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

[Vlastimil Babka]

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.
>