Re: [PATCH v3 0/6] percpu: partial chunk depopulation

From: Pratik Sampat
Date: Fri Apr 16 2021 - 08:56:34 EST

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:             1952 kB
Percpu:           219648 kB
Percpu:           219648 kB

5.12-rc6 + with patchset applied
# ./
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

Vanilla 5.12-rc6
# ./
Percpu:             5888 kB
Percpu:           118272 kB
Percpu:           118272 kB

5.12-rc6 + with patchset applied
# ./
Percpu:             6144 kB
Percpu:           119040 kB
Percpu:           119040 kB

I'm wondering if there's any architectural specific code that needs plumbing

I will also look through the code to find the reason why POWER isn't
depopulating pages.

Thank you,

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:


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}"

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}"

sleep 10

cat /proc/meminfo | grep Percpu

for i in `seq 1 1000`; do
rmdir percpu_test/cg_"${i}"

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.

Percpu: 7488 kB
Percpu: 481152 kB
Percpu: 481152 kB

with this patchset applied:
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.

- introduced pcpu_check_chunk_hint()
- fixed a bug related to the hint check
- minor cosmetic changes
- s/pretends/fixes (cc Vlastimil)

- depopulated chunks are sidelined
- depopulation happens in the reverse order
- depopulate list made per-chunk type
- better results due to better heuristics

- 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


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(-)