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

[Tvrtko Ursulin]

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 at chris-wilson.co.uk>
>>> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
>>> Cc: Michał Winiarski <michal.winiarski at intel.com>
>>> Cc: Jon Bloomfield <jon.bloomfield at intel.com>
>>> Reviewed-by: Jon Bloomfield <jon.bloomfield at 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?

>> 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.

Regards,

Tvrtko