Re: [PATCH 12/15] drm/i915/gem: Cancel non-persistent contexts on close

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 14/10/2019 14:34, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-10-14 14:10:30)
>>
>> On 14/10/2019 13:21, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2019-10-14 13:11:46)
>>>>
>>>> On 14/10/2019 10:07, Chris Wilson wrote:
>>>>> Normally, we rely on our hangcheck to prevent persistent batches from
>>>>> hogging the GPU. However, if the user disables hangcheck, this mechanism
>>>>> breaks down. Despite our insistence that this is unsafe, the users are
>>>>> equally insistent that they want to use endless batches and will disable
>>>>> the hangcheck mechanism. We are looking at perhaps replacing hangcheck
>>>>> with a softer mechanism, that sends a pulse down the engine to check if
>>>>> it is well. We can use the same preemptive pulse to flush an active
>>>>> persistent context off the GPU upon context close, preventing resources
>>>>> being lost and unkillable requests remaining on the GPU after process
>>>>> termination. To avoid changing the ABI and accidentally breaking
>>>>> existing userspace, we make the persistence of a context explicit and
>>>>> enable it by default (matching current ABI). Userspace can opt out of
>>>>> persistent mode (forcing requests to be cancelled when the context is
>>>>> closed by process termination or explicitly) by a context parameter. To
>>>>> facilitate existing use-cases of disabling hangcheck, if the modparam is
>>>>> disabled (i915.enable_hangcheck=0), we disable persistence mode by
>>>>> default.  (Note, one of the outcomes for supporting endless mode will be
>>>>> the removal of hangchecking, at which point opting into persistent mode
>>>>> will be mandatory, or maybe the default perhaps controlled by cgroups.)
>>>>>
>>>>> v2: Check for hangchecking at context termination, so that we are not
>>>>> left with undying contexts from a crafty user.
>>>>> v3: Force context termination even if forced-preemption is disabled.
>>>>>
>>>>> Testcase: igt/gem_ctx_persistence
>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>>>>> Cc: Michał Winiarski <michal.winiarski@intel.com>
>>>>> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
>>>>> Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/gem/i915_gem_context.c   | 182 ++++++++++++++++++
>>>>>     drivers/gpu/drm/i915/gem/i915_gem_context.h   |  15 ++
>>>>>     .../gpu/drm/i915/gem/i915_gem_context_types.h |   1 +
>>>>>     .../gpu/drm/i915/gem/selftests/mock_context.c |   2 +
>>>>>     include/uapi/drm/i915_drm.h                   |  15 ++
>>>>>     5 files changed, 215 insertions(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>>> index 5d8221c7ba83..70b72456e2c4 100644
>>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>>> @@ -70,6 +70,7 @@
>>>>>     #include <drm/i915_drm.h>
>>>>>     
>>>>>     #include "gt/intel_lrc_reg.h"
>>>>> +#include "gt/intel_engine_heartbeat.h"
>>>>>     #include "gt/intel_engine_user.h"
>>>>>     
>>>>>     #include "i915_gem_context.h"
>>>>> @@ -269,6 +270,128 @@ void i915_gem_context_release(struct kref *ref)
>>>>>                 schedule_work(&gc->free_work);
>>>>>     }
>>>>>     
>>>>> +static inline struct i915_gem_engines *
>>>>> +__context_engines_static(const struct i915_gem_context *ctx)
>>>>> +{
>>>>> +     return rcu_dereference_protected(ctx->engines, true);
>>>>> +}
>>>>> +
>>>>> +static bool __reset_engine(struct intel_engine_cs *engine)
>>>>> +{
>>>>> +     struct intel_gt *gt = engine->gt;
>>>>> +     bool success = false;
>>>>> +
>>>>> +     if (!intel_has_reset_engine(gt))
>>>>> +             return false;
>>>>> +
>>>>> +     if (!test_and_set_bit(I915_RESET_ENGINE + engine->id,
>>>>> +                           &gt->reset.flags)) {
>>>>> +             success = intel_engine_reset(engine, NULL) == 0;
>>>>> +             clear_and_wake_up_bit(I915_RESET_ENGINE + engine->id,
>>>>> +                                   &gt->reset.flags);
>>>>> +     }
>>>>> +
>>>>> +     return success;
>>>>> +}
>>>>> +
>>>>> +static void __reset_context(struct i915_gem_context *ctx,
>>>>> +                         struct intel_engine_cs *engine)
>>>>> +{
>>>>> +     intel_gt_handle_error(engine->gt, engine->mask, 0,
>>>>> +                           "context closure in %s", ctx->name);
>>>>> +}
>>>>> +
>>>>> +static bool __cancel_engine(struct intel_engine_cs *engine)
>>>>> +{
>>>>> +     /*
>>>>> +      * Send a "high priority pulse" down the engine to cause the
>>>>> +      * current request to be momentarily preempted. (If it fails to
>>>>> +      * be preempted, it will be reset). As we have marked our context
>>>>> +      * as banned, any incomplete request, including any running, will
>>>>> +      * be skipped following the preemption.
>>>>> +      */
>>>>> +     if (CONFIG_DRM_I915_PREEMPT_TIMEOUT && !intel_engine_pulse(engine))
>>>>> +             return true;
>>>>
>>>> Maybe I lost the train of thought here.. But why not even try with the
>>>> pulse even if forced preemption is not compiled in? There is a chance it
>>>> may preempt normally, no?
>>>
>>> If there is no reset-on-preemption failure and no hangchecking, there is
>>> no reset and we are left with the denial-of-service that we are seeking
>>> to close.
>>
>> Because there is no mechanism to send a pulse, see if it managed to
>> preempt, but if it did not come come back later and reset?
> 
> What are you going to preempt with? The mechanism you describe is what
> the pulse + forced-preempt is meant to be handling. (I was going to use
> a 2 birds with one stone allegory for the various features all pulling
> together, but it's more like a flock with a grenade.)

I meant try to preempt with idle pulse first. If it doesn't let go then 
go for a reset. It has as chance of working even without forced 
preemption, no?

>>>> Hm, or from the other angle, why bother with preemption and not just
>>>> reset? What is the value in letting the closed context complete if at
>>>> the same time, if it is preemptable, we will cancel all outstanding work
>>>> anyway?
>>>
>>> The reset is the elephant gun; it is likely to cause collateral damage.
>>> So we try with a bit of finesse first.
>>
>> How so? Isn't our per-engine reset supposed to be fast and reliable? But
>> yes, I have no complaints of trying preemption first, just trying to
>> connect all the dots.
> 
> Fast and reliable! Even if were, we still the challenge of ensuring we
> reset the right context. But in terms of being fast and reliable, we
> have actually talked about using this type of preempt mechanism to avoid
> taking a risk with the reset! :)

:) Ok, makes sense. I did not look into how reset victims are picked in 
detail. So just a question if we want to try to send an idle pulse first 
in any case.

Regards,

Tvrtko
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