Re: [RFC PATCH v7] sched/fair: select idle cpu from idle cpumask for task wakeup

Peter Zijlstra
2020-12-11T23:14:10Z

On Fri, Dec 11, 2020 at 08:43:37PM +0000, Mel Gorman wrote:
> One bug is in __select_idle_core() though. It's scanning the SMT mask,
> not select_idle_mask so it can return an idle candidate that is not in
> p->cpus_ptr.

D'0h.. luckily the benchmarks don't hit that :-)

> There are some other potential caveats.
>
> This is a single pass so when test_idle_cores() is true, __select_idle_core
> is used to to check all the siblings even if the core is not idle. That
> could have been cut short if __select_idle_core checked *idle_cpu ==
> 1 and terminated the SMT scan if an idle candidate had already been found.

So I did that on purpose, so as to track the last/most-recent idle cpu,
with the thinking that that cpu has the higher chance of still being
idle vs one we checked earlier/longer-ago.

I suppose we benchmark both and see which is liked best.

> Second downside is related. If test_idle_cpus() was true but no idle
> CPU is found then __select_idle_core has been called enough to scan
> the entire domain. In this corner case, the new code does *more* work
> because the old code would have failed select_idle_core() quickly and
> then select_idle_cpu() would be throttled by SIS_PROP. I think this will
> only be noticable in the heavily overloaded case but if the corner case
> hits enough then the new code will be slower than the old code for the
> over-saturated case (i.e. hackbench with lots of groups).

Right, due to scanning siblings, even if the first inspected thread is
not idle, we scan more.

> The third potential downside is that the SMT sibling is not guaranteed to
> be checked due to SIS_PROP throttling but in the old code, that would have
> been checked by select_idle_smt(). That might result in premature stacking
> of runnable tasks on the same CPU. Similarly, as __select_idle_core may
> find multiple idle candidates, it will not pick the targets SMT sibling
> if it is idle like select_idle_smt would have.
>
> That said, I am skeptical that select_idle_smt() matters all that often.

This, I didn't really believe in it either.

The benchmarks I started are mostly noise, with a few wins for
TCP_STREAM and UDP_RR around the 50% mark. Although I should run
longer variants to make sure.