linux-kernel - Re: [RFC PATCH] mm: memcg: Do not count memory.low reclaim if it does not happen

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Date:   Fri, 25 Mar 2022 11:31:18 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Richard Palethorpe <rpalethorpe@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Muchun Song <songmuchun@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Yang Shi <shy828301@...il.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tejun Heo <tj@...nel.org>, Chris Down <chris@...isdown.name>
Subject: Re: [RFC PATCH] mm: memcg: Do not count memory.low reclaim if it
 does not happen

On Thu, Mar 24, 2022 at 11:17:14AM -0700, Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> Ok, so it’s not really about the implementation details of the reclaim
> mechanism (I mean rounding up to the batch size etc),

Actually, that was what I deemed more serious first.
It's the point 2 of RFCness:

| 2) The observed behavior slightly impacts distribution of parent's memory.low.
|    Constructed example is a passive protected workload in s1 and active in s2
|    (active ~ counteracts the reclaim with allocations). It could strip
|    protection from s1 one by one (one:=SWAP_CLUSTER_MAX/2^sc.priority).
|    That may be considered both wrong (s1 should have been more protected) or
|    correct s2 deserves protection due to its activity.
|    I don't have (didn't collect) data for this, so I think just masking the
|    false events is sufficient (or independent).

> Idk, I don’t have a strong argument against this change (except that
> it changes the existing behavior), but I also don’t see why such
> events are harmful. Do you mind elaborating a bit more?

So I've collected some demo data now.

	systemd-run \
	        -u precious.service --slice=test-protected.slice \
	        -p MemoryLow=50M \
	        /root/memeater 50 # allocates 50M anon, doesn't use it
	
	systemd-run \
	        -u victim.service --slice=test-protected.slice \
	        -p MemoryLow=0M \
	        /root/memeater -m 50 50 # allocates 50M anon, uses it
	
	echo "Started workloads"
	
	systemctl set-property --runtime test.slice MemoryMax=200M
	systemctl set-property --runtime test-protected.slice MemoryLow=50M
	
	sleep 5
	
	systemd-run \
	        -u pressure.service --slice=test.slice \
	        -p MemorySwapMax=0M \ # to push test-protected.slice to swap
	        /root/memeater -m 170 170
	
	sleep 5
	systemd-cgtop -b -1 -m test.slice

Result with memory_recursiveprot

> Control Group                                                        Tasks   %CPU   Memory  Input/s Output/s
> test.slice                                                               3      -   199.9M        -        -
> test.slice/pressure.service                                              1      -   170.5M        -        -
> test.slice/test-protected.slice                                          2      -    29.4M        -        -
> test.slice/test-protected.slice/victim.service                           1      -    29.1M        -        -
> test.slice/test-protected.slice/precious.service                         1      -   292.0K        -        -

Result without memory_recursiveprot

> Control Group                                                        Tasks   %CPU   Memory  Input/s Output/s
> test.slice                                                               3      -   199.8M        -        -
> test.slice/pressure.service                                              1      -   170.5M        -        -
> test.slice/test-protected.slice                                          2      -    29.3M        -        -
> test.slice/test-protected.slice/precious.service                         1      -    28.7M        -        -
> test.slice/test-protected.slice/victim.service                           1      -   560.0K        -        -

(kernel 5.17.0, systemd 249.10)

So with this result, I'd say the event reporting is an independent change
(admiteddly, thanks to the current implementation (not the proposal of
mine) I noticed this issue).
/me scratches head, let me review my other approaches...


Michal