On 26/06/2015 14:34, Chris Wilson wrote:
On Fri, Jun 26, 2015 at 01:58:11PM +0100, John.C.Harrison@xxxxxxxxx wrote:
From: John Harrison <John.C.Harrison@xxxxxxxxx>
The intended usage model for struct fence is that the signalled status should be
set on demand rather than polled. That is, there should not be a need for a
'signaled' function to be called everytime the status is queried. Instead,
'something' should be done to enable a signal callback from the hardware which
will update the state directly. In the case of requests, this is the seqno
update interrupt. The idea is that this callback will only be enabled on demand
when something actually tries to wait on the fence.
This change removes the polling test and replaces it with the callback scheme.
Each fence is added to a 'please poke me' list at the start of
i915_add_request(). The interrupt handler then scans through the 'poke me' list
when a new seqno pops out and signals any matching fence/request. The fence is
then removed from the list so the entire request stack does not need to be
scanned every time. Note that the fence is added to the list before the commands
to generate the seqno interrupt are added to the ring. Thus the sequence is
guaranteed to be race free if the interrupt is already enabled.
One complication here is that the 'poke me' system requires holding a reference
count on the request to guarantee that it won't be freed prematurely.
Unfortunately, it is unsafe to decrement the reference count from the interrupt
handler because if that is the last reference, the clean up code gets run and
the clean up code is not IRQ friendly. Hence, the request is added to a 'please
clean me' list that gets processed at retire time. Any request in this list
simply has its count decremented and is then removed from that list.
Note that the interrupt is only enabled on demand (i.e. when __wait_request() is
called). Thus there is still a potential race when enabling the interrupt as the
request may already have completed. However, this is simply solved by calling
the interrupt processing code immediately after enabling the interrupt and
thereby checking for already completed requests.
Lastly, the ring clean up code has the possibility to cancel outstanding
requests (e.g. because TDR has reset the ring). These requests will never get
signalled and so must be removed from the signal list manually. This is done by
setting a 'cancelled' flag and then calling the regular notify/retire code path
rather than attempting to duplicate the list manipulatation and clean up code in
multiple places. This also avoid any race condition where the cancellation
request might occur after/during the completion interrupt actually arriving.
-nightly nop:
Time to exec x 1: 15.000µs (ring=render)
Time to exec x 1: 2.000µs (ring=blt)
Time to exec x 131072: 1.827µs (ring=render)
Time to exec x 131072: 1.555µs (ring=blt)
rq tuning patches nop:
Time to exec x 1: 12.200µs (ring=render)
Time to exec x 1: 1.600µs (ring=blt)
Time to exec x 131072: 1.516µs (ring=render)
Time to exec x 131072: 0.812µs (ring=blt)
interrupt driven nop:
Time to exec x 1: 19.200µs (ring=render)
Time to exec x 1: 5.200µs (ring=blt)
Time to exec x 131072: 2.381µs (ring=render)
Time to exec x 131072: 2.009µs (ring=blt)
So the basic question that is left unanswered from last time is why
would we want to slow down __i915_wait_request? And enabling IRQs still
generates very high system load when processing the 30-40k IRQs per
second found under some workloads.
-Chris
As previously stated, the scheduler requires enabling interrupts for
each batch buffer as it needs to know when something more needs sending
to the hardware. Android requires enabling interrupts for each batch
buffer as it uses the sync framework to wait on batch buffer completion
asynchronously to the driver (i.e. without calling __i915_wait_request
or any other driver code). I presume much of the slow down to
wait_request itself is because it has to check for missed interrupts. It
should be possible to optimise that somewhat although it was completely
unnecessary in the original version as you can't miss interrupts if they
are always on.
How do you get consistent results from gem_exec_nop? For the x1 case, I
see random variation from one run to the next of the order of 10us ->
over 100us. And that is with a straight nightly build.
John.
_______________________________________________
Intel-gfx mailing list
Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
http://lists.freedesktop.org/mailman/listinfo/intel-gfx
