Re: [PATCH/RFC] core/nfsd: allow kernel threads to use task_work.

[Jeff Layton]

On Tue, 2023-11-28 at 14:51 +0100, Christian Brauner wrote:
> [Reusing the trimmed Cc]
> 
> On Tue, Nov 28, 2023 at 11:16:06AM +1100, NeilBrown wrote:
> > On Tue, 28 Nov 2023, Chuck Lever wrote:
> > > On Tue, Nov 28, 2023 at 09:05:21AM +1100, NeilBrown wrote:
> > > > 
> > > > I have evidence from a customer site of 256 nfsd threads adding files to
> > > > delayed_fput_lists nearly twice as fast they are retired by a single
> > > > work-queue thread running delayed_fput().  As you might imagine this
> > > > does not end well (20 million files in the queue at the time a snapshot
> > > > was taken for analysis).
> > > > 
> > > > While this might point to a problem with the filesystem not handling the
> > > > final close efficiently, such problems should only hurt throughput, not
> > > > lead to memory exhaustion.
> > > 
> > > I have this patch queued for v6.8:
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/commit/?h=nfsd-next&id=c42661ffa58acfeaf73b932dec1e6f04ce8a98c0
> > > 
> > 
> > Thanks....
> > I think that change is good, but I don't think it addresses the problem
> > mentioned in the description, and it is not directly relevant to the
> > problem I saw ... though it is complicated.
> > 
> > The problem "workqueue ...  hogged cpu..." probably means that
> > nfsd_file_dispose_list() needs a cond_resched() call in the loop.
> > That will stop it from hogging the CPU whether it is tied to one CPU or
> > free to roam.
> > 
> > Also that work is calling filp_close() which primarily calls
> > filp_flush().
> > It also calls fput() but that does minimal work.  If there is much work
> > to do then that is offloaded to another work-item.  *That* is the
> > workitem that I had problems with.
> > 
> > The problem I saw was with an older kernel which didn't have the nfsd
> > file cache and so probably is calling filp_close more often.  So maybe
> > my patch isn't so important now.  Particularly as nfsd now isn't closing
> > most files in-task but instead offloads that to another task.  So the
> > final fput will not be handled by the nfsd task either.
> > 
> > But I think there is room for improvement.  Gathering lots of files
> > together into a list and closing them sequentially is not going to be as
> > efficient as closing them in parallel.
> > 
> > > 
> > > > For normal threads, the thread that closes the file also calls the
> > > > final fput so there is natural rate limiting preventing excessive growth
> > > > in the list of delayed fputs.  For kernel threads, and particularly for
> > > > nfsd, delayed in the final fput do not impose any throttling to prevent
> > > > the thread from closing more files.
> > > 
> > > I don't think we want to block nfsd threads waiting for files to
> > > close. Won't that be a potential denial of service?
> > 
> > Not as much as the denial of service caused by memory exhaustion due to
> > an indefinitely growing list of files waiting to be closed by a single
> > thread of workqueue.
> 
> It seems less likely that you run into memory exhausting than a DOS
> because nfsd() is busy closing fds. Especially because you default to
> single nfsd thread afaict.
> 

The default is currently 8 threads (which is ridiculously low for most
uses, but that's another discussion). That policy is usually set by
userland nfs-utils though.

This is another place where we might want to reserve a "rescuer" thread
that avoids doing work that can end up blocked. Maybe we could switch
back to queuing them to the list when we're below a certain threshold of
available threads (1? 2? 4?).

> > I think it is perfectly reasonable that when handling an NFSv4 CLOSE,
> > the nfsd thread should completely handle that request including all the
> > flush and ->release etc.  If that causes any denial of service, then
> > simple increase the number of nfsd threads.
> 
> But isn't that a significant behavioral change? So I would expect to
> make this at configurable via a module- or Kconfig option?
> 

I struggle to think about how we would document a new option like this. 

> > For NFSv3 it is more complex.  On the kernel where I saw a problem the
> > filp_close happen after each READ or WRITE (though I think the customer
> > was using NFSv4...).  With the file cache there is no thread that is
> > obviously responsible for the close.
> > To get the sort of throttling that I think is need, we could possibly
> > have each "nfsd_open" check if there are pending closes, and to wait for
> > some small amount of progress.
> > 
> > But don't think it is reasonable for the nfsd threads to take none of
> > the burden of closing files as that can result in imbalance.
> 
> It feels that this really needs to be tested under a similar workload in
> question to see whether this is a viable solution.

Definitely. I'd also like to see how this behaves with NFS or Ceph
reexport. Closing can be quite slow on those filesystems, so that might
be a good place to try and break this.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>