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

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Date:   Wed, 3 Feb 2021 16:40:09 +0530
From:   Bharata B Rao <bharata@...ux.ibm.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Lameter <cl@...ux.com>, Will Deacon <will@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, guro@...com,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        aneesh.kumar@...ux.ibm.com, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Catalin Marinas <Catalin.Marinas@....com>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the
 slub page order

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@suse.cz/

So what is preferrable here now? Above or other quick fix or reverting
the original commit?

Regards,
Bharata.