Re: [PATCH] sched/topology: Provide arch_llc_mask for cache aware scheduling

[Srikar Dronamraju]

* Shrikanth Hegde <sshegde@xxxxxxxxxxxxx> [2026-05-29 13:27:12]:

> Venkat Reported a boot kernel panic next-20260522. Git bisect pointed to
> b5ea300a17e3 ("sched/cache: Make LLC id continuous")
> 
> Stacktrace points to llc_mask being null.
> 
> NIP [c000000000e58504] _find_first_bit+0x44/0x130
> LR [c000000000e58500] _find_first_bit+0x40/0x130
> Call Trace:
> build_sched_domains+0xad8/0xe50
> sched_init_smp+0xa8/0x164
> kernel_init_freeable+0x250/0x370
> ret_from_kernel_user_thread+0x14/0x1c
> 
> On powerpc, cpu_coregroup_mask is available only when the underlying
> hardware support coregroup. In shared LPAR, QEMU guest or power9 etc
> coregroup isn't supported. In such cases llc_mask was being referenced
> when it was null leading to panic.
> 
> On powerpc, LLC is at SMT core level. So assumption that coregroup(MC)
> domain point to LLC is wrong. Provide a way for archs to say where its
> LLC is if it not at MC domain. 
> 
> Based on tip/master at 5c89783224e9 ("Merge branch into tip/master: 'x86/tdx'")
> Cc: stable@xxxxxxxxxxxxxxx
> 
> Fixes: b5ea300a17e3 ("sched/cache: Make LLC id continuous")
> Reported-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/51154de7-3700-4cb4-82f2-1b3a8fa427f7@xxxxxxxxxxxxx/
> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> Tested-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxx> 
> Tested-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> Co-developed-by: Chen, Yu C <yu.c.chen@xxxxxxxxx>
> Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> ---
>  arch/powerpc/include/asm/topology.h |  6 ++++++
>  kernel/sched/topology.c             | 13 +++++++++++--
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
> index 66ed5fe1b718..e3de0f3d8b86 100644
> --- a/arch/powerpc/include/asm/topology.h
> +++ b/arch/powerpc/include/asm/topology.h
> @@ -135,6 +135,12 @@ struct cpumask *cpu_coregroup_mask(int cpu);
>  const struct cpumask *cpu_die_mask(int cpu);
>  int cpu_die_id(int cpu);
>  
> +/* Points to where the LLC is. On power9 this will point at CACHE
> + * domain, On others it will point to SMT domain. In all cases
> + * cpu_l2_cache_mask points to where LLC is
> + */

Nit: Regular comment style could have been better.

> +#define arch_llc_mask(cpu)     cpu_l2_cache_mask(cpu)
> +
>  #ifdef CONFIG_PPC64
>  #include <asm/smp.h>
>  
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index df2ceb54c970..622e2e01974c 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2063,12 +2063,21 @@ const struct cpumask *tl_mc_mask(struct sched_domain_topology_level *tl, int cpu
>  	return cpu_coregroup_mask(cpu);
>  }
>  
> -#define llc_mask(cpu) cpu_coregroup_mask(cpu)
> +/*
> + * Majority of architectures have LLC at MC domain level with exception
> + * such as powerpc. Provide a way for arch to specify where its LLC is
> + * if it falls in exception category
> + */
> +# ifndef arch_llc_mask
> +#define arch_llc_mask(cpu) cpu_coregroup_mask(cpu)
> +# endif
>  
>  #else
> -#define llc_mask(cpu) cpumask_of(cpu)
> +#define arch_llc_mask(cpu) cpumask_of(cpu)
>  #endif
>  
> +#define llc_mask(cpu) arch_llc_mask(cpu)
> +

Instead of having another define, could we have modified current users of
llc_mask() to arch_llc_mask()? Again its not a problem, but why have 2
defines since both point to the same thing.

>  const struct cpumask *tl_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
>  {
>  	return cpu_node_mask(cpu);
> -- 
> 2.47.3
> 

Otherwise, looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxx>

-- 
Thanks and Regards
Srikar Dronamraju