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. > > IOW there is some parallelism there already, especially now that > > nfsd_filecache_wq is UNBOUND. > > No there is not. And UNBOUND makes no difference to parallelism in this > case. It allows the one work item to migrate between CPUs while it is > running, but it doesn't allow it to run concurrently on two different > CPUs. Right. The laundrette can now run in parallel with other work by moving to a different core, but there still can be only one laundrette running per namespace. > (UNBOUND can improve parallelism when multiple different work items are > submitted all from the same CPU. Without UNBOUND all the work would > happen on the same CPU, though if the work sleeps, the different work > items can be interleaved. With UNBOUND the different work items can > enjoy true parallelism when needed). > > > > > > > > > > > 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. > > > > The cache garbage collector is single-threaded, but nfsd_filecache_wq > > has a max_active setting of zero. > > This allows parallelism between network namespaces, but not within a > network namespace. > > > > > > > > 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. > > > > > > 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. > > > > Well nfsd_open() in particular appears to be used only for readdir. > > > > But maybe nfsd_file_acquire() could wait briefly, in the garbage- > > collected case, if the nfsd_net's disposal queue is long. > > > > > > > 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. > > > > > > I'll need to give this more thought. > > > > > > -- > > Chuck Lever > > > > Thanks, > NeilBrown -- Chuck Lever
