Re: [PATCH] sched/arm64: store cpu topology before notify_cpu_starting

[Sudeep Holla]

On Thu, Apr 09, 2020 at 11:32:12AM +0100, Valentin Schneider wrote:
>
> On 09/04/20 10:59, Sudeep Holla wrote:
> > On Wed, Apr 01, 2020 at 02:23:33PM +0100, Valentin Schneider wrote:
> >>
> >> (+LAKML, +Sudeep)
> >>
> >
> > Thanks Valentin.
> >
> >> On Wed, Apr 01 2020, Cheng Jian wrote:
> >> > when SCHED_CORE enabled, sched_cpu_starting() uses thread_sibling as
> >> > SMT_MASK to initialize rq->core, but only after store_cpu_topology(),
> >> > the thread_sibling is ready for use.
> >> >
> >> >       notify_cpu_starting()
> >> >           -> sched_cpu_starting()	# use thread_sibling
> >> >
> >> >       store_cpu_topology(cpu)
> >> >           -> update_siblings_masks	# set thread_sibling
> >> >
> >> > Fix this by doing notify_cpu_starting later, just like x86 do.
> >> >
> >>
> >> I haven't been following the sched core stuff closely; can't this
> >> rq->core assignment be done in sched_cpu_activate() instead? We already
> >> look at the cpu_smt_mask() in there, and it is valid (we go through the
> >> entirety of secondary_start_kernel() before getting anywhere near
> >> CPUHP_AP_ACTIVE).
> >>
> >
> > I too came to same conclusion. Did you see any issues ? Or is it
> > just code inspection in parity with x86 ?
> >
>
> With mainline this isn't a problem; with the core scheduling stuff there is
> an expectation that we can use the SMT masks in sched_cpu_starting().
>

Ah, OK. I prefer this to be specified in the commit message as it is not
obvious.

> >> I don't think this breaks anything, but without this dependency in
> >> sched_cpu_starting() then there isn't really a reason for this move.
> >>
> >
> > Based on the commit message, had a quick look at x86 code and I agree
> > this shouldn't break anything. However the commit message does make
> > complete sense to me, especially reference to sched_cpu_starting
> > while smt_masks are accessed in sched_cpu_activate. Or am I missing
> > to understand something here ?
>
> As stated above, it's not a problem for mainline, and AIUI we can change
> the core scheduling bits to only use the SMT mask in sched_cpu_activate()
> instead, therefore not requiring any change in the arch code.
>

Either way is fine. If it is already set expectation that SMT masks needs
to be set before sched_cpu_starting, then let us just stick with that.

> I'm not aware of any written rule that the topology masks should be usable
> from a given hotplug state upwards, only that right now we need them in
> sched_cpu_(de)activate() for SMT scheduling - and that is already working
> fine.
>

Sure, we can at-least document as part of this change even if it is just
in ARM64 so that someone need not wonder the same in future.

> So really this should be considering as a simple neutral cleanup; I don't
> really have any opinion on picking it up or not.

I am fine with the change too, just need some tweaking in the commit
message.

--
Regards,
Sudeep