* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > There's far more normal mutex fastpath use during an AIM7 run than
> > any BKL use. So if it's due to any direct fastpath overhead and the
> > resulting widening of the window for the real slowdown, we should
> > see a severe slowdown on AIM7 with CONFIG_MUTEX_DEBUG=y. Agreed?
>
> Not agreed.
>
> The BKL is special because it is a *single* lock.
ok, indeed my suggestion is wrong and this would not be a good
comparison.
another idea: my trial-baloon patch should test your theory too, because
the generic down_trylock() is still the 'fat' version, it does:
spin_lock_irqsave(&sem->lock, flags);
count = sem->count - 1;
if (likely(count >= 0))
sem->count = count;
spin_unlock_irqrestore(&sem->lock, flags);
if there is a noticeable performance difference between your
trial-ballon patch and mine, then the micro-cost of the BKL very much
matters to this workload. Agreed about that?
but i'd be _hugely_ surprised about it. The tty code's BKL use should i
think only happen when a task exits and releases the tty - and a task
exit - even if this is a threaded test (which AIM7 can be - not sure
which exact parameters Yanmin used) - the costs of thread creation and
thread exit are just not in the same ballpark as any BKL micro-costs.
Dunno, maybe i overlooked some high-freq BKL user. (but any such site
would have shown up before) Even assuming a widening of the critical
path and some catastrophic domino effect (that does show up as increased
scheduling) i've never seen a 40% drop like this.
this regression, to me, has "different scheduling behavior" written all
over it - but that's just an impression. I'm not going to bet against
you though ;-)
Ingo
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