On 11/12/2017 17:08, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2017-12-11 16:10:49)
On 09/12/2017 12:47, Chris Wilson wrote:
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
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@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/intel_breadcrumbs.c | 39 ++++++++++++++++++++++++--------
1 file changed, 30 insertions(+), 9 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 24c6fefdd0b1..76e6f8e7cfd4 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -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
+
I kind of remember the on_cpu from before and I was probably complaining
about it. Sigh, if it helps ok..
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;
}
@@ -47,12 +65,15 @@ static unsigned int __intel_breadcrumbs_wakeup(struct intel_breadcrumbs *b)
unsigned int intel_engine_wakeup(struct intel_engine_cs *engine)
{
struct intel_breadcrumbs *b = &engine->breadcrumbs;
- unsigned long flags;
- unsigned int result;
+ unsigned int result = 0;
- spin_lock_irqsave(&b->irq_lock, flags);
- result = __intel_breadcrumbs_wakeup(b);
- spin_unlock_irqrestore(&b->irq_lock, flags);
+ if (READ_ONCE(b->irq_wait)) {
+ unsigned long flags;
+
+ spin_lock_irqsave(&b->irq_lock, flags);
+ result = __intel_breadcrumbs_wakeup(b);
+ spin_unlock_irqrestore(&b->irq_lock, flags);
+ }
This hunk I'd leave out from the fix.
And if I postpone that hunk to tomorrow, would r-b the rest?
Yep.
Regards,
Tvrtko
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