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Fri, 17 Oct 2025 04:51:04 +0000 Message-ID: <4ec19969-831c-4d9e-b585-fc02db31b343@intel.com> Date: Fri, 17 Oct 2025 12:50:47 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 06/19] sched/fair: Assign preferred LLC ID to processes To: Peter Zijlstra CC: Tim Chen , Ingo Molnar , "K Prateek Nayak" , "Gautham R . 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Can the kernel detect this, or get the hint from user space? > > This needs the PMU, then you can steer using cache-miss ratios. But then > people will hate us for using counters. > >> Yes, the numa_group in NUMA load balancing indicates >> that several tasks manipulate the same page, which could be an >> indicator. Besides, if task A frequently wakes up task B, does it >> mean A and B have the potential to share data? Furthermore, if >> task A wakes up B via a pipe, it might also indicate that A has >> something to share with B. I just wonder if we can introduce a >> structure to gather this information together. > > The wakeup or pipe relation might be small relative to the working set. > Consider a sharded in memory database, where the query comes in through > the pipe/socket/wakeup. This query is small, but then it needs to go > trawl through its memory to find the answer. > > Something we *could* look at -- later -- is an interface to create > thread groups, such that userspace that is clever enough can communicate > this. But then there is the ago old question, will there be sufficient > users to justify the maintenance of said interface. I did not intend to digress too far, but since this issue has been brought up, a wild guess came to me - could the "interface to create thread groups" here refer to something like the filesystem for memory cgroup v2 thread mode? I just heard that some cloud users might split the threads of a single process into different thread groups, where threads within each group share data with one another (for example, when performing K-V hashing operations). Using cgroup for this purpose might be a bit overkill, though, considering that cgroup itself is designed for resource partitioning rather than identifying tasks sharing data. Meanwhile, the hierarchy of cgroup could also cause some overhead. If there were a single-layer thread partitioning mechanism - similar to the resctrl filesystem - wouldn’t that allow us to avoid modifying too much user business code while minimizing coupling with existing kernel components? thanks, Chenyu