[Intel-gfx] [PATCH v2] drm/i915/gt: update request engine before removing virtual GuC engine
Andrzej Hajda
andrzej.hajda at intel.com
Wed Jul 19 10:41:10 UTC 2023
On 18.07.2023 17:48, Tvrtko Ursulin wrote:
>
> On 17/07/2023 19:03, John Harrison wrote:
>> On 7/13/2023 05:11, Tvrtko Ursulin wrote:
>>> On 13/07/2023 12:09, Andrzej Hajda wrote:
>>>> Hi,
>>>>
>>>> On 13.07.2023 09:39, Tvrtko Ursulin wrote:
>>>>> On 12/07/2023 19:54, John Harrison wrote:
>>>>>> On 7/12/2023 09:27, Andrzej Hajda wrote:
>>>>>>> On 12.07.2023 14:35, Tvrtko Ursulin wrote:
>>>>>>>> On 12/07/2023 13:18, Andrzej Hajda wrote:
>>>>>>>>> On 11.07.2023 17:27, Tvrtko Ursulin wrote:
>>>>>>>>>> On 11/07/2023 14:58, Andrzej Hajda wrote:
>>>>>>>>>>> On 11.07.2023 13:34, Andi Shyti wrote:
>>>>>>>>>>>> Hi Andrzej,
>>>>>>>>>>>>
>>>>>>>>>>>>> drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 11
>>>>>>>>>>>>> +++++++++++
>>>>>>>>>>>>> 1 file changed, 11 insertions(+)
>>>>>>>>>>>>>
>>>>>>>>>>>>> diff --git
>>>>>>>>>>>>> a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
>>>>>>>>>>>>> b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
>>>>>>>>>>>>> index a0e3ef1c65d246..2c877ea5eda6f0 100644
>>>>>>>>>>>>> ---
>>>>>>>>>>>>> a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
>>>>>>>>>>>>> +++
>>>>>>>>>>>>> b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
>>>>>>>>>>>>> @@ -3461,6 +3461,8 @@ static void
>>>>>>>>>>>>> guc_prio_fini(struct i915_request *rq, struct
>>>>>>>>>>>>> intel_context *ce)
>>>>>>>>>>>>> static void remove_from_context(struct
>>>>>>>>>>>>> i915_request *rq)
>>>>>>>>>>>>> {
>>>>>>>>>>>>> struct intel_context *ce =
>>>>>>>>>>>>> request_to_scheduling_context(rq);
>>>>>>>>>>>>> + struct intel_engine_cs *engine;
>>>>>>>>>>>>> + intel_engine_mask_t tmp;
>>>>>>>>>>>>>
>>>>>>>>>>>>> GEM_BUG_ON(intel_context_is_child(ce));
>>>>>>>>>>>>>
>>>>>>>>>>>>> @@ -3478,6 +3480,15 @@ static void
>>>>>>>>>>>>> remove_from_context(struct i915_request *rq)
>>>>>>>>>>>>>
>>>>>>>>>>>>> atomic_dec(&ce->guc_id.ref);
>>>>>>>>>>>>> i915_request_notify_execute_cb_imm(rq);
>>>>>>>>>>>>> +
>>>>>>>>>>>>> + /*
>>>>>>>>>>>>> + * GuC virtual engine can disappear after
>>>>>>>>>>>>> this call, so let's assign
>>>>>>>>>>>>> + * something valid, as driver expects
>>>>>>>>>>>>> this to be always valid pointer.
>>>>>>>>>>>>> + */
>>>>>>>>>>>>> + for_each_engine_masked(engine, rq->engine->gt,
>>>>>>>>>>>>> rq->execution_mask, tmp) {
>>>>>>>>>>>>> + rq->engine = engine;
>>>>>>>>>>>>>
>>>>>>>>>>>>> yes... here the context might lose the virtual
>>>>>>>>>>>>> engine... I wonder
>>>>>>>>>>>>> whether this is the rigth solution, though. Maybe we
>>>>>>>>>>>>> should set
>>>>>>>>>>>>> rq->engine = NULL; and check for NULL? Don't know.
