linux-kernel - Re: [PATCH] sched/arm64: store cpu topology before notify_cpu

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Message-ID: <20200409095941.GA25948@bogus>
Date:   Thu, 9 Apr 2020 10:59:41 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Cheng Jian <cj.chengjian@...wei.com>, vpillai@...italocean.com,
        aaron.lwe@...il.com, aubrey.intel@...il.com,
        aubrey.li@...ux.intel.com, fweisbec@...il.com,
        jdesfossez@...italocean.com, joel@...lfernandes.org,
        joelaf@...gle.com, keescook@...omium.org, kerrnel@...gle.com,
        linux-kernel@...r.kernel.org, mgorman@...hsingularity.net,
        mingo@...nel.org, naravamudan@...italocean.com, pauld@...hat.com,
        pawan.kumar.gupta@...ux.intel.com, pbonzini@...hat.com,
        peterz@...radead.org, pjt@...gle.com, tglx@...utronix.de,
        tim.c.chen@...ux.intel.com, torvalds@...ux-foundation.org,
        xiexiuqi@...wei.com, huawei.libin@...wei.com, w.f@...wei.com,
        linux-arm-kernel@...ts.infradead.org,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH] sched/arm64: store cpu topology before
 notify_cpu_starting

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