Re: [PATCH v2 0/7] make slab shrink lockless

From: Qi Zheng
Date: Thu Feb 23 2023 - 23:09:23 EST



On 2023/2/24 02:19, Paul E. McKenney wrote:
On Thu, Feb 23, 2023 at 09:27:18PM +0800, Qi Zheng wrote:
Hi all,

This patch series aims to make slab shrink lockless.

1. Background
=============

On our servers, we often find the following system cpu hotspots:

44.16% [kernel] [k] down_read_trylock
14.12% [kernel] [k] up_read
13.43% [kernel] [k] shrink_slab
5.25% [kernel] [k] count_shadow_nodes
3.42% [kernel] [k] idr_find

Then we used bpftrace to capture its calltrace as follows:

@[
down_read_trylock+5
shrink_slab+292
shrink_node+640
do_try_to_free_pages+211
try_to_free_mem_cgroup_pages+266
try_charge_memcg+386
charge_memcg+51
__mem_cgroup_charge+44
__handle_mm_fault+1416
handle_mm_fault+260
do_user_addr_fault+459
exc_page_fault+104
asm_exc_page_fault+38
clear_user_rep_good+18
read_zero+100
vfs_read+176
ksys_read+93
do_syscall_64+62
entry_SYSCALL_64_after_hwframe+114
]: 1868979

It is easy to see that this is caused by the frequent failure to obtain the
read lock of shrinker_rwsem when reclaiming slab memory.

Currently, the shrinker_rwsem is a global lock. And the following cases may
cause the above system cpu hotspots:

a. the write lock of shrinker_rwsem was held for too long. For example, there
are many memcgs in the system, which causes some paths to hold locks and
traverse it for too long. (e.g. expand_shrinker_info())
b. the read lock of shrinker_rwsem was held for too long, and a writer came at
this time. Then this writer will be forced to wait and block all subsequent
readers.
For example:
- be scheduled when the read lock of shrinker_rwsem is held in
do_shrink_slab()
- some shrinker are blocked for too long. Like the case mentioned in the
patchset[1].

[1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@xxxxxxxxxxxxx/

And all the down_read_trylock() hotspots caused by the above cases can be
solved by replacing the shrinker_rwsem trylocks with SRCU.

Hi Paul,


Glad to see that making SRCU unconditional was helpful! And I do very
much like the idea of the shrinker running better!

+1 :)


The main thing that enabled unconditional SRCU was the code added in
v5.19 to dynamically allocate SRCU's srcu_node combining tree. This is
important for a number of Linux distributions that have NR_CPUS up in the
thousands, for which this combining tree is quite large. In v5.19 and
later, srcu_struct structures without frequent call_srcu() invocations
never allocate that combining tree. Even srcu_struct structures that
have enough call_srcu() activity to cause the lock contention that in
turn forces the combining tree to be allocated, that combining tree
is sized for the actual number of CPUs present, which is usually way
smaller than NR_CPUS.

Thank you very much for such a detailed background introduction. :)


So if you are going to backport this back past v5.19, you might also
need those SRCU changes. Or not, depending on how much memory your
systems are equipped with. ;-)

Got it.

Thanks,
Qi


Thanx, Paul

2. Survey
=========

Before doing the code implementation, I found that there were many similar
submissions in the community:

a. Davidlohr Bueso submitted a patch in 2015.
Subject: [PATCH -next v2] mm: srcu-ify shrinkers
Link: https://lore.kernel.org/all/1437080113.3596.2.camel@xxxxxxxxxxxx/
Result: It was finally merged into the linux-next branch, but failed on arm
allnoconfig (without CONFIG_SRCU)

b. Tetsuo Handa submitted a patchset in 2017.
Subject: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.
Link: https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx/
Result: Finally chose to use the current simple way (break when
rwsem_is_contended()). And Christoph Hellwig suggested to using SRCU,
but SRCU was not unconditionally enabled at the time.

c. Kirill Tkhai submitted a patchset in 2018.
Subject: [PATCH RFC 00/10] Introduce lockless shrink_slab()
Link: https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
Result: At that time, SRCU was not unconditionally enabled, and there were
some objections to enabling SRCU. Later, because Kirill's focus was
moved to other things, this patchset was not continued to be updated.

d. Sultan Alsawaf submitted a patch in 2021.
Subject: [PATCH] mm: vmscan: Replace shrinker_rwsem trylocks with SRCU protection
Link: https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@xxxxxxxxxxxxxxx/
Result: Rejected because SRCU was not unconditionally enabled.