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Setting NULL causes occasional null page de-reference in
>>>>>>>>>>>>>
>>>>>>>>>>>>> i915_request_wait_timeout:
>>>>>>>>>>>>>
>>>>>>>>>>>>> mutex_release(&rq->engine->gt->reset.mutex.dep_map,
>>>>>>>>>>>>> _THIS_IP_)
>>>>>>>>>>>>>
>>>>>>>>>>>>> rq->engine after removing rq from context is (IMHO) used
>>>>>>>>>>>>> as a set of aliases
>>>>>>>>>>>>> for gt and i915 (despite rq itself contains the alias to
>>>>>>>>>>>>> i915).
>>>>>>>>>>>> without investigating further, but maybe that code is not even
>>>>>>>>>>>> supposed to be executed, at this point, if the request's
>>>>>>>>>>>> assigned
>>>>>>>>>>>> virtual engine is removed.
>>>>>>>>>>>
>>>>>>>>>>> Real tests show it is executed and the function
>>>>>>>>>>> i915_request_wait_timeout is quite generic
>>>>>>>>>>> I guess it is quite typical use-case, the only question is
>>>>>>>>>>> about timings - what happens earlier -
>>>>>>>>>>> finalization of i915_request_wait_timeout or context removal.
>>>>>>>>>>>
>>>>>>>>>>> The other point rq->engine is accessed after context removal
>>>>>>>>>>> is i915_fence_release -
>>>>>>>>>>> there is long comment there regarding virtual context and
>>>>>>>>>>> reuse retired rq.
>>>>>>>>>>> Anyway calling there "intel_engine_is_virtual(rq->engine)"
>>>>>>>>>>> is risky without this patch and KASAN complains clearly
>>>>>>>>>>> about it:
>>>>>>>>>>> http://gfx-ci.igk.intel.com/tree/drm-tip/kasan.html?testfilter=gem_exec_balancer
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Looks like a bug introduced in bcb9aa45d5a0 ("Revert
>>>>>>>>>> "drm/i915: Hold reference to intel_context over life of
>>>>>>>>>> i915_request""), which was a partial revert of 1e98d8c52ed5
>>>>>>>>>> ("drm/i915: Hold reference to intel_context over life of
>>>>>>>>>> i915_request").
>>>>>>>>>>
>>>>>>>>>> Ie. if 1e98d8c52ed5 recognised the problem with disappearing
>>>>>>>>>> rq->engine, then I am confused how bcb9aa45d5a0 left the
>>>>>>>>>> rq->engine dereference in there after removing the extra
>>>>>>>>>> reference.
>>>>>>>>>>
>>>>>>>>>> Could it be that the intel_engine_is_virtual check simply
>>>>>>>>>> needs to be removed from i915_fence_release, restoring things
>>>>>>>>>> to how they were before 1e98d8c52ed5? Could you try it out?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I have already tried something similar [1] and KASAN bugs
>>>>>>>>> disappeared, or more precisely gem_exec_balance tests passed.
>>>>>>>>> But I have been warned by Nirmoy guc virtual engines can be
>>>>>>>>> created for only one real engine (ie.
>>>>>>>>> is_power_of_2(rq->execution_mask) is true but rq->engine
>>>>>>>>> points to virtual engine).
>>>>>>>>>
>>>>>>>>> [1]: https://patchwork.freedesktop.org/series/118879/
>>>>>>>>
>>>>>>>> Ugh.. Try involving media umd folks to see if they need a
>>>>>>>> single engine virtual engine? Or we could always just not
>>>>>>>> create it in the driver, I mean just use the physical one.
