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

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Date:   Thu, 4 Feb 2021 10:33:47 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     bharata@...ux.ibm.com
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 2/3/21 12:10 PM, Bharata B Rao wrote:
> On Wed, Jan 27, 2021 at 12:04:01PM +0100, Vlastimil Babka wrote:
>> 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?

I would try the above first. In case it doesn't work, revert. As the immediate
fix for the regression, that people can safely backport.
Anything more complex will take more time and would be more risky to backport.

> Regards,
> Bharata.
>