We can find that almost all these historical commits were abandoned because SRCU
was not unconditionally enabled. But now SRCU has been unconditionally enable
by Paul E. McKenney in 2023 [2], so it's time to replace shrinker_rwsem trylocks
with SRCU.

[2] https://lore.kernel.org/lkml/20230105003759.GA1769545@paulmck-ThinkPad-P17-Gen-1/

3. Reproduction and testing
===========================

We can reproduce the down_read_trylock() hotspot through the following script:

```
#!/bin/bash
DIR="/root/shrinker/memcg/mnt"

do_create()
{
mkdir /sys/fs/cgroup/memory/test
echo 200M > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
for i in `seq 0 $1`;
do
mkdir /sys/fs/cgroup/memory/test/$i;
echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
mkdir -p $DIR/$i;
done
}

do_mount()
{
for i in `seq $1 $2`;
do
mount -t tmpfs $i $DIR/$i;
done
}

do_touch()
{
for i in `seq $1 $2`;
do
echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 &
done
}

do_create 2000
do_mount 0 2000
do_touch 0 1000
```

Save the above script and execute it, we can get the following perf hotspots:

46.60% [kernel] [k] down_read_trylock
18.70% [kernel] [k] up_read
15.44% [kernel] [k] shrink_slab
4.37% [kernel] [k] _find_next_bit
2.75% [kernel] [k] xa_load
2.07% [kernel] [k] idr_find
1.73% [kernel] [k] do_shrink_slab
1.42% [kernel] [k] shrink_lruvec
0.74% [kernel] [k] shrink_node
0.60% [kernel] [k] list_lru_count_one

After applying this patchset, the hotspot becomes as follows:

19.53% [kernel] [k] _find_next_bit
14.63% [kernel] [k] do_shrink_slab
14.58% [kernel] [k] shrink_slab
11.83% [kernel] [k] shrink_lruvec
9.33% [kernel] [k] __blk_flush_plug
6.67% [kernel] [k] mem_cgroup_iter
3.73% [kernel] [k] list_lru_count_one
2.43% [kernel] [k] shrink_node
1.96% [kernel] [k] super_cache_count
1.78% [kernel] [k] __rcu_read_unlock
1.38% [kernel] [k] __srcu_read_lock
1.30% [kernel] [k] xas_descend

We can see that the slab reclaim is no longer blocked by shinker_rwsem trylock,
which realizes the lockless slab reclaim.

This series is based on next-20230217.

Comments and suggestions are welcome.

Thanks,
Qi.

Changelog in v1 -> v2:
- add a map_nr_max field to shrinker_info (suggested by Kirill)
- use shrinker_mutex in reparent_shrinker_deferred() (pointed by Kirill)

Qi Zheng (7):
mm: vmscan: add a map_nr_max field to shrinker_info
mm: vmscan: make global slab shrink lockless
mm: vmscan: make memcg slab shrink lockless
mm: shrinkers: make count and scan in shrinker debugfs lockless
mm: vmscan: hold write lock to reparent shrinker nr_deferred
mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()
mm: shrinkers: convert shrinker_rwsem to mutex

drivers/md/dm-cache-metadata.c | 2 +-
drivers/md/dm-thin-metadata.c | 2 +-
fs/super.c | 2 +-
include/linux/memcontrol.h | 1 +
mm/shrinker_debug.c | 38 ++++-----
mm/vmscan.c | 142 +++++++++++++++++----------------
6 files changed, 92 insertions(+), 95 deletions(-)

--
2.20.1


--
Thanks,
Qi