>>>>>>>
>>>>>>>
>>>>>>> In case there is single physical engine
>>>>>>> intel_engine_create_virtual falls back to intel_context_create
>>>>>>> (no virtual engine), but in case of parallel contexts there is
>>>>>>> special KMD flag FORCE_VIRTUAL which enforces virtual engine
>>>>>>> even for single physical engine. So it seems to be KMD concept.
>>>>>>>
>>>>>>> Anyway is it worth investigating how to make
>>>>>>> "is_power_of_2(rq->execution_mask)" indication of dangling
>>>>>>> engine pointer? It will not help in 1st case:
>>>>>>> mutex_release(&rq->engine->gt->reset.mutex.dep_map, _THIS_IP_)
>>>>>>>
>>>>>>>
>>>>>> There seems to be a fundamental problem here. Object 1 (rq) is
>>>>>> holding a pointer to a reference counted and transient object 2
>>>>>> (engine) but without taking a reference count for itself. That is
>>>>>> a Bad Thing(tm).
>>>>
>>>> Engine is not ref counted (at least directly), hardware engines
>>>> seems to be even persistent across whole life of i915.
>>>> I guess before introduction of virtual engines the assumption about
>>>> validity of rq->engine was correct and developers used it to access
>>>> rq->engine->gt, rq->engine->i915, etc
>>>> So the problems described here are probably leftovers of this change.
>>>> After virtual engines were introduced
>>>> "is_power_of_2(rq->execution_mask)" was used to detect rq->engine
>>>> is virtual.
>>>> And after adding parallel engines it does not seem to be valid
>>>> check anymore.
>>>>
>>>>>> I'm not following the description in the revert patch as to why
>>>>>> rq can't ref count the context/engine. Is there actually a
>>>>>> recursive counting problem? Or is it just a lifetime issue caused
>>>>>> by requests hanging around indefinitely because they are locked
>>>>>> by a user process?
>>>>>>
>>>>>> Either way, jumping through convoluted hoops to ensure the code
>>>>>> does not attempt to dereference a dangling pointer seems like the
>>>>>> wrong fix. Removing the engine pointer when the request is
>>>>>> completed and no longer dependent upon an engine (but before the
>>>>>> engine can possibly be destroyed) seems like a much better
>>>>>> solution. And then making the request handling code check for and
>>>>>> cope with a null engine pointer. It sounds like the only problem
>>>>>> there is the above mutex, but there is an alternate route to
>>>>>> that? Although why a completed request would need access to a GT
>>>>>> reset mutex seems confusing. If the request is done, then what
>>>>>> connection does it still have to the GT?
>>>>>
>>>>> Agreed in principle but the question is how invasive would it be
>>>>> to change the rules.
>>>>>
>>>>> With the latest info that the issue is really just the GuC
>>>>> _parallel_ engine setup, and looking at the code, I wonder if we
>>>>> couldn't just flag the rq->flags with "kernel context request".
>>>>> The code in i915_fence_release claims the rq pool is only relevant
>>>>> for those so it sounds it would be safe to skip everything else
>>>>> based on that new flag.
>>>>>
>>>>> For the mutex_release path, presumable the bad deref is only
>>>>> _after_ the wait, right? (Only once the request has been retired.)
>>>>>
>>>>> In which case caching the gt pointer at the start of
>>>>> i915_request_wait_timeout would be sufficient.
>>>>>
>>>>> That should be a few lines fixup overall and then the idea of
>>>>> allowing rq->engine to be reset to NULL can be explored more
>>>>> leisurely.
>>>>
>>>> I guess:
>>>> - setting engine to NULL in remove_from_context,
>>>> - caching gt pointer,
>>>> - checking for null pointer in i915_fence_release
>>>>
>>>> should be enough to solve current issue. However I am not sure if
>>>> there are no more dragons hidden in other execution paths. I will
>>>> try inspect the code.
>>>
>>> What you have in this patch, cheat by replacing the engine, *might*
>>> work for the short term *if* you can make sure all parallel readers
>>> will see the updated rq->engine pointer in time, before the old one
>>> gets freed.
