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

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Date:   Mon, 6 Apr 2020 16:00:32 +0800
From:   "chengjian (D)" <cj.chengjian@...wei.com>
To:     Valentin Schneider <valentin.schneider@....com>
CC:     <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>,
        "chengjian (D)" <cj.chengjian@...wei.com>
Subject: Re: [PATCH] sched/arm64: store cpu topology before
 notify_cpu_starting


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