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

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

[Valentin Schneider]

(+LAKML, +Sudeep)

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 don't think this breaks anything, but without this dependency in
sched_cpu_starting() then there isn't really a reason for this move.

> Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
> ---
>  arch/arm64/kernel/smp.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 5407bf5d98ac..a427c14e82af 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -236,13 +236,18 @@ asmlinkage notrace void secondary_start_kernel(void)
>       cpuinfo_store_cpu();
>
>       /*
> -	 * Enable GIC and timers.
> +	 * Store cpu topology before notify_cpu_starting,
> +	 * CPUHP_AP_SCHED_STARTING requires SMT topology
> +	 * been initialized for SCHED_CORE.
>        */
> -	notify_cpu_starting(cpu);
> -
>       store_cpu_topology(cpu);
>       numa_add_cpu(cpu);
>
> +	/*
> +	 * Enable GIC and timers.
> +	 */
> +	notify_cpu_starting(cpu);
> +
>       /*
>        * OK, now it's safe to let the boot CPU continue.  Wait for
>        * the CPU migration code to notice that the CPU is online

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

[chengjian (D)]

On 2020/4/1 21:23, Valentin Schneider wrote:
> (+LAKML, +Sudeep)
>
> 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 don't think this breaks anything, but without this dependency in
> sched_cpu_starting() then there isn't really a reason for this move.

Yes, it is correct to put the rq-> core assignment in sched_cpu_active().

The cpu_smt_mask is already valid here.


I have made such an attempt on my own branch and passed the test.


Thank you.


     -- Cheng Jian



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

[Sudeep Holla]

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 ?

> 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 ?

--
Regards,
Sudeep

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

[Valentin Schneider]

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().

>> 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.

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.

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

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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

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