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 z0cMDsX3 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On Thu, Apr 24, 2025 at 05:07:48PM +0900, Harry Yoo wrote: > ... > > Eighteen years later, Mateusz Guzik proposed [1] re-introducing a slab > > constructor/destructor pair to mitigate the global serialization point > > (pcpu_alloc_mutex) that occurs when each slab object allocates and frees > > percpu memory during its lifetime. > > > > Consider mm_struct: it allocates two percpu regions (mm_cid and rss_stat), > > so each allocate–free cycle requires two expensive acquire/release on > > that mutex. > > When percpu allocator was first introduced, the use cases were a lot more > limited, so the single mutex and expensive alloc/free paths weren't a > problem. We keep using percpu memory for more and more things, and I always > thought we'd eventually need a more sophisticated allocator something with > object caching. Yeah, when you first write an allocator, it's an overkill to make it too scalable. But over time, as with other allocators, more users show up that require a more sophisticated allocator. > I don't exactly know what that should look like but maybe a > simplified version of sl*b serving power of two sizes should do or maybe it > needs to be smaller and more adaptive. We'd need to collect some data to > decide which way to go. I'm not sure what kind of data we need — maybe allocation size distributions, or more profiling data on workloads that contend on percpu allocator's locks? > Improving percpu allocator in general is obviously a heavier lift but that > may be a better long-term direction. Yeah, if that's doable. But until then I think it still makes sense to cache it within slab objects, ...or probably even after improving the percpu allocator? It's a still churn that's incurred during each object's lifetime regardless. (Need some data to see if justifiable) And, as Mateusz explained, the percpu allocator isn’t the only motivation for the ctor/dtor pair. Other expensive serializations like pgd_lock and percpu_counters_lock are other motivations to do this. -- Cheers, Harry / Hyeonggon