Re: [PATCH] arch/arm64: Fix topology initialization for core scheduling

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Phil Auld <pauld@redhat.com>, linux-kernel@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arch/arm64: Fix topology initialization for core scheduling
Date: Tue, 29 Mar 2022 16:02:22 +0200	[thread overview]
Message-ID: <1a546197-872b-7762-68ac-d5e6bb6d19aa@arm.com> (raw)
In-Reply-To: <20220322160304.26229-1-pauld@redhat.com>

On 22/03/2022 17:03, Phil Auld wrote:
> Some arm64 rely on store_cpu_topology() to setup the real topology.
> This needs to be done before the call to notify_cpu_starting() which
> tell the scheduler about the cpu 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@redhat.com>
> ---
> This is a similar issue to 
>   f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
> which fixed it for MIPS.

I assume this is for a machine which relies on MPIDR-based setup
(package_id == -1)? I.e. it doesn't have proper ACPI/(DT) data for
topology setup.

Tried on a ThunderX2 by disabling parse_acpi_topology() but then I end
up with a machine w/o SMT, so `stress-ng --prctl N` doesn't show this issue.

Which machine were you using?

>  arch/arm64/kernel/smp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 27df5c1e6baa..3b46041f2b97 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -234,6 +234,7 @@ asmlinkage notrace void secondary_start_kernel(void)
>  	 * Log the CPU info before it is marked online and might get read.
>  	 */
>  	cpuinfo_store_cpu();
> +	store_cpu_topology(cpu);
>  
>  	/*
>  	 * Enable GIC and timers.
> @@ -242,7 +243,6 @@ asmlinkage notrace void secondary_start_kernel(void)
>  
>  	ipi_setup(cpu);
>  
> -	store_cpu_topology(cpu);
>  	numa_add_cpu(cpu);
>  
>  	/*