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@xxxxxxxxxxxxxxxxxx>
> > Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> > Cc: Michał Winiarski <michal.winiarski@xxxxxxxxx>
> > Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
> > Reviewed-by: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
> > ---
> > 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,
> > + >->reset.flags)) {
> > + success = intel_engine_reset(engine, NULL) == 0;
> > + clear_and_wake_up_bit(I915_RESET_ENGINE + engine->id,
> > + >->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.
> 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.
-Chris
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