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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2023-11-28 at 10:34 -0500, Chuck Lever wrote:
> On Tue, Nov 28, 2023 at 01:57:30PM +1100, NeilBrown wrote:
> > 
> > (trimmed cc...)
> > 
> > On Tue, 28 Nov 2023, Chuck Lever wrote:
> > > 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.
> > > 
> > > Without the file cache, the filp_close() should be handled directly
> > > by the nfsd thread handling the RPC, IIRC.
> > 
> > Yes - but __fput() is handled by a workqueue.
> > 
> > > 
> > > 
> > > > 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.
> > > 
> > > I believe the file cache passes the filps to the work queue one at
> > 
> > nfsd_file_close_inode() does.  nfsd_file_gc() and nfsd_file_lru_scan()
> > can pass multiple.
> > 
> > > a time, but I don't think there's anything that forces the work
> > > queue to handle each flush/close completely before proceeding to the
> > > next.
> > 
> > Parallelism with workqueues is controlled by the work items (struct
> > work_struct).  Two different work items can run in parallel.  But any
> > given work item can never run parallel to itself.
> > 
> > The only work items queued on nfsd_filecache_wq are from
> >   nn->fcache_disposal->work.
> > There is one of these for each network namespace.  So in any given
> > network namespace, all work on nfsd_filecache_wq is fully serialised.
> 
> OIC, it's that specific case you are concerned with. The per-
> namespace laundrette was added by:
> 
>   9542e6a643fc ("nfsd: Containerise filecache laundrette")
> 
> It's purpose was to confine the close backlog to each container.
> 
> Seems like it would be better if there was a struct work_struct
> in each struct nfsd_file. That wouldn't add real backpressure to
> nfsd threads, but it would enable file closes to run in parallel.
> 

I like this idea. That seems a lot simpler than all of this weirdo
queueing of delayed closes that we do.

-- 
Jeff Layton <jlayton@kernel.org>