Re: nfs: avoid i_lock contention in nfs_clear_invalid_mapping

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Oct 30, 2024 at 03:51:44PM -0400, Anna Schumaker wrote:
> Hi Mike,
> 
> On 10/18/24 5:37 PM, Mike Snitzer wrote:
> > 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'm having some issues with non-localio NFS after applying this patch:
> 
> - cthon basic tests fail with NFS v3
> - cthon general tests fail with NFS v4.1 and v4.2
> - xfstests generic/080, generic/472, generic/615, and generic/633 fail with NFS v4.1 and v4.2
> - xfstests generic/683, and generic/684 fail with NFS v4.2
> 
> I think the problem is the call to smp_store_release(). It's overwriting nfsi->cache_validity
> with the value of 'flags', losing anything set there but not in 'flags'. Could we instead do
> something like:
>    smp_store_release(&nfsi->cache_validity, nfsi->cache_validity | flags)
> ?
> 
> Anna

Hi,

v2 addressed this issue like Jeff suggested with:

> >>>
> >>>     flags |= nfsi->cache_validity;
> >>>     smp_store_release(&nfsi->cache_validity, flags);
> >>
> >> Ah good catch, sorry about that, will fix.

I think you must not be using v2?

https://lore.kernel.org/all/20241018211541.42705-1-snitzer@kernel.org/

Jeff also provided his Reviewed-by for v2.

If you are using v2 that'll be weird (because I'm not seeing any
issues with xfstests, etc).

Thanks,
Mike