Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

[Mateusz Guzik]

On Tue, Apr 08, 2025 at 04:58:34PM -0400, Eric Chanudet wrote:
> Defer releasing the detached file-system when calling namespace_unlock()
> during a lazy umount to return faster.
> 
> When requesting MNT_DETACH, the caller does not expect the file-system
> to be shut down upon returning from the syscall. Calling
> synchronize_rcu_expedited() has a significant cost on RT kernel that
> defaults to rcupdate.rcu_normal_after_boot=1. Queue the detached struct
> mount in a separate list and put it on a workqueue to run post RCU
> grace-period.
> 
> w/o patch, 6.15-rc1 PREEMPT_RT:
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>     0.02455 +- 0.00107 seconds time elapsed  ( +-  4.36% )
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
>     0.02555 +- 0.00114 seconds time elapsed  ( +-  4.46% )
> 
> w/ patch, 6.15-rc1 PREEMPT_RT:
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>     0.026311 +- 0.000869 seconds time elapsed  ( +-  3.30% )
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
>     0.003194 +- 0.000160 seconds time elapsed  ( +-  5.01% )
> 

Christian wants the patch done differently and posted his diff, so I'm
not going to comment on it.

I do have some feedback about the commit message though.

In v1 it points out a real user which runs into it, while this one does
not. So I would rewrite this and put in bench results from the actual
consumer -- as it is one is left to wonder why patching up lazy unmount
is of any significance.

I had to look up what rcupdate.rcu_normal_after_boot=1 is. Docs claim it
makes everyone use normal grace-periods, which explains the difference.
But without that one is left to wonder if perhaps there is a perf bug in
RCU instead where this is taking longer than it should despite the
option. Thus I would also denote how the delay shows up.

v1 for reference:
> v1: https://lore.kernel.org/all/20230119205521.497401-1-echanude@xxxxxxxxxx/