[RFC PATCH 0/1] fs/namespace: defer free_mount from namespace_unlock

[Eric Chanudet]

We noticed a significant slow down when running a containers on an
Aarch64 system with the RT patch set using the following test:
# mkdir -p "rootfs/bin"
# printf "int main(){return 0;}" | gcc  -x c - -static -o rootfs/bin/sh
# crun spec
# perf stat -r 10 --table --null -- crun run test

 Performance counter stats for 'crun run test' (10 runs):

            # Table of individual measurements:
            0.3902 (-0.1941) ##########
            0.5791 (-0.0053) #
            0.5785 (-0.0058) #
            0.5891 (+0.0047) #
            0.6682 (+0.0839) ###
            0.5507 (-0.0337) ##
            0.5888 (+0.0044) #
            0.5797 (-0.0047) #
            0.5977 (+0.0133) #
            0.7217 (+0.1374) ####

            # Final result:
            0.5844 +- 0.0269 seconds time elapsed  ( +-  4.60% )

A 6.2 non-RT kernel results on the same hardware for comparison:

 Performance counter stats for 'crun run test' (10 runs):

            # Table of individual measurements:
            0.1680 (+0.1375) #################
            0.0074 (-0.0231) ###############################################################
            0.0073 (-0.0232) ################################################################
            0.0070 (-0.0235) ####################################################################
            0.0072 (-0.0233) #################################################################
            0.0794 (+0.0489) #############
            0.0078 (-0.0227) ##########################################################
            0.0070 (-0.0235) ###################################################################
            0.0070 (-0.0235) ####################################################################
            0.0068 (-0.0237) ######################################################################

            # Final result:
            0.0305 +- 0.0169 seconds time elapsed  ( +- 55.33% )

It looks like there is one bottleneck in:
-> do_umount 
 -> namespace_unlock
  -> synchronize_rcu_expedited

With the following patch, namespace_unlock will queue up the resources
that needs to be released and defer the operation through call_rcu to
return without waiting for the grace period.

Alexander Larsson (1):
  fs/namespace: defer free_mount from namespace_unlock

 fs/namespace.c | 42 +++++++++++++++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 7 deletions(-)

-- 
2.39.0