On Wed, Jan 27, 2021 at 12:04:01PM +0100, Vlastimil Babka wrote: > 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/ So what is preferrable here now? Above or other quick fix or reverting the original commit? Regards, Bharata.
