On 04/16/2018 11:19 AM, Daniel Vetter wrote:
On Mon, Apr 16, 2018 at 10:23 AM, Thomas Hellstrom
<thellstrom@xxxxxxxxxx> wrote:
On 04/14/2018 10:33 AM, Daniel Vetter wrote:
Hi Thomas,
On Fri, Apr 13, 2018 at 10:23 PM, Thomas Hellstrom
<thellstrom@xxxxxxxxxx> wrote:
On 04/13/2018 07:13 PM, Daniel Vetter wrote:
On Wed, Apr 11, 2018 at 10:27:06AM +0200, Thomas Hellstrom wrote:
2) Should we add a *real* wound-wait choice to our wound-wait mutexes.
Otherwise perhaps rename them or document that they're actually doing
wait-die.
I think a doc patch would be good at least. Including all the data you
assembled here.
Actually, a further investigation appears to indicate that manipulating
the
lock state under a local spinlock is about fast as using atomic
operations
even for the completely uncontended cases.
This means that we could have a solution where you decide on a per-mutex
or
per-reservation object basis whether you want to manipulate lock-state
under
a "batch group" spinlock, meaning certain performance characteristics or
traditional local locking, meaning other performance characteristics.
Like, vmwgfx could choose batching locks, radeon traditional locks, but
the
same API would work for both and locks could be shared between drivers..
Don't we need to make this decision at least on a per-class level?
No, I was thinking more in the line of the ww_mutex having a pointer to the
spinlock. It could either be the local mutex "wait_lock", or a
per-batch-group lock. The mutex code wouldn't care. We do need a special API
for batched locking, though, but not for ordinary locking.
Both APIs should be able to handle local or grouped spinlocks.
Note that this would of course require that there would be no performance
loss for users that don't use batch groups.
I guess the most efficient use for GPU command submission would be to use
per-process batch-groups. Then when the batch encounters a ww_mutex with a
different batch group (for example the display server shared surface, it'll
just switch batch lock), and this way the contention for
the batch spinlock will be mostly eliminated.
But won't this force us through the spinlock case for all ww_mutex?
The core mutex code goes to extreme lengths to avoid that for the
uncontended fast path. That's why I meant the batched and non-batch
ww_mutex look fundamentally incompatible. Or maybe I missed something
somewhere.
Yes, this would require the assumption to hold that a _local_ spinlock
path, that is, taking a local spinlock also in the fastpath would be as
fast as using the atomic operations directly. And that's what I'm
seeing, (or perhaps a percent or so slower with 20000 simulated CS'es
taking 800 uncontended locks each). Both running sequentially and in
parallel. Guess I need to verify this on my rpi as well, so it's not an
Intel specific thing.
This is of course a prerequisite for the idea to work.
Basically the spinlock fastpath appears to be an inlined atomic_cmpxchg,
so even theoretically there should not be any noticeable performance
loss. I'm not sure why the sleeping locks insist using atomic operations
over spinlocks, but with the qspinlock implementation (seems to be
2014-ish), the atomic exchange on spin_unlock was elminated, and I guess
that changed the playground...
With a _shared_ spinlock, like with a batch group, we would see
different performance characteristics, though.
Or
how will the spinlock/batch-lock approach interact with the normal
ww_mutex_lock path (which does require the atomics/ordered stores
we're trying to avoid)?
We can use the same code with some extra
if (use_ww_ctx) in the common locking and unlocking path.
Note that the "use_ww_ctx" parameter is defined at compile-time so the
ordinary mutex path (binary) shouldn't change at all after optimization but
I need to verify that, of course.
Agreed, no issue there.
What you can't do with such a change is to lock / unlock a ww_mutex using
the standard mutex API, like mutex_lock(&ww_mutex->base), but I guess that
would be OK?
Yeah right now that works, but I don't care about that. The point of
merging ww_mutex into the core mutex was that we can fully reuse the
__mutex_trylock_fast. Same for the unlock path.
Once we don't share that code anymore, there's imo not that much point
in having ww_mutex interleaved with core mutex code.
If we can't mix them I'm kinda leaning towards a
ww_batch_mutex/ww_batch_acquire_ctx, but exactly matching api
otherwise. We probably do need the new batch_start/end api, since
ww_acquire_done isn't quite the right place ...
I'll see if I get time to put together an RFC.
Yeah I think there's definitely some use for batched ww locks, where
parallelism is generally low, or at least the ratio between "time
spent acquiring locks" and "time spent doing stuff while holding
locks" small enough to not make the reduced parallelism while
acquiring an issue.
Yes. At least it's worth bringing up for discussion. The reduced parallelism
shouldn't be an issue if per-process batch-groups are used, or, like for
vmwgfx the command submission itself is serialized, due to a single FIFO.
Random aside: We do seem to already implement wound semantics (or I'm
terribly confused). See __ww_mutex_add_waiter plus related wakeup code
(in __ww_mutex_lock_check_stamp).
-Daniel
From what I can see, that's only the wait-die case, that is processes
that don't hold the lock kill their transaction. What's implemented is
also a multi-contender logic so that if we can deduce that a waiter will
need to die once another waiter with higher priority becomes the lock
holder, we wake it up early to kill its transaction. Wound-wait is when
we preempt the lock holder. That never happens with the current code.
/Thomas
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