On Thu, May 03, 2018 at 12:26:35AM +0200, Miklos Szeredi wrote: > When multiple shrinkers are operating on a directory containing many > dentries, it takes much longer than if only one shrinker is operating on > the directory. > > Call the shrinker instances A and B, which shrink DIR containing NUM > dentries. > > Assume A wins the race for locking DIR's d_lock, then it goes onto moving > all unlinked dentries to its dispose list. When it's done, then B will > scan the directory once again, but will find that all dentries are already > being shrunk, so it will have an empty dispose list. Both A and B will > have found NUM dentries (data.found == NUM). > > Now comes the interesting part: A will proceed to shrink the dispose list > by killing individual dentries and decrementing the refcount of the parent > (which is DIR). NB: decrementing DIR's refcount will block if DIR's d_lock > is held. B will shrink a zero size list and then immediately restart > scanning the directory, where it will lock DIR's d_lock, scan the remaining > dentries and find no dentry to dispose. > > So that results in B doing the directory scan over and over again, holding > d_lock of DIR, while A is waiting for a chance to decrement refcount of DIR > and making very slow progress because of this. B is wasting time and > holding up progress of A at the same time. > > Proposed fix is to check this situation in B (found some dentries, but > all are being shrunk already) and just sleep for some time, before retrying > the scan. The sleep is proportional to the number of found dentries. The thing is, the majority of massive shrink_dcache_parent() can be killed. Let's do that first and see if anything else is really needed. As it is, rmdir() and rename() are ridiculously bad - they should only call shrink_dcache_parent() after successful ->rmdir() or ->rename(). Sure, there are other places where we do large shrink_dcache_parent() runs, but those won't trigger in parallel on the same tree. IOW, let's wait adding complexity until we fix the sources of those calls.
