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Tue, 16 Dec 2025 06:12:30 +0000 Message-ID: <7e4640a2-f79f-4f14-b099-d97bfd842b37@intel.com> Date: Tue, 16 Dec 2025 14:12:11 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/23] sched/cache: Introduce infrastructure for cache-aware load balancing To: Vern Hao CC: Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Madadi Vineeth Reddy , "Hillf Danton" , Shrikanth Hegde , "Jianyong Wu" , Yangyu Chen , Tingyin Duan , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Aaron Lu , Tim Chen , , Peter Zijlstra , Ingo Molnar , K Prateek Nayak , Vincent Guittot , "Gautham R . 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This reduces cache misses and improves >> overall data access efficiency. >> >> In this initial implementation, threads belonging to the same process >> are treated as entities that likely share working sets. The mechanism >> tracks per-process CPU occupancy across cache domains and attempts to >> migrate threads toward cache-hot domains where their process already >> has active threads, thereby enhancing locality. >> >> This provides a basic model for cache affinity. While the current code >> targets the last-level cache (LLC), the approach could be extended to >> other domain types such as clusters (L2) or node-internal groupings. >> >> At present, the mechanism selects the CPU within an LLC that has the >> highest recent runtime. Subsequent patches in this series will use this >> information in the load-balancing path to guide task placement toward >> preferred LLCs. >> >> In the future, more advanced policies could be integrated through NUMA >> balancing-for example, migrating a task to its preferred LLC when spare >> capacity exists, or swapping tasks across LLCs to improve cache affinity. >> Grouping of tasks could also be generalized from that of a process >> to be that of a NUMA group, or be user configurable. >> >> Originally-by: Peter Zijlstra (Intel) >> Signed-off-by: Chen Yu >> Signed-off-by: Tim Chen >> --- >> >> Notes: >>      v1->v2: >>         Restore the original CPU scan to cover all online CPUs, >>         rather than scanning within the preferred NUMA node. >>         (Peter Zijlstra) >>         Use rq->curr instead of rq->donor. (K Prateek Nayak) >>         Minor fix in task_tick_cache() to use >>         if (mm->mm_sched_epoch >= rq->cpu_epoch) >>         to avoid mm_sched_epoch going backwards. >> >>   include/linux/mm_types.h |  44 +++++++ >>   include/linux/sched.h    |  11 ++ >>   init/Kconfig             |  11 ++ >>   kernel/fork.c            |   6 + >>   kernel/sched/core.c      |   6 + >>   kernel/sched/fair.c      | 258 +++++++++++++++++++++++++++++++++++++++ >>   kernel/sched/sched.h     |   8 ++ >>   7 files changed, 344 insertions(+) >> >> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h >> index 90e5790c318f..1ea16ef90566 100644 >> --- a/include/linux/mm_types.h >> +++ b/include/linux/mm_types.h >> @@ -939,6 +939,11 @@ typedef struct { >>       DECLARE_BITMAP(__mm_flags, NUM_MM_FLAG_BITS); >>   } __private mm_flags_t; >> +struct mm_sched { >> +    u64 runtime; >> +    unsigned long epoch; >> +}; >> + >>   struct kioctx_table; >>   struct iommu_mm_data; >>   struct mm_struct { >> @@ -1029,6 +1034,17 @@ struct mm_struct { >>            */ >>           raw_spinlock_t cpus_allowed_lock; >>   #endif >> +#ifdef CONFIG_SCHED_CACHE >> +        /* >> +         * Track per-cpu-per-process occupancy as a proxy for cache >> residency. >> +         * See account_mm_sched() and ... >> +         */ >> +        struct mm_sched __percpu *pcpu_sched; >> +        raw_spinlock_t mm_sched_lock; >> +        unsigned long mm_sched_epoch; >> +        int mm_sched_cpu; > As we discussed earlier，I continue to believe that dedicating > 'mm_sched_cpu' to handle the aggregated hotspots of all threads is > inappropriate, as the multiple threads lack a necessary correlation in > our real application. > > So, I was wondering if we could put this variable into struct > task_struct, That allows us to better monitor the hotspot CPU of each > thread, despite some details needing consideration. > I suppose you are suggesting a fine-grained control for a set of tasks. Process-scope aggregation could be a start as the default strategy( conservative, benefit multi-thread workloads that share data per process, not introduce regression). On top of that, I wonder if we could provide task-scope control like sched_setattr(), similar to core-scheduling cookie mechanism, for users that want aggressive aggregation. But before doing that, we need a mechanism that that leverages a monitor system(like PMU) to figure out if putting these tasks together would bring benefit(if I understand Steven's suggestion correctly on LPC), or detection tasks that share resource, then maybe leverage QOS interfaces to enable the cache-aware aggregation(something Qias mentioned on the LPC). thanks, Chenyu