[PATCH 0/2] kernfs: speed up concurrency performance

[Fox Chen <foxhlchen@gmail.com>]

Hello,

kernfs is an important facillity to support pseudo file systems and cgroup. 
Currently, with a global mutex, reading files concurrently from kernfs (e.g. /sys) 
is very slow.

This problem is reported by Brice Goglin on thread:
Re: [PATCH 1/4] drivers core: Introduce CPU type sysfs interface
https://lore.kernel.org/lkml/X60dvJoT4fURcnsF@kroah.com/

I independently comfirmed this on a 96-core AWS c5.metal server.
Do open+read+write on /sys/devices/system/cpu/cpu15/topology/core_id 1000 times.
With a single thread it takes ~2.5 us for each open+read+close.
With one thread per core, 96 threads running simultaneously takes 540 us 
for each of the same operation (without much variation) -- 200x slower than the 
single thread one. 

The problem can only be observed in large machines (>=16 cores).
The more cores you have the slower it can be.

Perf shows that CPUs spend most of the time (>80%) waiting on mutex locks in 
kernfs_iop_permission and kernfs_dop_revalidate.

This patchset contains the following 2 patches:
0001-kernfs-replace-the-mutex-in-kernfs_iop_permission-wi.patch
0002-kernfs-remove-mutex-in-kernfs_dop_revalidate.patch

0001 replace the mutex lock in kernfs_iop_permission with a new rwlock and 
0002 removes the mutex lock in kernfs_dop_revalidate.

After applying this patchset, the multi-thread performance becomes linear with 
the fastest one at ~30 us to the worst at ~150 us, very similar as I tested it
on a normal ext4 file system with fastest one at ~20 us to slowest at ~100 us. 
And I believe that is largely due to spin_locks in filesystems which are normal.

Although it's still slower than single thread, users can benefit from this 
patchset, especially ones working on HPC realm with lots of cpu cores and want to
fetch system information from sysfs.

I tried my best to solve this problem. If there is stupid mistake, please kindly
point out. I would appreciate it greatly.

Fox

 fs/kernfs/dir.c        |  9 +++------
 fs/kernfs/inode.c      | 16 ++++++++--------
 include/linux/kernfs.h |  1 +
 3 files changed, 12 insertions(+), 14 deletions(-)

-- 
2.29.2