Messages in this thread | | | Date | Thu, 9 Apr 2020 12:08:43 +0100 | From | Sudeep Holla <> | Subject | Re: [PATCH] sched/arm64: store cpu topology before notify_cpu_starting |
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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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