Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Date:   Fri, 17 Nov 2023 11:13:43 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     "Huang, Ying" <ying.huang@intel.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        David Hildenbrand <david@redhat.com>,
        akpm@linux-foundation.org, wangkefeng.wang@huawei.com,
        willy@infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, John Hubbard <jhubbard@nvidia.com>
Subject: Re: [RFC PATCH] mm: support large folio numa balancing
Message-ID: <20231117101343.GH3818@noisy.programming.kicks-ass.net>
References: <a71a478ce404e93683023dbb7248dd95f11554f4.1699872019.git.baolin.wang@linux.alibaba.com>
 <606d2d7a-d937-4ffe-a6f2-dfe3ae5a0c91@redhat.com>
 <871qctf89m.fsf@yhuang6-desk2.ccr.corp.intel.com>
 <f22001bb-e474-4ddb-8440-2668e6cec000@linux.alibaba.com>
 <87sf57en8n.fsf@yhuang6-desk2.ccr.corp.intel.com>
 <20231117100745.fnpijbk4xgmals3k@techsingularity.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20231117100745.fnpijbk4xgmals3k@techsingularity.net>
Precedence: bulk

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?