linux-kernel - Re: [RFC PATCH v1 0/4] Kernel thread based async batch migration

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Message-ID: <87bjqi3ohx.fsf@DESKTOP-5N7EMDA>
Date: Fri, 20 Jun 2025 14:39:54 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Bharata B Rao <bharata@....com>
Cc: <linux-kernel@...r.kernel.org>,  <linux-mm@...ck.org>,
  <Jonathan.Cameron@...wei.com>,  <dave.hansen@...el.com>,
  <gourry@...rry.net>,  <hannes@...xchg.org>,
  <mgorman@...hsingularity.net>,  <mingo@...hat.com>,
  <peterz@...radead.org>,  <raghavendra.kt@....com>,  <riel@...riel.com>,
  <rientjes@...gle.com>,  <sj@...nel.org>,  <weixugc@...gle.com>,
  <willy@...radead.org>,  <ziy@...dia.com>,  <dave@...olabs.net>,
  <nifan.cxl@...il.com>,  <xuezhengchu@...wei.com>,  <yiannis@...corp.com>,
  <akpm@...ux-foundation.org>,  <david@...hat.com>
Subject: Re: [RFC PATCH v1 0/4] Kernel thread based async batch migration

Bharata B Rao <bharata@....com> writes:

> Hi,
>
> This is a continuation of the earlier post[1] that attempted to
> convert migrations from NUMA Balancing to be async and batched.
> In this version, per-node kernel threads are created to handle
> migrations in an async manner.
>
> This adds a few fields to the extended page flags that can be
> used both by the sub-systems that request migrations and kmigrated
> which migrates the pages. Some of the fields are potentially defined
> to be used by kpromoted-like subsystem to manage hot page metrics,
> but are unused right now.
>
> Currently only NUMA Balancing is changed to make use of the async
> batched migration. It does so by recording the target NID and the
> readiness of the page to be migrated in the extended page flags
> fields.
>
> Each kmigrated routinely scans its PFNs, identifies the pages
> marked for migration and batch-migrates them. Unlike the previous
> approach, the responsibility of isolating the pages is now with
> kmigrated.
>
> The major difference between this approach and the way kpromoted[2]
> tracked hot pages is the elimination of heavy synchronization points
> between producers(sub-systems that request migrations or report
> a hot page) and the consumer (kmigrated or kpromoted).
> Instead of tracking only the list of hot pages in an orthogonal
> manner, this approach ties the hot page or migration infomation
> to the struct page.

I don't think page flag + scanning is a good idea.  If the
synchronization is really a problem for you (based on test results),
some per-CPU data structure can be used to record candidate pages.

[snip]

---
Best Regards,
Huang, Ying