Quoting Mika Kuoppala (2018-05-24 15:40:47)
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
>
> > We were not very carefully checking to see if an older request on the
> > engine was an earlier switch-to-kernel-context before deciding to emit a
> > new switch. The end result would be that we could get into a permanent
> > loop of trying to emit a new request to perform the switch simply to
> > flush the existing switch.
> >
> > What we need is a means of tracking the completion of each timeline
> > versus the kernel context, that is to detect if a more recent request
> > has been submitted that would result in a switch away from the kernel
> > context. To realise this, we need only to look in our syncmap on the
> > kernel context and check that we have synchronized against all active
> > rings.
> >
> > v2: Since all ringbuffer clients currently share the same timeline, we do
> > have to use the gem_context to distinguish clients.
> >
> > As a bonus, include all the tracing used to debug the death inside
> > suspend.
> >
> > v3: Test, test, test. Construct a selftest to exercise and assert the
> > expected behaviour that multiple switch-to-contexts do not emit
> > redundant requests.
> >
> > Reported-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
> > Fixes: a89d1f921c15 ("drm/i915: Split i915_gem_timeline into individual timelines")
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/i915_gem.c | 7 +
> > drivers/gpu/drm/i915/i915_gem.h | 3 +
> > drivers/gpu/drm/i915/i915_gem_context.c | 86 +++++++++--
> > drivers/gpu/drm/i915/i915_request.c | 5 +-
> > .../gpu/drm/i915/selftests/i915_gem_context.c | 144 ++++++++++++++++++
> > .../drm/i915/selftests/i915_mock_selftests.h | 1 +
> > 6 files changed, 231 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 03874b50ada9..05f44ca35a06 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -3703,6 +3703,9 @@ static int wait_for_engines(struct drm_i915_private *i915)
> >
> > int i915_gem_wait_for_idle(struct drm_i915_private *i915, unsigned int flags)
> > {
> > + GEM_TRACE("flags=%x (%s)\n",
> > + flags, flags & I915_WAIT_LOCKED ? "locked" : "unlocked");
> > +
> > /* If the device is asleep, we have no requests outstanding */
> > if (!READ_ONCE(i915->gt.awake))
> > return 0;
> > @@ -3719,6 +3722,7 @@ int i915_gem_wait_for_idle(struct drm_i915_private *i915, unsigned int flags)
> > return err;
> > }
> > i915_retire_requests(i915);
> > + GEM_BUG_ON(i915->gt.active_requests);
> >
> > return wait_for_engines(i915);
> > } else {
> > @@ -4901,6 +4905,7 @@ static void assert_kernel_context_is_current(struct drm_i915_private *i915)
> > struct intel_engine_cs *engine;
> > enum intel_engine_id id;
> >
> > + GEM_BUG_ON(i915->gt.active_requests);
> > for_each_engine(engine, i915, id) {
> > GEM_BUG_ON(__i915_gem_active_peek(&engine->timeline.last_request));
> > GEM_BUG_ON(engine->last_retired_context->gem_context != kctx);
> > @@ -4932,6 +4937,8 @@ int i915_gem_suspend(struct drm_i915_private *dev_priv)
> > struct drm_device *dev = &dev_priv->drm;
> > int ret;
> >
> > + GEM_TRACE("\n");
> > +
> > intel_runtime_pm_get(dev_priv);
> > intel_suspend_gt_powersave(dev_priv);
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.h b/drivers/gpu/drm/i915/i915_gem.h
> > index 5bf24cfc218c..62ee4e385365 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.h
> > +++ b/drivers/gpu/drm/i915/i915_gem.h
> > @@ -63,9 +63,12 @@ struct drm_i915_private;
> > #if IS_ENABLED(CONFIG_DRM_I915_TRACE_GEM)
> > #define GEM_TRACE(...) trace_printk(__VA_ARGS__)
> > #define GEM_TRACE_DUMP() ftrace_dump(DUMP_ALL)
> > +#define GEM_TRACE_DUMP_ON(expr) \
> > + do { if (expr) ftrace_dump(DUMP_ALL); } while (0)
> > #else
> > #define GEM_TRACE(...) do { } while (0)
> > #define GEM_TRACE_DUMP() do { } while (0)
> > +#define GEM_TRACE_DUMP_ON(expr) BUILD_BUG_ON_INVALID(expr)
> > #endif
> >
> > #define I915_NUM_ENGINES 8
> > diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> > index b69b18ef8120..45393f6e0208 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> > @@ -576,30 +576,72 @@ last_request_on_engine(struct i915_timeline *timeline,
> > {
> > struct i915_request *rq;
> >
> > - if (timeline == &engine->timeline)
> > - return NULL;
> > + GEM_BUG_ON(timeline == &engine->timeline);
> >
> > rq = i915_gem_active_raw(&timeline->last_request,
> > &engine->i915->drm.struct_mutex);
> > - if (rq && rq->engine == engine)
> > + if (rq && rq->engine == engine) {
> > + GEM_TRACE("last request for %s on engine %s: %llx:%d\n",
> > + timeline->name, engine->name,
> > + rq->fence.context, rq->fence.seqno);
> > + GEM_BUG_ON(rq->timeline != timeline);
> > return rq;
> > + }
> >
> > return NULL;
> > }
> >
> > -static bool engine_has_idle_kernel_context(struct intel_engine_cs *engine)
> > +static bool engine_has_kernel_context_barrier(struct intel_engine_cs *engine)
> > {
> > - struct list_head * const active_rings = &engine->i915->gt.active_rings;
> > + struct drm_i915_private *i915 = engine->i915;
> > + const struct intel_context * const ce =
> > + to_intel_context(i915->kernel_context, engine);
> > + struct i915_timeline *barrier = ce->ring->timeline;
> > struct intel_ring *ring;
> > + bool any_active = false;
> >
> > - lockdep_assert_held(&engine->i915->drm.struct_mutex);
> > + lockdep_assert_held(&i915->drm.struct_mutex);
> > + list_for_each_entry(ring, &i915->gt.active_rings, active_link) {
> > + struct i915_request *rq;
> > +
> > + rq = last_request_on_engine(ring->timeline, engine);
> > + if (!rq)
> > + continue;
> >
> > - list_for_each_entry(ring, active_rings, active_link) {
> > - if (last_request_on_engine(ring->timeline, engine))
> > + any_active = true;
> > +
> > + if (rq->gem_context == i915->kernel_context)
> > + continue;
> > +
> > + /*
> > + * Was this request submitted after the previous
> > + * switch-to-kernel-context?
