linux-kernel - Re: [PATCH 02/10] cacheinfo: calculate per-CPU data cache size

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Date:   Fri, 22 Sep 2023 15:56:41 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Arjan Van De Ven <arjan@...ux.intel.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        "David Hildenbrand" <david@...hat.com>,
        Johannes Weiner <jweiner@...hat.com>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        "Pavel Tatashin" <pasha.tatashin@...een.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 02/10] cacheinfo: calculate per-CPU data cache size

Sudeep Holla <sudeep.holla@....com> writes:

> On Wed, Sep 20, 2023 at 02:18:48PM +0800, Huang Ying wrote:
>> Per-CPU data cache size is useful information.  For example, it can be
>> used to determine per-CPU cache size.  So, in this patch, the data
>> cache size for each CPU is calculated via data_cache_size /
>> shared_cpu_weight.
>>
>> A brute-force algorithm to iterate all online CPUs is used to avoid
>> to allocate an extra cpumask, especially in offline callback.
>>
>
> You have not mentioned who will use this information ? Looking at the
> change, it is not exposed to the user-space. Also I see this is actually
> part of the series [1]. Is this info used in any of those patches ? Can you
> point me to the same ?

Yes.  It is used by [PATCH 03/10] of the series.  If the per-CPU data
cache size is large enough, we will cache more pages in the per-CPU
pageset to reduce the zone lock contention.

> Not all architecture use cacheinfo yet. How will the mm changes affect those
> platforms ?

If cacheinfo isn't available, we will fallback to the original
behavior.  That is, we will drain per-CPU pageset more often (that is,
cache less to improve cache-hot pages sharing between CPUs).

> --
> Regards,
> Sudeep
>
> [1] https://lore.kernel.org/all/20230920061856.257597-1-ying.huang@intel.com/

--
Best Regards,
Huang, Ying