Quoting Tvrtko Ursulin (2018-02-15 11:59:06)
>
> On 15/02/2018 11:47, Chris Wilson wrote:
> > 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...
>
> Oh yeah.. it was irqsave only because of perf. I am pretty sure you're
> right and I can downgrade write locking to _bh - after the recent
> changes we got a writer in process context (enable/disable), and a
> writer in tasklet context (context in/out).
First look says that the seqlock has lockdep magic to tell us if we are
wrong. (Or at least we need to be creative ;)
-Chris
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