Re: nfs: avoid i_lock contention in nfs_clear_invalid_mapping

[Mike Snitzer <snitzer@kernel.org>]

On Fri, Oct 18, 2024 at 03:49:50PM -0400, Mike Snitzer wrote:
> On Fri, Oct 18, 2024 at 03:39:13PM -0400, Jeff Layton wrote:
> > On Fri, 2024-10-18 at 13:03 -0400, Mike Snitzer wrote:
> > > Multi-threaded buffered reads to the same file exposed significant
> > > inode spinlock contention in nfs_clear_invalid_mapping().
> > > 
> > > Eliminate this spinlock contention by checking flags without locking,
> > > instead using smp_rmb and smp_load_acquire accordingly, but then take
> > > spinlock and double-check these inode flags.
> > > 
> > > Also refactor nfs_set_cache_invalid() slightly to use
> > > smp_store_release() to pair with nfs_clear_invalid_mapping()'s
> > > smp_load_acquire().
> > > 
> > > While this fix is beneficial for all multi-threaded buffered reads
> > > issued by an NFS client, this issue was identified in the context of
> > > surprisingly low LOCALIO performance with 4K multi-threaded buffered
> > > read IO.  This fix dramatically speeds up LOCALIO performance:
> > > 
> > > before: read: IOPS=1583k, BW=6182MiB/s (6482MB/s)(121GiB/20002msec)
> > > after:  read: IOPS=3046k, BW=11.6GiB/s (12.5GB/s)(232GiB/20001msec)
> > > 
> > > Fixes: 17dfeb911339 ("NFS: Fix races in nfs_revalidate_mapping")
> > > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > > ---
> > >  fs/nfs/inode.c | 19 ++++++++++++++-----
> > >  1 file changed, 14 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
> > > index 542c7d97b235..130d7226b12a 100644
> > > --- a/fs/nfs/inode.c
> > > +++ b/fs/nfs/inode.c
> > > @@ -205,12 +205,14 @@ void nfs_set_cache_invalid(struct inode *inode, unsigned long flags)
> > >  		nfs_fscache_invalidate(inode, 0);
> > >  	flags &= ~NFS_INO_REVAL_FORCED;
> > >  
> > > -	nfsi->cache_validity |= flags;
> > > +	if (inode->i_mapping->nrpages == 0)
> > > +		flags &= ~NFS_INO_INVALID_DATA;
> > >  
> > > -	if (inode->i_mapping->nrpages == 0) {
> > > -		nfsi->cache_validity &= ~NFS_INO_INVALID_DATA;
> > > -		nfs_ooo_clear(nfsi);
> > > -	} else if (nfsi->cache_validity & NFS_INO_INVALID_DATA) {
> > > +	/* pairs with nfs_clear_invalid_mapping()'s smp_load_acquire() */
> > > +	smp_store_release(&nfsi->cache_validity, flags);
> > > +
> > 
> > I don't know this code that well, but it used to do an |= of flags into
> > cache_validity. Now you're replacing cache_validity wholesale with
> > flags. Maybe that should do something like this?
> > 
> >     flags |= nfsi->cache_validity;
> >     smp_store_release(&nfsi->cache_validity, flags);
> 
> Ah good catch, sorry about that, will fix.
> 
> This will allow further cleanup too, will let v2 speak to that.

I just posted v2, but for completeness: I decided to leave well enough
alone and not do any further cleanup.

(my thinking was relative to nfs_ooo_clear, and possibly moving it
before the smp_store_release, so that nfsi->cache_validity only
changed once using smp_store_release.  The interlock between
nfs_set_cache_invalid and nfs_clear_invalid_mapping is focused on
NFS_INO_INVALID_DATA, which nfs_ooo_clear doesn't touch).