On 10/01/2023 15:55, Matthew Brost wrote:
On Tue, Jan 10, 2023 at 12:19:35PM +0000, Tvrtko Ursulin wrote:
On 10/01/2023 11:28, Tvrtko Ursulin wrote:
On 09/01/2023 17:27, Jason Ekstrand wrote:
[snip]
>>> AFAICT it proposes to have 1:1 between *userspace* created
contexts (per
>>> context _and_ engine) and drm_sched. I am not sure avoiding
invasive changes
>>> to the shared code is in the spirit of the overall idea and
instead
>>> opportunity should be used to look at way to refactor/improve
drm_sched.
Maybe? I'm not convinced that what Xe is doing is an abuse at all
or really needs to drive a re-factor. (More on that later.)
There's only one real issue which is that it fires off potentially a
lot of kthreads. Even that's not that bad given that kthreads are
pretty light and you're not likely to have more kthreads than
userspace threads which are much heavier. Not ideal, but not the
end of the world either. Definitely something we can/should
optimize but if we went through with Xe without this patch, it would
probably be mostly ok.
>> Yes, it is 1:1 *userspace* engines and drm_sched.
>>
>> I'm not really prepared to make large changes to DRM scheduler
at the
>> moment for Xe as they are not really required nor does Boris
seem they
>> will be required for his work either. I am interested to see
what Boris
>> comes up with.
>>
>>> Even on the low level, the idea to replace drm_sched threads
with workers
>>> has a few problems.
>>>
>>> To start with, the pattern of:
>>>
>>> while (not_stopped) {
>>> keep picking jobs
>>> }
>>>
>>> Feels fundamentally in disagreement with workers (while
obviously fits
>>> perfectly with the current kthread design).
>>
>> The while loop breaks and worker exists if no jobs are ready.
I'm not very familiar with workqueues. What are you saying would fit
better? One scheduling job per work item rather than one big work
item which handles all available jobs?
Yes and no, it indeed IMO does not fit to have a work item which is
potentially unbound in runtime. But it is a bit moot conceptual mismatch
because it is a worst case / theoretical, and I think due more
fundamental concerns.
If we have to go back to the low level side of things, I've picked this
random spot to consolidate what I have already mentioned and perhaps
expand.
To start with, let me pull out some thoughts from workqueue.rst:
"""
Generally, work items are not expected to hog a CPU and consume many
cycles. That means maintaining just enough concurrency to prevent work
processing from stalling should be optimal.
"""
For unbound queues:
"""
The responsibility of regulating concurrency level is on the users.
"""
Given the unbound queues will be spawned on demand to service all queued
work items (more interesting when mixing up with the system_unbound_wq),
in the proposed design the number of instantiated worker threads does
not correspond to the number of user threads (as you have elsewhere
stated), but pessimistically to the number of active user contexts. That
is the number which drives the maximum number of not-runnable jobs that
can become runnable at once, and hence spawn that many work items, and
in turn unbound worker threads.
Several problems there.
It is fundamentally pointless to have potentially that many more threads
than the number of CPU cores - it simply creates a scheduling storm.
To make matters worse, if I follow the code correctly, all these per user
context worker thread / work items end up contending on the same lock or
circular buffer, both are one instance per GPU:
guc_engine_run_job
-> submit_engine
a) wq_item_append
-> wq_wait_for_space
-> msleep
a) is dedicated per xe_engine
Hah true, what its for then? I thought throttling the LRCA ring is done via:
drm_sched_init(&ge->sched, &drm_sched_ops,
e->lrc[0].ring.size / MAX_JOB_SIZE_BYTES,
Is there something more to throttle other than the ring? It is
throttling something using msleeps..
Also you missed the step of programming the ring which is dedicated per xe_engine
I was trying to quickly find places which serialize on something in the
backend, ringbuffer emission did not seem to do that but maybe I missed
something.
b) xe_guc_ct_send
-> guc_ct_send
-> mutex_lock(&ct->lock);
-> later a potential msleep in h2g_has_room
Techincally there is 1 instance per GT not GPU, yes this is shared but
in practice there will always be space in the CT channel so contention
on the lock should be rare.
Yeah I used the term GPU to be more understandable to outside audience.
I am somewhat disappointed that the Xe opportunity hasn't been used to
improve upon the CT communication bottlenecks. I mean those backoff
sleeps and lock contention. I wish there would be a single thread in
charge of the CT channel and internal users (other parts of the driver)
would be able to send their requests to it in a more efficient manner,
with less lock contention and centralized backoff.
I haven't read your rather long reply yet, but also FWIW using a
workqueue has suggested by AMD (original authors of the DRM scheduler)
when we ran this design by them.
Commit message says nothing about that. ;)
Regards,
Tvrtko
