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Thu, 2 Oct 2025 11:31:55 +0000 Message-ID: <89c777b7-33bd-400d-8b6f-4e6d697dc632@intel.com> Date: Thu, 2 Oct 2025 19:31:40 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v4 07/28] sched: Add helper function to decide whether to allow cache aware scheduling To: Peter Zijlstra CC: Ingo Molnar , K Prateek Nayak , "Gautham R . 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One side effect is that when the preferred LLC is saturated, >> more threads will continue to be stacked on it, degrading the workload's >> latency. A strategy is needed to prevent this aggregation from going too >> far such that the preferred LLC is too overloaded. > > So one of the ideas was to extend the preferred llc number to a mask. > Update the preferred mask with (nr_threads / llc_size) bits, indicating > the that many top llc as sorted by occupancy. > > Having more than one preferred LLC helps prevent aggregation from going too far on a single preferred LLC. One question would be: if one LLC cannot hold all the threads of a process, does a second preferred LLC help in this use case? Currently, this patch gives up task aggregation and falls back to legacy load balancing if the preferred LLC is overloaded. If we place threads across two preferred LLCs, these threads might encounter cross-LLC latency anyway - so we may as well let legacy load balancing spread them out IMO. Another issue that Patch 7 tries to address is avoiding task bouncing between preferred LLCs and non-preferred LLCs. If we introduce a preferred LLC priority list, logic to prevent task bouncing between different preferred LLCs might be needed in load balancing, which could become complicated. Currently, we mainly implement cache-aware scheduling in load balancing rather than during task wakeup, because the wakeup path conflicts with the load balance path and causes task migration bouncing. thanks, Chenyu