[PATCH intro] nfsd: add scheduling point in nfsd_file_gc()

[NeilBrown <neilb@suse.de>]

This patch is intended to fix the problem:
    However, I've seen the laundrette running for multiple milliseconds
    on some workloads, delaying other work. 

reported by Chuck in 
   [PATCH v1 2/2] NFSD: Change the filecache laundrette workqueue again

I believe this problem is mostly likely caused by a lack of scheduling
points in nfsd_file_gc() so this patches adds them as needed.

On reflecting, I think that the approach here is wrong but that would
need a bigger fix.

We generally expect a given file to be used repeatedly for a while, and
then for accesses to stop when the client closes the file.  The
nfsd_file_gc() call happens every two seconds with the aim of discarding
all the files that haven't been used since two seconds ago.

To do this, it scans all the files currently on the list - many of which
are likely to have been used recently.  This seems like a waste of
effort.

I think it would be better to have two lists. A and B.
When refcount reaches zero for a GC file, it is added to A.
When the refcount is incremented we removed from wherever it is.
Every 2 seconds we free everything on B and then splice A across to B.

This completely avoids walking through all the still-active files,
moving them to the end of the LRU.

However this would make the shrinker a little more complex as we
wouldn't be able to use list_lru.  So I'm not proposing that immediately
but would like to know what others think first.

Thanks,
NeilBrown




 [PATCH] nfsd: add scheduling point in nfsd_file_gc()