linux-kernel - Re: [RFC PATCH] mm/page_reporting: Adjust threshold according to MAX

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Date:   Tue, 1 Jun 2021 10:01:25 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Gavin Shan <gshan@...hat.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, alexander.h.duyck@...ux.intel.com,
        akpm@...ux-foundation.org, shan.gavin@...il.com,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [RFC PATCH] mm/page_reporting: Adjust threshold according to
 MAX_ORDER

On 01.06.21 05:33, Gavin Shan wrote:
> The PAGE_REPORTING_MIN_ORDER is equal to @pageblock_order, taken as
> minimal order (threshold) to trigger page reporting. The page reporting
> is never triggered with the following configurations and settings on
> aarch64. In the particular scenario, the page reporting won't be triggered
> until the largest (2 ^ (MAX_ORDER-1)) free area is achieved from the
> page freeing. The condition is very hard, or even impossible to be met.
> 
>    CONFIG_ARM64_PAGE_SHIFT:              16
>    CONFIG_HUGETLB_PAGE:                  Y
>    CONFIG_HUGETLB_PAGE_SIZE_VARIABLE:    N
>    pageblock_order:                      13
>    CONFIG_FORCE_MAX_ZONEORDER:           14
>    MAX_ORDER:                            14
> 
> The issue can be reproduced in VM, running kernel with above configurations
> and settings. The 'memhog' is used inside the VM to access 512MB anonymous
> area. The QEMU's RSS doesn't drop accordingly after 'memhog' exits.
> 
>    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64          \
>    -accel kvm -machine virt,gic-version=host                        \
>    -cpu host -smp 8,sockets=2,cores=4,threads=1 -m 4096M,maxmem=64G \
>    -object memory-backend-ram,id=mem0,size=2048M                    \
>    -object memory-backend-ram,id=mem1,size=2048M                    \
>    -numa node,nodeid=0,cpus=0-3,memdev=mem0                         \
>    -numa node,nodeid=1,cpus=4-7,memdev=mem1                         \
>      :                                                              \
>    -device virtio-balloon-pci,id=balloon0,free-page-reporting=yes
> 
> This tries to fix the issue by adjusting the threshold to the smaller value
> of @pageblock_order and (MAX_ORDER/2). With this applied, the QEMU's RSS
> drops after 'memhog' exits.

IIRC, we use pageblock_order to

a) Reduce the free page reporting overhead. Reporting on small chunks 
can make us report constantly with little system activity.

b) Avoid splitting THP in the hypervisor, avoiding downgraded VM 
performance.

c) Avoid affecting creation of pageblock_order pages while hinting is 
active. I think there are cases where "temporary pulling sub-pageblock 
pages" can negatively affect creation of pageblock_order pages. 
Concurrent compaction would be one of these cases.

The monstrosity called aarch64 64k is really special in that sense, 
because a) does not apply because pageblocks are just very big, b) does 
sometimes not apply because either our VM isn't backed by (rare) 512MB 
THP or uses 4k with 2MB THP and c) similarly doesn't apply in smallish 
VMs because we don't really happen to create 512MB THP either way.

For example, going on x86-64 from reporting 2MB to something like 32KB 
is absolutely undesired.

I think if we want to go down that path (and I am not 100% sure yet if 
we want to), we really want to treat only the special case in a special 
way. Note that even when doing it only for aarch64 with 64k, you will 
still end up splitting THP in a hypervisor if it uses 64k base pages 
(b)) and can affect creation of THP, for example, when compacting (c), 
so there is a negative side to that.

-- 
Thanks,

David / dhildenb