Quoting Tvrtko Ursulin (2018-02-15 11:13:33)
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
>
> We can convert engine stats from a spinlock to seqlock to ensure interrupt
> processing is never even a tiny bit delayed by parallel readers.
>
> There is a smidgen bit more cost on the write lock side, and an extremely
> unlikely chance that readers will have to retry a few times in face of
> heavy interrupt load.Bbut it should be extremely unlikely given how
> lightweight read side section is compared to the interrupt processing side,
> and also compared to the rest of the code paths which can lead into it.
>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Suggested-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> ---
> @@ -2028,12 +2028,13 @@ static ktime_t __intel_engine_get_busy_time(struct intel_engine_cs *engine)
> */
> ktime_t intel_engine_get_busy_time(struct intel_engine_cs *engine)
> {
> + unsigned int seq;
> ktime_t total;
> - unsigned long flags;
>
> - spin_lock_irqsave(&engine->stats.lock, flags);
> - total = __intel_engine_get_busy_time(engine);
> - spin_unlock_irqrestore(&engine->stats.lock, flags);
> + do {
> + seq = read_seqbegin(&engine->stats.lock);
> + total = __intel_engine_get_busy_time(engine);
> + } while (read_seqretry(&engine->stats.lock, seq));
Hmm, only the reader is inside a hard-irq context right? (Thanks perf!)
I was hoping we could avoid disabling irqs around the writer. Now that
requires some coffee...
-Chris
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