[Intel-gfx] [PATCH 13/13] drm/i915: Cache last IRQ seqno to reduce IRQ overhead

[John Harrison]

On 11/12/2015 14:28, Tvrtko Ursulin wrote:
> On 11/12/15 13:12, John.C.Harrison at Intel.com wrote:
>> From: John Harrison <John.C.Harrison at Intel.com>
>>
>> The notify function can be called many times without the seqno
>> changing. A large number of duplicates are to prevent races due to the
>> requirement of not enabling interrupts until requested. However, when
>> interrupts are enabled the IRQ handle can be called multiple times
>> without the ring's seqno value changing. This patch reduces the
>> overhead of these extra calls by caching the last processed seqno
>> value and early exiting if it has not changed.
>>
>> v3: New patch for series.
>>
>> For: VIZ-5190
>> Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
>> ---
>>   drivers/gpu/drm/i915/i915_gem.c         | 14 +++++++++++---
>>   drivers/gpu/drm/i915/intel_ringbuffer.h |  1 +
>>   2 files changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem.c 
>> b/drivers/gpu/drm/i915/i915_gem.c
>> index 279d79f..3c88678 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.c
>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>> @@ -2457,6 +2457,8 @@ i915_gem_init_seqno(struct drm_device *dev, u32 
>> seqno)
>>
>>           for (j = 0; j < ARRAY_SIZE(ring->semaphore.sync_seqno); j++)
>>               ring->semaphore.sync_seqno[j] = 0;
>> +
>> +        ring->last_irq_seqno = 0;
>>       }
>>
>>       return 0;
>> @@ -2788,11 +2790,14 @@ void i915_gem_request_notify(struct 
>> intel_engine_cs *ring, bool fence_locked)
>>           return;
>>       }
>>
>> -    if (!fence_locked)
>> -        spin_lock_irqsave(&ring->fence_lock, flags);
>> -
>>       seqno = ring->get_seqno(ring, false);
>>       trace_i915_gem_request_notify(ring, seqno);
>> +    if (seqno == ring->last_irq_seqno)
>> +        return;
>> +    ring->last_irq_seqno = seqno;
>
> Hmmm.. do you want to make the check "seqno <= ring->last_irq_seqno" ?
>
> Is there a possibility for some weird timing or caching issue where 
> two callers get in and last_irq_seqno goes backwards? Not sure that it 
> would cause a problem, but pattern is unusual and hard to understand 
> for me.
The check is simply to prevent repeat processing of identical seqno 
values. The 'last_' value is never used for anything more complicated. 
If there is a very rare race condition where the repeat processing can 
still happen, it doesn't really matter too much.

> Also check and the assignment would need to be under the spinlock I 
> think.

The whole point is to not grab the spinlock if there is no work to do. 
Hence the seqno read and test must be done first. The assignment could 
potentially be done after the lock but if two different threads have 
made it that far concurrently then it doesn't really matter who does the 
write first. Most likely they are both processing the same seqno and in 
the really rare case of two concurrent threads actually reading two 
different (and both new) seqno values then there is no guarantee about 
which will take the lock first. So you are into the above situation of 
it doesn't really matter if there is then a third time around later that 
finds an 'incorrect' last value and goes through the processing sequence 
but with no work to do.


>> +
>> +    if (!fence_locked)
>> +        spin_lock_irqsave(&ring->fence_lock, flags);
>>
>>       list_for_each_entry_safe(req, req_next, 
>> &ring->fence_signal_list, signal_link) {
>>           if (!req->cancelled) {
>> @@ -3163,7 +3168,10 @@ static void i915_gem_reset_ring_cleanup(struct 
>> drm_i915_private *dev_priv,
>>        * Tidy up anything left over. This includes a call to
>>        * i915_gem_request_notify() which will make sure that any 
>> requests
>>        * that were on the signal pending list get also cleaned up.
>> +     * NB: The seqno cache must be cleared otherwise the notify call 
>> will
>> +     * simply return immediately.
>>        */
>> +    ring->last_irq_seqno = 0;
>>       i915_gem_retire_requests_ring(ring);
>>
>>       /* Having flushed all requests from all queues, we know that all
>> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h 
>> b/drivers/gpu/drm/i915/intel_ringbuffer.h
>> index 9d09edb..1987abd 100644
>> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
>> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
>> @@ -356,6 +356,7 @@ struct  intel_engine_cs {
>>       spinlock_t fence_lock;
>>       struct list_head fence_signal_list;
>>       struct list_head fence_unsignal_list;
>> +    uint32_t last_irq_seqno;
>>   };
>>
>>   bool intel_ring_initialized(struct intel_engine_cs *ring);
>>
>
> Regards,
>
> Tvrtko