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

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Date:   Thu, 31 Mar 2022 11:04:31 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org
Cc:     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 30/03/2022 17:56, Phil Auld wrote:
> Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
> to transfer the toplogy to the various cpu masks. This needs to be done 
> before the call to notify_cpu_starting() which tells the scheduler about 
> each cpu found, otherwise the core scheduling data structures are setup 
> in a way that does not match the actual topology.
> 
> Without this change stress-ng (which enables core scheduling in its prctl 
> tests) causes a warning and then a crash (trimmed for legibility):
> 
> [ 1853.805168] ------------[ cut here ]------------
> [ 1853.809784] task_rq(b)->core != rq->core
> [ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
> ...
> [ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> ...
> [ 1854.231256] Call trace:
> [ 1854.233689]  pick_next_task+0x3dc/0x81c
> [ 1854.237512]  __schedule+0x10c/0x4cc
> [ 1854.240988]  schedule_idle+0x34/0x54
> 
> Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
> Signed-off-by: Phil Auld <pauld@...hat.com>
> ---
> This is a similar issue to 
>   f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
> which fixed it for MIPS. 
> 
> v2: Fixed the commit message. No code change.

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.

LGTM. Tested on: HPE Apollo 70 X1

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@....com>

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