drm/i915: Only report a wakeup if the waiter was truly asleep (b92326a0) · Commits · 戴 / test

If we attempt to wake up a waiter, who is currently checking the seqno it will be in the TASK_INTERRUPTIBLE state and ttwu will report success. However, it is actually awake and functioning -- so delay reporting the actual wake up until it sleeps. This fixes some spurious claims of missed_breadcrumbs when running under heavy load; i.e. sufficient load to preempt away the newly woken waiter before they complete their checks. However, it does so at the cost of a rare false negative; where the waiter changes between the check and ttwu -- the only way to fix that would be to extend the reporting from ttwu where the check could be done atomically. v2: Defend against !CONFIG_SMP v3: Don't filter out calls to wake_up_process v4: Drop risky microoptimisation to skip wakeups Testcase: igt/drv_missed_irq # sanity check we do detect missed_breadcrumb() Testcase: igt/gem_concurrent_blit # for generating false positives References: https://bugs.freedesktop.org/show_bug.cgi?id=100007 Signed-off-by:

Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20171209124710.1606-1-chris@chris-wilson.co.uk Reviewed-by:

Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/intel_breadcrumbs.c

+22 −4

Original line number	Diff line number	Diff line
		@@ -27,6 +27,12 @@

		#include "i915_drv.h"

		#ifdef CONFIG_SMP
		#define task_asleep(tsk) ((tsk)->state & TASK_NORMAL && !(tsk)->on_cpu)
		#else
		#define task_asleep(tsk) ((tsk)->state & TASK_NORMAL)
		#endif

		static unsigned int __intel_breadcrumbs_wakeup(struct intel_breadcrumbs *b)
		{
		struct intel_wait *wait;
		@@ -36,8 +42,20 @@ static unsigned int __intel_breadcrumbs_wakeup(struct intel_breadcrumbs *b)

		wait = b->irq_wait;
		if (wait) {
		/*
		* N.B. Since task_asleep() and ttwu are not atomic, the
		* waiter may actually go to sleep after the check, causing
		* us to suppress a valid wakeup. We prefer to reduce the
		* number of false positive missed_breadcrumb() warnings
		* at the expense of a few false negatives, as it it easy
		* to trigger a false positive under heavy load. Enough
		* signal should remain from genuine missed_breadcrumb()
		* for us to detect in CI.
		*/
		bool was_asleep = task_asleep(wait->tsk);

		result = ENGINE_WAKEUP_WAITER;
		if (wake_up_process(wait->tsk))
		if (wake_up_process(wait->tsk) && was_asleep)
		result \|= ENGINE_WAKEUP_ASLEEP;
		}

		@@ -77,8 +95,8 @@ static noinline void missed_breadcrumb(struct intel_engine_cs *engine)

		static void intel_breadcrumbs_hangcheck(struct timer_list *t)
		{
		struct intel_engine_cs *engine = from_timer(engine, t,
		breadcrumbs.hangcheck);
		struct intel_engine_cs *engine =
		from_timer(engine, t, breadcrumbs.hangcheck);
		struct intel_breadcrumbs *b = &engine->breadcrumbs;

		if (!b->irq_armed)
		@@ -104,7 +122,7 @@ static void intel_breadcrumbs_hangcheck(struct timer_list *t)
		*/
		if (intel_engine_wakeup(engine) & ENGINE_WAKEUP_ASLEEP) {
		missed_breadcrumb(engine);
		mod_timer(&engine->breadcrumbs.fake_irq, jiffies + 1);
		mod_timer(&b->fake_irq, jiffies + 1);
		} else {
		mod_timer(&b->hangcheck, wait_timeout());
		}

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