Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order

[Bharata B Rao]

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.