Re: [PATCH 4/6] nfsd: filecache: introduce NFSD_FILE_RECENT

Jeff Layton
2025-02-11T11:38:32Z

On Tue, 2025-02-11 at 10:57 +1100, Dave Chinner wrote:
> On Mon, Feb 10, 2025 at 09:01:41AM -0500, Jeff Layton wrote:
> > On Fri, 2025-02-07 at 16:15 +1100, NeilBrown wrote:
> > > The filecache lru is walked in 2 circumstances for 2 different reasons.
> > >
> > > 1/ When called from the shrinker we want to discard the first few
> > > entries on the list, ignoring any with NFSD_FILE_REFERENCED set
> > > because they should really be at the end of the LRU as they have been
> > > referenced recently. So those ones are ROTATED.
> > >
> > > 2/ When called from the nfsd_file_gc() timer function we want to discard
> > > anything that hasn't been used since before the previous call, and
> > > mark everything else as unused at this point in time.
> > >
> > > Using the same flag for both of these can result in some unexpected
> > > outcomes. If the shrinker callback clears NFSD_FILE_REFERENCED then the
> > > nfsd_file_gc() will think the file hasn't been used in a while, while
> > > really it has.
> > >
> > > I think it is easier to reason about the behaviour if we instead have
> > > two flags.
> > >
> > > NFSD_FILE_REFERENCED means "this should be at the end of the LRU, please
> > > put it there when convenient"
> > > NFSD_FILE_RECENT means "this has been used recently - since the last
> > > run of nfsd_file_gc()
> > >
> > > When either caller finds an NFSD_FILE_REFERENCED entry, that entry
> > > should be moved to the end of the LRU and the flag cleared. This can
> > > safely happen at any time. The actual order on the lru might not be
> > > strictly least-recently-used, but that is normal for linux lrus.
> > >
> > > The shrinker callback can ignore the "recent" flag. If it ends up
> > > freeing something that is "recent" that simply means that memory
> > > pressure is sufficient to limit the acceptable cache age to less than
> > > the nfsd_file_gc frequency.
> > >
> > > The gc caller should primarily focus on NFSD_FILE_RECENT. It should
> > > free everything that doesn't have this flag set, and should clear the
> > > flag on everything else. When it clears the flag it is convenient to
> > > clear the "REFERENCED" flag and move to the end of the LRU too.
> > >
> > > With this, calls from the shrinker do not prematurely age files. It
> > > will focus only on freeing those that are least recently used.
> > >
> > > Signed-off-by: NeilBrown <neilb@suse.de>
> > > ---
> > > fs/nfsd/filecache.c | 21 +++++++++++++++++++--
> > > fs/nfsd/filecache.h | 1 +
> > > fs/nfsd/trace.h | 3 +++
> > > 3 files changed, 23 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > index 04588c03bdfe..9faf469354a5 100644
> > > --- a/fs/nfsd/filecache.c
> > > +++ b/fs/nfsd/filecache.c
> > > @@ -318,10 +318,10 @@ nfsd_file_check_writeback(struct nfsd_file *nf)
> > > mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK);
> > > }
> > >
> > > -
> > > static bool nfsd_file_lru_add(struct nfsd_file *nf)
> > > {
> > > set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
> > > + set_bit(NFSD_FILE_RECENT, &nf->nf_flags);
> >
> > Technically, I don't think you need the REFERENCED bit at all. This is
> > the only place it's set, and below this is calling list_lru_add_obj().
> > That returns false if the object was already on a per-node LRU.
> >
> > Instead of that, you could add a list_lru helper that will rotate the
> > object to the end of its nodelist if it's already on one. OTOH, that
> > might mean more cross NUMA-node accesses to the spinlocks than we get
> > by using a flag and doing this at GC time.
>
> No, please don't.
>
> Per-object reference bits are required to enable lazy LRU rotation.
> The LRU lists are -hot- objects; touching them every time we touch
> an object on the LRU is prohibitively expensive because of exclusive
> lock/cacheline contention. Hence we defer operations like rotation
> to a context where we already have the list locked and cached
> exclusively for some other reason (i.e. memory reclaim).
>
> This is the same reason we use lazy removal from LRUs - it avoids
> LRU list manipulations every time a hot cached object is accessed
> and/or dropped.
>
> IOWs, removing the per-object NFSD_FILE_REFERENCED bit will undo one
> of the necessary the optimisations that allow hot caches LRU
> management to work efficiently with minimal overhead.
>

Yep, that was the point of my "OTOH" comment. Keeping the REFERENCED
flag is better from a "let's minimize cacheline invalidations"
standpoint.
--
Jeff Layton <jlayton@kernel.org>