linux-kernel - Re: [RFC PATCH] mm: support large folio numa balancing

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Date:   Fri, 17 Nov 2023 11:13:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Hildenbrand <david@...hat.com>,
        akpm@...ux-foundation.org, wangkefeng.wang@...wei.com,
        willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC PATCH] mm: support large folio numa balancing

On Fri, Nov 17, 2023 at 10:07:45AM +0000, Mel Gorman wrote:

> This leads into a generic problem with large anything with NUMA
> balancing -- false sharing. As it stands, THP can be false shared by
> threads if thread-local data is split within a THP range. In this case,
> the ideal would be the THP is migrated to the hottest node but such
> support doesn't exist. The same applies for folios. If not handled
> properly, a large folio of any type can ping-pong between nodes so just
> migrating because we can is not necessarily a good idea. The patch
> should cover a realistic case why this matters, why splitting the folio
> is not better and supporting data.

Would it make sense to have THP merging conditional on all (most?) pages
having the same node?