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

[Phil Auld <pauld@redhat.com>]

On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
> 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@redhat.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.
> 

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?)


> LGTM. Tested on: HPE Apollo 70 X1
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> 

Thanks!



Cheers,
Phil


> [...]
> 

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