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    SubjectRe: [PATCH v3 0/6] percpu: partial chunk depopulation
    On Fri, Apr 16, 2021 at 02:18:10PM +0000, Dennis Zhou wrote:
    > Hello,
    >
    > On Fri, Apr 16, 2021 at 06:26:15PM +0530, Pratik Sampat wrote:
    > > Hello Roman,
    > >
    > > I've tried the v3 patch series on a POWER9 and an x86 KVM setup.
    > >
    > > My results of the percpu_test are as follows:
    > > Intel KVM 4CPU:4G
    > > Vanilla 5.12-rc6
    > > # ./percpu_test.sh
    > > Percpu:             1952 kB
    > > Percpu:           219648 kB
    > > Percpu:           219648 kB
    > >
    > > 5.12-rc6 + with patchset applied
    > > # ./percpu_test.sh
    > > Percpu:             2080 kB
    > > Percpu:           219712 kB
    > > Percpu:            72672 kB
    > >
    > > I'm able to see improvement comparable to that of what you're see too.
    > >
    > > However, on POWERPC I'm unable to reproduce these improvements with the patchset in the same configuration
    > >
    > > POWER9 KVM 4CPU:4G
    > > Vanilla 5.12-rc6
    > > # ./percpu_test.sh
    > > Percpu:             5888 kB
    > > Percpu:           118272 kB
    > > Percpu:           118272 kB
    > >
    > > 5.12-rc6 + with patchset applied
    > > # ./percpu_test.sh
    > > Percpu:             6144 kB
    > > Percpu:           119040 kB
    > > Percpu:           119040 kB
    > >
    > > I'm wondering if there's any architectural specific code that needs plumbing
    > > here?
    > >
    >
    > There shouldn't be. Can you send me the percpu_stats debug output before
    > and after?

    Btw, sidelined chunks are not listed in the debug output. It was actually on my
    to-do list, looks like I need to prioritize it a bit.

    >
    > > I will also look through the code to find the reason why POWER isn't
    > > depopulating pages.
    > >
    > > Thank you,
    > > Pratik
    > >
    > > On 08/04/21 9:27 am, Roman Gushchin wrote:
    > > > In our production experience the percpu memory allocator is sometimes struggling
    > > > with returning the memory to the system. A typical example is a creation of
    > > > several thousands memory cgroups (each has several chunks of the percpu data
    > > > used for vmstats, vmevents, ref counters etc). Deletion and complete releasing
    > > > of these cgroups doesn't always lead to a shrinkage of the percpu memory,
    > > > so that sometimes there are several GB's of memory wasted.
    > > >
    > > > The underlying problem is the fragmentation: to release an underlying chunk
    > > > all percpu allocations should be released first. The percpu allocator tends
    > > > to top up chunks to improve the utilization. It means new small-ish allocations
    > > > (e.g. percpu ref counters) are placed onto almost filled old-ish chunks,
    > > > effectively pinning them in memory.
    > > >
    > > > This patchset solves this problem by implementing a partial depopulation
    > > > of percpu chunks: chunks with many empty pages are being asynchronously
    > > > depopulated and the pages are returned to the system.
    > > >
    > > > To illustrate the problem the following script can be used:
    > > >
    > > > --
    > > > #!/bin/bash
    > > >
    > > > cd /sys/fs/cgroup
    > > >
    > > > mkdir percpu_test
    > > > echo "+memory" > percpu_test/cgroup.subtree_control
    > > >
    > > > cat /proc/meminfo | grep Percpu
    > > >
    > > > for i in `seq 1 1000`; do
    > > > mkdir percpu_test/cg_"${i}"
    > > > for j in `seq 1 10`; do
    > > > mkdir percpu_test/cg_"${i}"_"${j}"
    > > > done
    > > > done
    > > >
    > > > cat /proc/meminfo | grep Percpu
    > > >
    > > > for i in `seq 1 1000`; do
    > > > for j in `seq 1 10`; do
    > > > rmdir percpu_test/cg_"${i}"_"${j}"
    > > > done
    > > > done
    > > >
    > > > sleep 10
    > > >
    > > > cat /proc/meminfo | grep Percpu
    > > >
    > > > for i in `seq 1 1000`; do
    > > > rmdir percpu_test/cg_"${i}"
    > > > done
    > > >
    > > > rmdir percpu_test
    > > > --
    > > >
    > > > It creates 11000 memory cgroups and removes every 10 out of 11.
    > > > It prints the initial size of the percpu memory, the size after
    > > > creating all cgroups and the size after deleting most of them.
    > > >
    > > > Results:
    > > > vanilla:
    > > > ./percpu_test.sh
    > > > Percpu: 7488 kB
    > > > Percpu: 481152 kB
    > > > Percpu: 481152 kB
    > > >
    > > > with this patchset applied:
    > > > ./percpu_test.sh
    > > > Percpu: 7488 kB
    > > > Percpu: 481408 kB
    > > > Percpu: 135552 kB
    > > >
    > > > So the total size of the percpu memory was reduced by more than 3.5 times.
    > > >
    > > > v3:
    > > > - introduced pcpu_check_chunk_hint()
    > > > - fixed a bug related to the hint check
    > > > - minor cosmetic changes
    > > > - s/pretends/fixes (cc Vlastimil)
    > > >
    > > > v2:
    > > > - depopulated chunks are sidelined
    > > > - depopulation happens in the reverse order
    > > > - depopulate list made per-chunk type
    > > > - better results due to better heuristics
    > > >
    > > > v1:
    > > > - depopulation heuristics changed and optimized
    > > > - chunks are put into a separate list, depopulation scan this list
    > > > - chunk->isolated is introduced, chunk->depopulate is dropped
    > > > - rearranged patches a bit
    > > > - fixed a panic discovered by krobot
    > > > - made pcpu_nr_empty_pop_pages per chunk type
    > > > - minor fixes
    > > >
    > > > rfc:
    > > > https://lwn.net/Articles/850508/
    > > >
    > > >
    > > > Roman Gushchin (6):
    > > > percpu: fix a comment about the chunks ordering
    > > > percpu: split __pcpu_balance_workfn()
    > > > percpu: make pcpu_nr_empty_pop_pages per chunk type
    > > > percpu: generalize pcpu_balance_populated()
    > > > percpu: factor out pcpu_check_chunk_hint()
    > > > percpu: implement partial chunk depopulation
    > > >
    > > > mm/percpu-internal.h | 4 +-
    > > > mm/percpu-stats.c | 9 +-
    > > > mm/percpu.c | 306 +++++++++++++++++++++++++++++++++++--------
    > > > 3 files changed, 261 insertions(+), 58 deletions(-)
    > > >
    > >
    >
    > Roman, sorry for the delay. I'm looking to apply this today to for-5.14.

    Great, thanks!

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