Amir Goldstein <amir73il@xxxxxxxxx> writes: > On Fri, May 10, 2024 at 6:21 PM Stephen Brennan > <stephen.s.brennan@xxxxxxxxxx> wrote: >> >> Hi Amir, Jan, et al, > > Hi Stephen, > >> >> It's been a while since I worked with you on the patch series[1] that aimed to >> make __fsnotify_update_child_dentry_flags() a sleepable function. That work got >> to a point that it was close to ready, but there were some locking issues which >> Jan found, and the kernel test robot reported, and I didn't find myself able to >> tackle them in the amount of time I had. >> >> But looking back on that series, I think I threw out the baby with the >> bathwater. While I may not have resolved the locking issues associated with the >> larger change, there was one patch which Amir shared, that probably resolves >> more than 90% of the issues that people may see. I'm sending that here, since it >> still applies to the latest master branch, and I think it's a very good idea. >> >> To refresh you, the underlying issue I was trying to resolve was when >> directories have many dentries (frequently, a ton of negative dentries), the >> __fsnotify_update_child_dentry_flags() operation can take a while, and it >> happens under spinlock. >> >> Case #1 - if the directory has tens of millions of dentries, then you could get >> a soft lockup from a single call to this function. I have seen some cases where >> a single directory had this many dentries, but it's pretty rare. >> >> Case #2 - suppose you have a system with many CPUs and a busy directory. Suppose >> the directory watch is removed. The caller will begin executing >> __fsnotify_update_child_dentry_flags() to clear the PARENT_WATCHED flag, but in >> parallel, many other CPUs could wind up in __fsnotify_parent() and decide that >> they, too, must call __fsnotify_update_child_dentry_flags() to clear the flags. >> These CPUs will all spin waiting their turn, at which point they'll re-do the >> long (and likely, useless) call. Even if the original call only took a second or >> two, if you have a dozen or so CPUs that end up in that call, some CPUs will >> spin a long time. >> >> Amir's patch to clear PARENT_WATCHED flags lazily resolves that easily. In >> __fsnotify_parent(), if callers notice that the parent is no longer watching, >> they merely update the flags for the current dentry (not all the other >> children). The __fsnotify_recalc_mask() function further avoids excess calls by >> only updating children if the parent started watching. This easily handles case >> #2 above. Perhaps case #1 could still cause issues, for the cases of truly huge >> dentry counts, but we shouldn't let "perfect" get in the way of "good enough" :) >> > > The story sounds good :) > Only thing I am worried about is: was case #2 tested to prove that > the patch really imploves in practice and not only in theory? > > I am not asking that you write a test for this or even a reproducer > just evidence that you collected from a case where improvement is observed > and measurable. I had not done so when you sent this, but I should have done it beforehand. In any case, now I have. I got my hands on a 384-CPU machine and extended my negative dentry creation tool so that it can run a workload in which it constantly runs "open()" followed by "close()" on 1000 files in the same directory, per thread (so a total of 384,000 files, a large but not unreasonable amount of dentries). Then I simply run "inotifywait /path/to/dir" a few times. Without the patch, softlockups are easy to reproduce. With the patch, I haven't been able to get a single soft lockup. https://github.com/brenns10/kernel_stuff/tree/master/negdentcreate make mkdir test # create 384k files inside "test" ./negdentcreate -p test -c 384000 -t 384 -o create # start a loop opening and closing those files negdentcreate -p test -c 384000 -t 384 -o open -l # in another window: inotifywait test Stephen > > Thanks, > Amir. > >> [1]: https://lore.kernel.org/all/20221013222719.277923-1-stephen.s.brennan@xxxxxxxxxx/ >> >> Amir Goldstein (1): >> fsnotify: clear PARENT_WATCHED flags lazily >> >> fs/notify/fsnotify.c | 26 ++++++++++++++++++++------ >> fs/notify/fsnotify.h | 3 ++- >> fs/notify/mark.c | 32 +++++++++++++++++++++++++++++--- >> include/linux/fsnotify_backend.h | 8 +++++--- >> 4 files changed, 56 insertions(+), 13 deletions(-) >> >> -- >> 2.43.0 >>
