mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
synced 2025-12-28 20:49:11 -05:00
02d954c0fd
When a process reduces its number of threads or clears bits in its CPU affinity mask, the mm_cid allocation should eventually converge towards smaller values. However, the change introduced by: commit7e019dcc47("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads") adds a per-mm/CPU recent_cid which is never unset unless a thread migrates. This is a tradeoff between: A) Preserving cache locality after a transition from many threads to few threads, or after reducing the hamming weight of the allowed CPU mask. B) Making the mm_cid upper bounds wrt nr threads and allowed CPU mask easy to document and understand. C) Allowing applications to eventually react to mm_cid compaction after reduction of the nr threads or allowed CPU mask, making the tracking of mm_cid compaction easier by shrinking it back towards 0 or not. D) Making sure applications that periodically reduce and then increase again the nr threads or allowed CPU mask still benefit from good cache locality with mm_cid. Introduce the following changes: * After shrinking the number of threads or reducing the number of allowed CPUs, reduce the value of max_nr_cid so expansion of CID allocation will preserve cache locality if the number of threads or allowed CPUs increase again. * Only re-use a recent_cid if it is within the max_nr_cid upper bound, else find the first available CID. Fixes:7e019dcc47("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads") Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Gabriele Monaco <gmonaco@redhat.com> Link: https://lkml.kernel.org/r/20250210153253.460471-2-gmonaco@redhat.com