Am 23.02.20 um 17:54 schrieb Thomas Hellström (VMware):
On 2/23/20 4:45 PM, Christian König wrote:
Am 21.02.20 um 18:12 schrieb Daniel Vetter:
[SNIP]
Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly
degenerating
into essentially a global lock. But only when there's actual contention
and thrashing.
Yes exactly. A really big problem in TTM is currently that we drop
the lock after evicting BOs because they tend to move in again
directly after that.
From practice I can also confirm that there is exactly zero benefit
from dropping locks early and reacquire them for example for the VM
page tables. That's just makes it more likely that somebody needs to
roll back and this is what we need to avoid in the first place.
If you have a benchmarking setup available it would be very
interesting for future reference to see how changing from WD to WW
mutexes affects the roll back frequency. WW is known to cause
rollbacks much less frequently but there is more work associated with
each rollback.
Not of hand. To be honest I still have a hard time to get a grip on the
difference between WD and WW from the algorithm point of view. So I
can't judge that difference at all.
Contention on BO locks during command submission is perfectly fine as
long as this is as lightweight as possible while we don't have
trashing. When we have trashing multi submission performance is best
archived to just favor a single process to finish its business and
block everybody else.
Hmm. Sounds like we need a per-manager ww_rwsem protecting manager
allocation, taken in write-mode then there's thrashing. In read-mode
otherwise. That would limit the amount of "unnecessary" locks we'd
have to keep and reduce unwanted side-effects, (see below):
Well per-manager (you mean per domain here don't you?) doesn't sound
like that useful because we rarely use only one domain, but I'm actually
questioning for quite a while if the per BO lock scheme was the right
approach.
See from the performance aspect the closest to ideal solution I can
think of would be a ww_rwsem per user of a resource.
In other words we don't lock BOs, but instead a list of all their users
and when you want to evict a BO you need to walk that list and inform
all users that the BO will be moving.
During command submission you then have the fast path which rather just
grabs the read side of the user lock and check if all BOs are still in
the expected place.
If some BOs were evicted you back off and start the slow path, e.g.
maybe even copy additional data from userspace then grab the write side
of the lock etc.. etc...
That approach is similar to what we use in amdgpu with the per-VM BOs,
but goes a step further. Problem is that we are so used to per BO locks
in the kernel that this is probably not doable any more.
Because of this I would actually vote for forbidding to release
individual ww_mutex() locks in a context.
Yes, I see the problem.
But my first reaction is that this might have undersirable
side-effects. Let's say somebody wanted to swap the evicted BOs out?
Please explain further, I off hand don't see the problem here.
In general I actually wanted to re-work TTM in a way that BOs in the
SYSTEM/SWAPABLE domain are always backed by a shmem file instead of the
struct page array we currently have.
Or cpu-writes to them causing faults, that might also block the
mm_sem, which in turn blocks hugepaged?
Mhm, I also only have a higher level view how hugepaged works so why
does it grabs the mm_sem on the write side?
Thanks,
Christian.
Still it's a fairly simple solution to a problem that seems otherwise
hard to solve efficiently.
Thanks,
Thomas
Regards,
Christian.
-Daniel
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