Re: [PATCH] drm/i915: Only report a wakeup if the waiter was truly asleep

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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@chris-wilson.co.uk>
>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>>> ---
>>>    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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