> > + */
> > + if (!i915_timeline_sync_is_later(barrier, &rq->fence)) {
> > + GEM_TRACE("%s needs barrier for %llx:%d\n",
> > + ring->timeline->name,
> > + rq->fence.context,
> > + rq->fence.seqno);
> > return false;
> > + }
> > +
> > + GEM_TRACE("%s has barrier after %llx:%d\n",
> > + ring->timeline->name,
> > + rq->fence.context,
> > + rq->fence.seqno);
> > }
> >
> > - return intel_engine_has_kernel_context(engine);
> > + /*
> > + * If any other timeline was still active and behind the last barrier,
> > + * then our last switch-to-kernel-context must still be queued and
> > + * will run last (leaving the engine in the kernel context when it
> > + * eventually idles).
> > + */
> > + if (any_active)
> > + return true;
> > +
> > + /* The engine is idle; check that it is idling in the kernel context. */
> > + return engine->last_retired_context == ce;
> > }
> >
> > int i915_gem_switch_to_kernel_context(struct drm_i915_private *i915)
> > @@ -607,7 +649,10 @@ int i915_gem_switch_to_kernel_context(struct drm_i915_private *i915)
> > struct intel_engine_cs *engine;
> > enum intel_engine_id id;
> >
> > + GEM_TRACE("\n");
> > +
> > lockdep_assert_held(&i915->drm.struct_mutex);
> > + GEM_BUG_ON(!i915->kernel_context);
> >
> > i915_retire_requests(i915);
> >
> > @@ -615,9 +660,12 @@ int i915_gem_switch_to_kernel_context(struct drm_i915_private *i915)
> > struct intel_ring *ring;
> > struct i915_request *rq;
> >
> > - if (engine_has_idle_kernel_context(engine))
> > + GEM_BUG_ON(!to_intel_context(i915->kernel_context, engine));
> > + if (engine_has_kernel_context_barrier(engine))
> > continue;
> >
> > + GEM_TRACE("emit barrier on %s\n", engine->name);
> > +
> > rq = i915_request_alloc(engine, i915->kernel_context);
> > if (IS_ERR(rq))
> > return PTR_ERR(rq);
> > @@ -627,10 +675,20 @@ int i915_gem_switch_to_kernel_context(struct drm_i915_private *i915)
> > struct i915_request *prev;
> >
> > prev = last_request_on_engine(ring->timeline, engine);
> > - if (prev)
> > - i915_sw_fence_await_sw_fence_gfp(&rq->submit,
> > - &prev->submit,
> > - I915_FENCE_GFP);
> > + if (!prev)
> > + continue;
> > +
> > + if (prev->gem_context == i915->kernel_context)
> > + continue;
> > +
> > + GEM_TRACE("add barrier on %s for %llx:%d\n",
> > + engine->name,
> > + prev->fence.context,
> > + prev->fence.seqno);
> > + i915_sw_fence_await_sw_fence_gfp(&rq->submit,
> > + &prev->submit,
> > + I915_FENCE_GFP);
> > + i915_timeline_sync_set(rq->timeline, &prev->fence);
> > }
> >
> > /*
> > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > index fc499bcbd105..f187250e60c6 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -320,6 +320,7 @@ static void advance_ring(struct i915_request *request)
> > * is just about to be. Either works, if we miss the last two
> > * noops - they are safe to be replayed on a reset.
