Re: nfs: avoid i_lock contention in nfs_clear_invalid_mapping

[Mike Snitzer <snitzer@kernel.org>]

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.

Thanks!
Mike