From: Srikar Dronamraju <srikar@linux•ibm.com>
To: Shrikanth Hegde <sshegde@linux•ibm.com>
Cc: maddy@linux•ibm.com, linuxppc-dev@lists•ozlabs.org,
peterz@infradead•org, mingo@kernel•org,
christophe.leroy@csgroup•eu, linux-kernel@vger•kernel.org,
venkat88@linux•ibm.com, yu.c.chen@intel•com,
tim.c.chen@linux•intel.com, kprateek.nayak@amd•com,
riteshh@linux•ibm.com, stable@vger•kernel.org,
"Ritesh Harjani (IBM)" <ritesh.list@gmail•com>
Subject: Re: [PATCH] sched/topology: Provide arch_llc_mask for cache aware scheduling
Date: Fri, 29 May 2026 15:45:18 +0530 [thread overview]
Message-ID: <ahlnNsaQH-zg5tV2@linux.ibm.com> (raw)
In-Reply-To: <20260529075712.1181039-1-sshegde@linux.ibm.com>
* Shrikanth Hegde <sshegde@linux•ibm.com> [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@vger•kernel.org
>
> Fixes: b5ea300a17e3 ("sched/cache: Make LLC id continuous")
> Reported-by: Venkat Rao Bagalkote <venkat88@linux•ibm.com>
> Closes: https://lore.kernel.org/all/51154de7-3700-4cb4-82f2-1b3a8fa427f7@linux.ibm.com/
> Reviewed-by: Chen Yu <yu.c.chen@intel•com>
> Tested-by: Venkat Rao Bagalkote <venkat88@linux•ibm.com>
> Tested-by: Ritesh Harjani (IBM) <ritesh.list@gmail•com>
> Co-developed-by: Chen, Yu C <yu.c.chen@intel•com>
> Signed-off-by: Shrikanth Hegde <sshegde@linux•ibm.com>
> ---
> 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@linux•ibm.com>
--
Thanks and Regards
Srikar Dronamraju
prev parent reply other threads:[~2026-05-29 10:15 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-29 7:57 [PATCH] sched/topology: Provide arch_llc_mask for cache aware scheduling Shrikanth Hegde
2026-05-29 9:17 ` Peter Zijlstra
2026-05-29 10:15 ` Srikar Dronamraju [this message]
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