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/
