linux-kernel - Re: RFC: Memory Tiering Kernel Interfaces (v2)

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Date:   Fri, 13 May 2022 11:25:34 +0800
From:   "ying.huang@...el.com" <ying.huang@...el.com>
To:     Wei Xu <weixugc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Tim C Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alistair Popple <apopple@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Feng Tang <feng.tang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dan Williams <dan.j.williams@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Linux MM <linux-mm@...ck.org>,
        Brice Goglin <brice.goglin@...il.com>,
        Hesham Almatary <hesham.almatary@...wei.com>
Subject: Re: RFC: Memory Tiering Kernel Interfaces (v2)

On Wed, 2022-05-11 at 23:22 -0700, Wei Xu wrote:
> 
> Memory Allocation for Demotion
> ==============================
> 
> To allocate a new page as the demotion target for a page, the kernel
> calls the allocation function (__alloc_pages_nodemask) with the
> source page node as the preferred node and the union of all lower
> tier nodes as the allowed nodemask.  The actual target node selection
> then follows the allocation fallback order that the kernel has
> already defined.
> 
> The pseudo code looks like:
> 
>     targets = NODE_MASK_NONE;
>     src_nid = page_to_nid(page);
>     src_tier = node_tier_map[src_nid];
>     for (i = src_tier + 1; i < MAX_MEMORY_TIERS; i++)
>             nodes_or(targets, targets, memory_tiers[i]);
>     new_page = __alloc_pages_nodemask(gfp, order, src_nid, targets);
> 
> The memopolicy of cpuset, vma and owner task of the source page can
> be set to refine the demotion target nodemask, e.g. to prevent
> demotion or select a particular allowed node as the demotion target.

Consider a system with 3 tiers, if we want to demote some pages from
tier 0, the desired behavior is,

- Allocate pages from tier 1
- If there's no enough free pages in tier 1, wakeup kswapd of tier 1 so
demote some pages from tier 1 to tier 2
- If there's still no enough free pages in tier 1, allocate pages from
tier 2.

In this way, tier 0 will have the hottest pages, while tier 1 will have
the coldest pages.

With your proposed method, the demoting from tier 0 behavior is,

- Allocate pages from tier 1
- If there's no enough free pages in tier 1, allocate pages in tier 2

The kswapd of tier 1 will not be waken up until there's no enough free
pages in tier 2.  In quite long time, there's no much hot/cold
differentiation between tier 1 and tier 2.

This isn't hard to be fixed, just call __alloc_pages_nodemask() for each
tier one by one considering page allocation fallback order.

Best Regards,
Huang, Ying