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Fri, 26 Sep 2025 13:51:24 +0000 Message-ID: <1ec61212-24d5-4147-898a-f93c03c6ac94@intel.com> Date: Fri, 26 Sep 2025 21:51:09 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v4 08/28] sched: Set up LLC indexing To: Adam Li CC: Vincent Guittot , Juri Lelli , Dietmar Eggemann , "Steven Rostedt" , Ben Segall , Mel Gorman , Valentin Schneider , Libo Chen , Madadi Vineeth Reddy , "Hillf Danton" , Shrikanth Hegde , "Jianyong Wu" , Yangyu Chen , Tingyin Duan , Vern Hao , Len Brown , Tim Chen , Aubrey Li , Zhao Liu , Chen Yu , , Peter Zijlstra , Ingo Molnar , K Prateek Nayak , "Gautham R . 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So *no* schedule domain > has flag 'SD_SHARE_LLC'. > Good catch! And many thanks for your detailed testing and analysis. Is this issue triggered with CONFIG_SCHED_CLUSTER disabled? > With this topology: > per_cpu(sd_llc_id, cpu) is actually the cpu id (0-191). > > And kernel bug will be triggered at: > 'BUG_ON(idx > MAX_LLC)' > Yes, the sd_llc_idx thing is a bit tricky - we want to use it to index into the static array struct sg_lb_stat.nr_pref_llc, and we have to limit its range. A better approach would be to dynamically allocate the buffer, so we could get rid of the 'idx > MAX_LLC' check, but that might complicate the code. > Please see details bellow. > > The bug will disappear if setting 'MAX_LLC' to 192. > But I think we might disable CAS(cache aware scheduling) > if no domain has 'SD_SHARE_LLC'. > I agree with you. Simply disabling cache-aware scheduling if there is no SD_SHARE_LLC would be simpler. > On 8/9/2025 1:03 PM, Chen Yu wrote: > A draft patch like bellow can fix the kernel BUG: > 1) Do not call update_llc_idx() if domain has no SD_SHARE_LLC > 2) Disable CAS if domain has no SD_SHARE_LLC > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c > index 8483c02b4d28..cde9b6cdb1de 100644 > --- a/kernel/sched/topology.c > +++ b/kernel/sched/topology.c > @@ -704,7 +704,8 @@ static void update_top_cache_domain(int cpu) > per_cpu(sd_llc_size, cpu) = size; > per_cpu(sd_llc_id, cpu) = id; > rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds); > - update_llc_idx(cpu); > + if (sd) > + update_llc_idx(cpu); > OK, that make sense. > sd = lowest_flag_domain(cpu, SD_CLUSTER); > if (sd) > @@ -2476,6 +2477,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att > int i, ret = -ENOMEM; > bool has_asym = false; > bool has_cluster = false; > + bool has_llc = false; > bool llc_has_parent_sd = false; > unsigned int multi_llcs_node = 1; > > @@ -2621,6 +2623,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att > > if (lowest_flag_domain(i, SD_CLUSTER)) > has_cluster = true; > + > + if (highest_flag_domain(i, SD_SHARE_LLC)) > + has_llc = true; > } > rcu_read_unlock(); > > @@ -2631,7 +2636,8 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att > static_branch_inc_cpuslocked(&sched_cluster_active); > > #ifdef CONFIG_SCHED_CACHE > - if (llc_has_parent_sd && multi_llcs_node && !sched_asym_cpucap_active()) > + if (has_llc && llc_has_parent_sd && multi_llcs_node && multi_llcs_node will be false if there is no SD_SHARE_LLC domain on the platform, so I suppose we don’t have to introduce has_llc? multi_llcs is set to true iff there are more than 1 SD_SHARE_LLC domains under its SD_SHARE_LLC parent domain. thanks, Chenyu > + !sched_asym_cpucap_active()) > static_branch_inc_cpuslocked(&sched_cache_present); > #endif > > > Thanks, > -adam