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

From: Shakeel Butt <shakeelb@google.com>
To: SeongJae Park <sjpark@amazon.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	SeongJae Park <sjpark@amazon.de>,
	Jonathan.Cameron@huawei.com,
	Andrea Arcangeli <aarcange@redhat.com>,
	acme@kernel.org, alexander.shishkin@linux.intel.com,
	amit@kernel.org, benh@kernel.crashing.org,
	brendan.d.gregg@gmail.com,
	Brendan Higgins <brendanhiggins@google.com>,
	Qian Cai <cai@lca.pw>, Colin Ian King <colin.king@canonical.com>,
	Jonathan Corbet <corbet@lwn.net>,
	David Hildenbrand <david@redhat.com>,
	dwmw@amazon.com, "Du, Fan" <fan.du@intel.com>,
	foersleo@amazon.de, Greg Thelen <gthelen@google.com>,
	Ian Rogers <irogers@google.com>,
	jolsa@redhat.com, "Kirill A. Shutemov" <kirill@shutemov.name>,
	mark.rutland@arm.com, Mel Gorman <mgorman@suse.de>,
	Minchan Kim <minchan@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	namhyung@kernel.org,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Rik van Riel <riel@surriel.com>,
	David Rientjes <rientjes@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	rppt@kernel.org, sblbir@amazon.com, shuah@kernel.org,
	sj38.park@gmail.com, snu@amazon.de,
	Vlastimil Babka <vbabka@suse.cz>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Huang Ying <ying.huang@intel.com>,
	zgf574564920@gmail.com, linux-damon@amazon.com,
	Linux MM <linux-mm@kvack.org>,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC v7 00/10] DAMON: Support Physical Memory Address Space Monitoring
Date: Wed, 19 Aug 2020 18:21:44 -0700	[thread overview]
Message-ID: <CALvZod6RS66aSFjWHvpbjuinz2mwbGDnz+gh5L7dp+c3D_Zy1w@mail.gmail.com> (raw)
In-Reply-To: <20200818072501.30396-1-sjpark@amazon.com>

On Tue, Aug 18, 2020 at 12:25 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> From: SeongJae Park <sjpark@amazon.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.