Re: [RFC PATCH v3 2/3] sched: Introduce cpus_share_l2c

[Aaron Lu]

On Mon, Aug 28, 2023 at 07:19:45PM +0800, Aaron Lu wrote:
> On Fri, Aug 25, 2023 at 09:51:19AM -0400, Mathieu Desnoyers wrote:
> > On 8/25/23 02:49, Aaron Lu wrote:
> > > On Thu, Aug 24, 2023 at 10:40:45AM -0400, Mathieu Desnoyers wrote:
> > [...]
> > > > > - task migrations dropped with this series for nr_group=20 and 32
> > > > >     according to 'perf stat'. migration number didn't drop for nr_group=10
> > > > >     but the two update functions' cost dropped which means fewer access to
> > > > >     tg->load_avg and thus, fewer task migrations. This is contradictory
> > > > >     and I can not explain yet;
> > > > 
> > > > Neither can I.
> > > > 
> > 
> > [...]
> > 
> > > > 
> > > > > It's not clear to me why this series can reduce task migrations. I doubt
> > > > > it has something to do with more wakelist style wakeup becasue for this
> > > > > test machine, only a single core with two SMT threads share L2 so more
> > > > > wakeups are through wakelist. In wakelist style wakeup, the target rq's
> > > > > ttwu_pending is set and that will make the target cpu as !idle_cpu();
> > > > > This is faster than grabbing the target rq's lock and then increase
> > > > > target rq's nr_running or set target rq's curr to something else than
> > > > > idle. So wakelist style wakeup can make target cpu appear as non idle
> > > > > faster, but I can't connect this with reduced migration yet, I just feel
> > > > > this might be the reason why task migration reduced.
> > > > 
> > > 
> > [...]
> > > > I've tried adding checks for rq->ttwu_pending in those code paths on top of
> > > > my patch and I'm still observing the reduction in number of migrations, so
> > > > it's unclear to me how doing more queued wakeups can reduce migrations the
> > > > way it does.
> > > 
> > > An interesting puzzle.
> > 
> > One metric that can help understand the impact of my patch: comparing
> > hackbench from a baseline where only your load_avg patch is applied
> > to a kernel with my l2c patch applied, I notice that the goidle
> > schedstat is cut in half. For a given CPU (they are pretty much alike),
> > it goes from 650456 to 353487.
> > 
> > So could it be that by doing queued wakeups, we end up batching
> > execution of the woken up tasks for a given CPU, rather than going
> > back and forth between idle and non-idle ? One important thing that
> > this changes is to reduce the number of newidle balance triggered.
> 
> I noticed the majority(>99%) migrations are from wakeup path on this
> Intel SPR when running hackbench: ttwu() -> set_task_cpu() ->
> migrate_task_rq_fair(), so while I think it's a good finding that
> newidle balance dropped, it's probably not the reason why migration
> number dropped...

I profiled select_idle_sibling() and found that with this series,
select_idle_cpu() tends to fail more and select_idle_sibling() fallbacks
to use target in the end, which equals to prev_cpu very often.

Initially I think the reason why select_idle_cpu() failed more with this
series is because "wake_list style enqueue" can make the target cpu appear
as busy earlier and thus, it will be harder for select_idle_cpu() to
find an idle cpu overall. But I also suspect SIS_UTIL makes a difference
here: in vanilla kernel, the idle% is 8% and with this series, the idle%
is only 2% and SIS_UTIL may simply skip doing any search for idle cpu.
Anyway, I think I'll also need to profile select_idle_cpu() to see
what's going on there too.

The above profile was done with below workload on a 2 sockets Intel SPR:
hackbench -g 20 -f 20 --pipe --threads -l 480000 -s 100

Thanks,
Aaron