linux-kernel - Re: [RFC v7 00/10] DAMON: Support Physical Memory Address Space Monitoring

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Message-ID: <CALvZod6RS66aSFjWHvpbjuinz2mwbGDnz+gh5L7dp+c3D_Zy1w@mail.gmail.com>
Date:   Wed, 19 Aug 2020 18:21:44 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     SeongJae Park <sjpark@...zon.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        SeongJae Park <sjpark@...zon.de>, Jonathan.Cameron@...wei.com,
        Andrea Arcangeli <aarcange@...hat.com>, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, amit@...nel.org,
        benh@...nel.crashing.org, brendan.d.gregg@...il.com,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Qian Cai <cai@....pw>,
        Colin Ian King <colin.king@...onical.com>,
        Jonathan Corbet <corbet@....net>,
        David Hildenbrand <david@...hat.com>, dwmw@...zon.com,
        "Du, Fan" <fan.du@...el.com>, foersleo@...zon.de,
        Greg Thelen <gthelen@...gle.com>,
        Ian Rogers <irogers@...gle.com>, jolsa@...hat.com,
        "Kirill A. Shutemov" <kirill@...temov.name>, mark.rutland@....com,
        Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, namhyung@...nel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        David Rientjes <rientjes@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>, rppt@...nel.org,
        sblbir@...zon.com, shuah@...nel.org, sj38.park@...il.com,
        snu@...zon.de, Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Huang Ying <ying.huang@...el.com>, zgf574564920@...il.com,
        linux-damon@...zon.com, Linux MM <linux-mm@...ck.org>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v7 00/10] DAMON: Support Physical Memory Address Space Monitoring

On Tue, Aug 18, 2020 at 12:25 AM SeongJae Park <sjpark@...zon.com> wrote:
>
> From: SeongJae Park <sjpark@...zon.de>
>
> Changes from Previous Version
> =============================
>
> - Use 42 as the fake target id for paddr instead of -1
> - Fix a typo
>
> Introduction
> ============
>
> DAMON[1] programming interface users can extend DAMON for any address space by
> configuring the address-space specific low level primitives with appropriate
> ones including their own implementations.  However, because the implementation
> for the virtual address space is only available now, the users should implement
> their own for other address spaces.  Worse yet, the user space users who rely
> on the debugfs interface and user space tool, cannot implement their own.
>
> This patchset implements another reference implementation of the low level
> primitives for the physical memory address space.  With this change, hence, the
> kernel space users can monitor both the virtual and the physical address spaces
> by simply changing the configuration in the runtime.  Further, this patchset
> links the implementation to the debugfs interface and the user space tool for
> the user space users.
>
> Note that the implementation supports only the user memory, as same to the idle
> page access tracking feature.
>
> [1] https://lore.kernel.org/linux-mm/20200706115322.29598-1-sjpark@amazon.com/
>

I am still struggling to find the benefit of this feature the way it
is implemented i.e. region based physical address space monitoring.
What exactly am I supposed to do for a given hot (or cold) physical
region? In a containerized world, that region can contain pages from
any cgroup. I can not really do anything about the accesses PHY-DAMON
provides me for a region.

Now if you give me per-page information that would be useful as I can
at least get per-cgroup accesses (idle or re-use data) but that would
be as costly as Page Idle Tracking.