Re: nfs: avoid i_lock contention in nfs_clear_invalid_mapping

[Anna Schumaker <anna.schumaker@oracle.com>]

On 10/30/24 4:03 PM, Mike Snitzer wrote:
> 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?

Oops! It looks like `b4` grabbed v1 of the patch from the mailing list instead of 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).

v2 does fix the issues I was seeing. Sorry about the noise!

Anna

> 
> Thanks,
> Mike