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

[Vincent Guittot]

On Wed, 3 Feb 2021 at 12:10, Bharata B Rao <bharata@xxxxxxxxxxxxx> wrote:
>
> 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?

I'm fine with whatever the solution as long as we can use keep using
nr_cpu_ids when other values like num_present_cpus, don't reflect
correctly the system

Regards,
Vincent

>
> Regards,
> Bharata.