>>>
>>> For the longer term solution - maybe making the engine reference
>>> counted?
>> That was the point of the original solution - having the request
>> reference count the context. The context is what owns the virtual
>> engine. So ensuring that the context can't be destroyed while there
>> are requests outstanding on it ensured that rq->engine would always
>> be valid. Nice simple and clean solution.So why did that get
>> reverted? What is the problem with VM_BIND and having requests ensure
>> that their resources stay alive for the lifetime of the request?
>
> Don't ask me, but it perhaps it does read a bit vague from the commit
> message on its own:
>
> commit bcb9aa45d5a0e11ef91245330c53cde214d15e8d (tag: intel/CI_DIGN_563)
> Author: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
> Date: Wed Jun 15 00:13:47 2022 +0530
>
> Revert "drm/i915: Hold reference to intel_context over life of
> i915_request"
> This reverts commit 1e98d8c52ed5dfbaf273c4423c636525c2ce59e7.
> The problem with this patch is that it makes i915_request to
> hold a
> reference to intel_context, which in turn holds a reference on the
> VM.
> This strong back referencing can lead to reference loops which leads
> to resource leak.
> An example is the upcoming VM_BIND work which requires VM to hold
> a reference to some shared VM specific BO. But this BO's dma-resv
> fences holds reference to the i915_request thus leading to reference
> loop.
> v2:
> Do not use reserved requests for virtual engines
>
> So I don't know what was the leak or was it permanent leak (?!) or not.
>
> Given VM_BIND has been abandoned maybe this could even be unreverted.
> I don't see a problem with holding a reference for the request
> lifetime right now but could be wrong..
unrevert or alternatively hold reference to context only in case of
virtual engines, as in this case their lifetime is the same?
Regards
Andrzej
>
> Regards,
>
> Tvrtko
>
>> John.
>>
>>
>>>
>>> Or if setting rq->engine to NULL then evaluating the paths which
>>> derefence it. AFAIR these request retirement races have been
>>> generally tricky/annoying.
>>>
>>> For instance under the i915_request_wait_timeout chain.
>>>
>>> 1)
>>> __i915_spin_request - could be racy if request gets retired between
>>> i915_request_wait_timeout/dma_fence_is_signaled check and
>>> __i915_spin_request dereferencing rq->engine.props?
>>>
>>> 2)
>>> intel_rps_boost - claims to be safe by serialising via
>>> i915_request_retire/I915_FENCE_FLAG_BOOST but is it? What prevents
>>> retire tearing down the engine between:
>>>
>>> if (!test_and_set_bit(I915_FENCE_FLAG_BOOST, &rq->fence.flags)) {
>>>
>>> ---> HERE - if whole retirement happens here <----
>>>
>>> struct intel_rps *rps = &READ_ONCE(rq->engine)->gt->rps;
>>>
>>> 3)
>>> __intel_engine_flush_submission - could be racy to? What if the
>>> whole process of consuming the request by the backend and retirement
>>> happens between these two lines:
>>>
>>> if (i915_request_is_ready(rq))
>>>
>>> ---> HERE <---
>>> __intel_engine_flush_submission(rq->engine, false);
>>>
>>> Then "is ready" can be true, but by the 2nd line request retired and
>>> rq->engine freed/NULL already.
>>>
>>> Lets hope I am just making unwarranted panic by being to away from
>>> this area of the driver for a while. :) But if I am not, then it
>>> could be that rq->engine is just overall too problematic and perhaps
>>> we should have a look into making it reference counted by the request.
>>>
>>>> Btw there is rq->i915 but code only uses "rq->engine->i915" which
>>>> way shall we go? remove former or latter?
>>>
>>> Simpler/shorter option should be better.
>>>
>>> Regards,
>>>
>>> Tvrtko
>>
More information about the Intel-gfx
mailing list