Felix
On 2019-11-27 10:02 a.m., Christian König wrote:
Hi Alejandro,
yes I'm very aware of this issue, but unfortunately can't give an
easy solution either.
I'm working for over a year now on getting this fixed, but
unfortunately it turned out that this problem is much bigger than
initially thought.
Setting the appropriate GFP flags for the job allocation is actually
the trivial part.
The really really hard thing is that we somehow need to add a lock to
prevent the page tables from being evicted. And as you also figured
out that lock can't be taken easily anywhere else.
I've already wrote a prototype for this, but didn't had time to
hammer it into shape for upstreaming yet.
Regards,
Christian.
Am 27.11.19 um 15:55 schrieb Sierra Guiza, Alejandro (Alex):
Hi Christian,
As you know, we’re working on the HMM enablement. Im working on the
dGPU page table entries invalidation on the userptr mapping case.
Currently, the MMU notifiers handle stops all user mode queues,
schedule a delayed worker to re-validate userptr mappings and
restart the queues.
Part of the HMM functionality, we need to invalidate the page table
entries instead of stopping the queues. At the same time we need to
move the revalidation of the userptr mappings into the page fault
handler.
We’re seeing a deadlock warning after we try to invalidate the PTEs
inside the MMU notifier handler. More specific, when we try to
update the BOs to invalidate PTEs using amdgpu_vm_bo_update. This
uses kmalloc on the amdgpu_job_alloc which seems to be causing this
problem.
Based on @Kuehling, Felix <mailto:Felix.Kuehling@xxxxxxx> comments,
kmalloc without any special flags can cause memory reclaim. Doing
that inside an MMU notifier is problematic, because an MMU notifier
may be called inside a memory-reclaim operation itself. That would
result in recursion. Also, reclaim shouldn't be done while holding a
lock that can be taken in an MMU notifier for the same reason. If
you cause a reclaim while holding that lock, then an MMU notifier
called by the reclaim can deadlock trying to take the same lock.
Please let us know if you have any advice to enable this the right
way
Thanks in advanced,
Alejandro
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