linux-kernel - Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

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Message-ID: <e93a7d66-7e43-d2c7-ad85-fb24d50effc5@arm.com>
Date:   Thu, 31 Mar 2022 16:37:50 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Phil Auld <pauld@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] arch/arm64: Fix topology initialization for core
 scheduling

On 31/03/2022 15:21, Phil Auld wrote:
> On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
>> On 30/03/2022 17:56, Phil Auld wrote:

[...]

>> Ah, the reason is that smt_mask is not correctly setup, so we bail on
>> `cpumask_weight(smt_mask) == 1` for !leaders in:
>>
>> notify_cpu_starting()
>>   cpuhp_invoke_callback_range()
>>     sched_cpu_starting()
>>       sched_core_cpu_starting()
>>
>> which leads to rq->core not being correctly set for !leader-rq's.
>>
> 
> Exactly, sorry I was not clearer.  smt_mask must be setup correctly 
> by the time sched_core_cpu_starting() is called. (Maybe I should crib
> some of the above lines into the commit message?)

Yeah, maybe, it wouldn't hurt I guess. IMHO mentioning stress-ng's prctl
needs PR_SCHED_CORE support could also be handy since today's stress-ng
packages don't seem to have this yet.