Re: [PATCH v2 hmm 05/11] mm/hmm: Remove duplicate condition test before wait_event_timeout

[Ralph Campbell <rcampbell@nvidia.com>]

On 6/7/19 1:44 PM, Jason Gunthorpe wrote:
> On Fri, Jun 07, 2019 at 01:21:12PM -0700, Ralph Campbell wrote:
> 
>>> What I want to get to is a pattern like this:
>>>
>>> pagefault():
>>>
>>>      hmm_range_register(&range);
>>> again:
>>>      /* On the slow path, if we appear to be live locked then we get
>>>         the write side of mmap_sem which will break the live lock,
>>>         otherwise this gets the read lock */
>>>      if (hmm_range_start_and_lock(&range))
>>>            goto err;
>>>
>>>      lockdep_assert_held(range->mm->mmap_sem);
>>>
>>>      // Optional: Avoid useless expensive work
>>>      if (hmm_range_needs_retry(&range))
>>>         goto again;
>>>      hmm_range_(touch vmas)
>>>
>>>      take_lock(driver->update);
>>>      if (hmm_range_end(&range) {
>>>          release_lock(driver->update);
>>>          goto again;
>>>      }
>>>      // Finish driver updates
>>>      release_lock(driver->update);
>>>
>>>      // Releases mmap_sem
>>>      hmm_range_unregister_and_unlock(&range);
>>>
>>> What do you think?
>>>
>>> Is it clear?
>>>
>>> Jason
>>>
>>
>> Are you talking about acquiring mmap_sem in hmm_range_start_and_lock()?
>> Usually, the fault code has to lock mmap_sem for read in order to
>> call find_vma() so it can set range.vma.
> 
>> If HMM drops mmap_sem - which I don't think it should, just return an
>> error to tell the caller to drop mmap_sem and retry - the find_vma()
>> will need to be repeated as well.
> 
> Overall I don't think it makes a lot of sense to sleep for retry in
> hmm_range_start_and_lock() while holding mmap_sem. It would be better
> to drop that lock, sleep, then re-acquire it as part of the hmm logic.
> 
> The find_vma should be done inside the critical section created by
> hmm_range_start_and_lock(), not before it. If we are retrying then we
> already slept and the additional CPU cost to repeat the find_vma is
> immaterial, IMHO?
> 
> Do you see a reason why the find_vma() ever needs to be before the
> 'again' in my above example? range.vma does not need to be set for
> range_register.

Yes, for the GPU case, there can be many faults in an event queue
and the goal is to try to handle more than one page at a time.
The vma is needed to limit the amount of coalescing and checking
for pages that could be speculatively migrated or mapped.

>> I'm also not sure about acquiring the mmap_sem for write as way to
>> mitigate thrashing. It seems to me that if a device and a CPU are
>> both faulting on the same page,
> 
> One of the reasons to prefer this approach is that it means we don't
> need to keep track of which ranges we are faulting, and if there is a
> lot of *unrelated* fault activity (unlikely?) we can resolve it using
> mmap sem instead of this elaborate ranges scheme and related
> locking.
> 
> This would reduce the overall work in the page fault and
> invalidate_start/end paths for the common uncontended cases.
> 
>> some sort of backoff delay is needed to let one side or the other
>> make some progress.
> 
> What the write side of the mmap_sem would do is force the CPU and
> device to cleanly take turns. Once the device pages are registered
> under the write side the CPU will have to wait in invalidate_start for
> the driver to complete a shootdown, then the whole thing starts all
> over again.
> 
> It is certainly imaginable something could have a 'min life' timer for
> a device mapping and hold mm invalidate_start, and device pagefault
> for that min time to promote better sharing.
> 
> But, if we don't use the mmap_sem then we can livelock and the device
> will see an unrecoverable error from the timeout which means we have
> risk that under load the system will simply obscurely fail. This seems
> unacceptable to me..
> 
> Particularly since for the ODP use case the issue is not trashing
> migration as a GPU might have, but simple system stability under swap
> load. We do not want the ODP pagefault to permanently fail due to
> timeout if the VMA is still valid..
> 
> Jason
> 

OK, I understand.
If you come up with a set of changes, I can try testing them.
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