On 1/27/21 10:10 AM, Christoph Lameter wrote: > On Tue, 26 Jan 2021, Will Deacon wrote: > >> > Hm, but booting the secondaries is just a software (kernel) action? They are >> > already physically there, so it seems to me as if the cpu_present_mask is not >> > populated correctly on arm64, and it's just a mirror of cpu_online_mask? >> >> I think the present_mask retains CPUs if they are hotplugged off, whereas >> the online mask does not. We can't really do any better on arm64, as there's >> no way of telling that a CPU is present until we've seen it. > > The order of each page in a kmem cache --and therefore also the number > of objects in a slab page-- can be different because that information is > stored in the page struct. > > Therefore it is possible to retune the order while the cache is in operaton. Yes, but it's tricky to do the retuning safely, e.g. if freelist randomization is enabled, see [1]. But as a quick fix for the regression, the heuristic idea could work reasonably on all architectures? - if num_present_cpus() is > 1, trust that it doesn't have the issue such as arm64, and use it - otherwise use nr_cpu_ids Long-term we can attempt to do the retuning safe, or decide that number of cpus shouldn't determine the order... [1] https://lore.kernel.org/linux-mm/d7fb9425-9a62-c7b8-604d-5828d7e6b1da@xxxxxxx/ > This means you can run an initcall after all cpus have been brought up to > set the order and number of objects in a slab page differently. > > The older slab pages will continue to exist with the old orders until they > are freed. >