> > */
> > + GEM_TRACE("marking %s as inactive\n", ring->timeline->name);
> > tail = READ_ONCE(request->tail);
> > list_del(&ring->active_link);
> > } else {
> > @@ -1095,8 +1096,10 @@ void __i915_request_add(struct i915_request *request, bool flush_caches)
> > i915_gem_active_set(&timeline->last_request, request);
> >
> > list_add_tail(&request->ring_link, &ring->request_list);
> > - if (list_is_first(&request->ring_link, &ring->request_list))
> > + if (list_is_first(&request->ring_link, &ring->request_list)) {
> > + GEM_TRACE("marking %s as active\n", ring->timeline->name);
> > list_add(&ring->active_link, &request->i915->gt.active_rings);
> > + }
> > request->emitted_jiffies = jiffies;
> >
> > /*
> > diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_context.c b/drivers/gpu/drm/i915/selftests/i915_gem_context.c
> > index ddb03f009232..98a33a8fb046 100644
> > --- a/drivers/gpu/drm/i915/selftests/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/selftests/i915_gem_context.c
> > @@ -26,6 +26,7 @@
> > #include "igt_flush_test.h"
> >
> > #include "mock_drm.h"
> > +#include "mock_gem_device.h"
> > #include "huge_gem_object.h"
> >
> > #define DW_PER_PAGE (PAGE_SIZE / sizeof(u32))
> > @@ -420,6 +421,130 @@ static int igt_ctx_exec(void *arg)
> > return err;
> > }
> >
> > +static const char *__engine_name(struct drm_i915_private *i915,
> > + unsigned int engines)
> > +{
> > + struct intel_engine_cs *engine;
> > + unsigned int tmp;
> > +
> > + if (engines == ALL_ENGINES)
> > + return "all";
> > +
> > + for_each_engine_masked(engine, i915, engines, tmp)
> > + return engine->name;
> > +
> > + return "none";
> > +}
> > +
> > +static int __igt_switch_to_kernel_context(struct drm_i915_private *i915,
> > + struct i915_gem_context *ctx,
> > + unsigned int engines)
> > +{
> > + struct intel_engine_cs *engine;
> > + unsigned int tmp;
> > + int err;
> > +
> > + GEM_TRACE("Testing %s\n", __engine_name(i915, engines));
> > + for_each_engine_masked(engine, i915, engines, tmp) {
> > + struct i915_request *rq;
> > +
> > + rq = i915_request_alloc(engine, ctx);
> > + if (IS_ERR(rq))
> > + return PTR_ERR(rq);
> > +
> > + i915_request_add(rq);
> > + }
> > +
> > + err = i915_gem_switch_to_kernel_context(i915);
> > + if (err)
> > + return err;
> > +
> > + for_each_engine_masked(engine, i915, engines, tmp) {
> > + if (!engine_has_kernel_context_barrier(engine)) {
> > + pr_err("kernel context not last on engine %s!\n",
> > + engine->name);
> > + return -EINVAL;
> > + }
> > + }
> > +
> > + err = i915_gem_wait_for_idle(i915, I915_WAIT_LOCKED);
> > + if (err)
> > + return err;
> > +
> > + GEM_BUG_ON(i915->gt.active_requests);
> > + for_each_engine_masked(engine, i915, engines, tmp) {
> > + if (engine->last_retired_context->gem_context != i915->kernel_context) {
> > + pr_err("engine %s not idling in kernel context!\n",
> > + engine->name);
> > + return -EINVAL;
> > + }
> > + }
> > +
> > + err = i915_gem_switch_to_kernel_context(i915);
> > + if (err)
> > + return err;
> > +
> > + if (i915->gt.active_requests) {
> > + pr_err("switch-to-kernel-context emitted %d requests even though it should already be idling in the kernel context\n",
> > + i915->gt.active_requests);
> > + return -EINVAL;
> > + }
> > +
> > + for_each_engine_masked(engine, i915, engines, tmp) {
> > + if (!intel_engine_has_kernel_context(engine)) {
> > + pr_err("kernel context not last on engine %s!\n",
> > + engine->name);
> > + return -EINVAL;
> > + }
> > + }
> > +
> > + return 0;
> > +}
> > +
> > +static int igt_switch_to_kernel_context(void *arg)
> > +{
> > + struct drm_i915_private *i915 = arg;
> > + struct intel_engine_cs *engine;
> > + struct i915_gem_context *ctx;
> > + enum intel_engine_id id;
> > + int err;
> > +
> > + /*
> > + * A core premise of switching to the kernel context is that
> > + * if an engine is already idling in the kernel context, we
> > + * do not emit another request and wake it up. The other being
> > + * that we do indeed end up idling in the kernel context.
> > + */
> > +
> > + mutex_lock(&i915->drm.struct_mutex);
>
> As discussed in irc,
>
> Add comment or rename the ctx here and in __igt_switch_to_kernel_context
> so that the reader knows that this mock kernel context is
> used as convenient way to inject test requests.
>
> Reviewed-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
>
>
> > + ctx = kernel_context(i915);
I can see how that might look confusing. Will try to address the issue
with the factory names.
Thank you for your patience in finding the bug, suggesting a fix and in
reviewing the aftermath. Pushed, for tomorrow we need to fix suspend!
-Chris
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