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>> >> I see long delays with fewer than 1024 items on the list. I might >> drop this number by one or two orders of magnitude. And make it a >> symbolic constant. > > In that case I seriously wonder if this is where the delays are coming > from. > > nfsd_file_dispose_list_delayed() does take and drop a spinlock > repeatedly (though it may not always be the same lock) and call > svc_wake_up() repeatedly - although the head of the queue might already > be woken. We could optimise that to detect runs with the same nn and > only take the lock once, and only wake_up once. > >> >> There's another naked integer (8) in nfsd_file_net_dispose() -- how does >> that relate to this new cap? Should that also be a symbolic constant? > > I don't think they relate. > The trade-off with "8" is: > a bigger number might block an nfsd thread for longer, > forcing serialising when the work can usefully be done in parallel. > a smaller number might needlessly wake lots of threads > to share out a tiny amount of work. > > The 1024 is simply about "don't hold a spinlock for too long". By that, I think you mean list_lru_walk() takes &l->lock for the duration of the scan? For a long scan, that would effectively block adding or removing LRU items for quite some time. So here's a typical excerpt from a common test: kworker/u80:7-206 [003] 266.985735: nfsd_file_unhash: ... kworker/u80:7-206 [003] 266.987723: nfsd_file_gc_removed: 1309 entries removed, 2972 remaining nfsd-1532 [015] 266.988626: nfsd_file_free: ... Here, the nfsd_file_unhash record marks the beginning of the LRU walk, and the nfsd_file_gc_removed record marks the end. The timestamps indicate the walk took two milliseconds. The nfsd_file_free record above marks the last disposal activity. That takes almost a millisecond, but as far as I can tell, it does not hold any locks for long. This seems to me like a strong argument for cutting the scan size down to no more than 32-64 items. Ideally spin locks are supposed to be held only for simple operations (eg, list_add); this seems a little outside that window (hence your remark that "a large nr_to_walk is always a bad idea" -- I now see what you meant). IMHO the patch description should single out that purpose: We want to significantly reduce the maximum amount of time that list_lru_walk() blocks foreground LRU activity such as list_lru_add_obj(). The cond_resched() in this case might be gravy. >>> + ret = list_lru_walk(&nfsd_file_lru, nfsd_file_lru_cb, >>> + &dispose, num_to_scan); >>> + trace_nfsd_file_gc_removed(ret, list_lru_count(&nfsd_file_lru)); >>> + nfsd_file_dispose_list_delayed(&dispose); >> >> I need to go back and review the function traces to see where the >> delays add up -- to make sure rescheduling here, rather than at some >> other point, is appropriate. It probably is, but my memory fails me >> these days. > > I would like to see those function traces too. Here's my reproducer: 1. On the client: Set up xfstests to use NFSv3 2. On the server: "sudo trace-cmd record -e nfsd &" 3. On the client: Run xfstests with "sudo ./check -nfs generic/750" -- Chuck Lever