Re: [viro-vfs:work.dcache2] [__dentry_kill()] 1b738f196e: stress-ng.sysinfo.ops_per_sec -27.2% regression

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Dec 01, 2023 at 04:09:51AM +0000, Al Viro wrote:
> On Fri, Dec 01, 2023 at 10:13:09AM +0800, Oliver Sang wrote:
> 
> > > Very interesting...  Out of curiosity, what effect would the following
> > > have on top of 1b738f196e?
> > 
> > I applied the patch upon 1b738f196e (as below fec356fd0c), but seems less
> > useful.
> 
> I would be rather surprised if it fixed anything; it's just that 1b738f196e
> changes two things - locking rules for __dentry_kill() and, in some cases,
> the order of dentry eviction in shrink_dentry_list().  That delta on top of
> it restores the original order in shrink_dentry_list(), leaving pretty much
> the changes in lock_for_kill()/dput()/__dentry_kill().
> 
> Interesting...  Looks like there are serious changes in context switch
> frequencies, but I don't see where could that have come from...

In principle it could be an effect of enforcing the ordering between __dentry_kill()
of child and parent, but if that's what is going on... we would've seen
more iterations of loop in shrink_dcache_parent() and/or d_walk() calls in
it having more work to do.  But... had that been what's going on, wouldn't we
see some of those functions in the changed part of profile?  

I'll try to split that thing into a series of steps, so we could at least narrow
the effect down, but that'll have to wait until tomorrow ;-/