Re: [PATCH/RFC] VFS: Improve fairness when locking the per-superblock s_anon list

[NeilBrown]

On Tue, Feb 02 2016, J. Bruce Fields wrote:

> On Fri, Jan 29, 2016 at 11:17:43AM +1100, NeilBrown wrote:
>> bit-spin-locks, as used for dcache hash chains, are not fair.
>> This is not a problem for the dcache hash table as different CPUs are
>> likely to access different entries in the hash table so high contention
>> is not expected.
>> However anonymous dentryies (created by NFSD) all live on a single hash
>> chain "s_anon" and the bitlock on this can be highly contended, resulting
>> in soft-lockup warnings.
>
> Just out of curiosity, because I can't recall seeing complaints about
> warnings, when do you see it happen?  Server reboot, maybe?

Soft-lockup warnings.  Possibly some client might notice delays longer
than they should be, but the only actual complaints have been about the warnings.

>
> It should be hitting that __d_obtain_alias() case only when a filehandle
> lookup finds a file without a cached dentry, which is an important case
> to handle, but not normally what I'd expect to be the common case.  Am I
> forgetting something?

I don't think you are missing anything significant.  I too was somewhat
surprised that there would be enough contention to cause problems, but
the evidence was fairly conclusive (at two separate sites), and the
proposed fix made the symptoms disappear.

Maybe there are a great many different files being accessed and a lot of
memory pressure on the server keeps pushing them out of cache.  I find
that customers often have loads that have quite different from what I
might expect...

Thanks,
NeilBrown


>
> --b.
>
>> 
>> So introduce a global (fair) spinlock and take it before grabing the
>> bitlock on s_anon.  This provides fairness and makes the warnings go away.
>> 
>> We could alternately use s_inode_list_lock, or add another spinlock
>> to struct super_block.  Suggestions?
>> 
>> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
>> ---
>> 
>> Dave: I'd guess you would be against using the new s_inode_list_lock
>> for this because it is already highly contended - yes?
>> Is it worth adding another spinlock to 'struct super_block' ?
>> 
>> Thanks,
>> NeilBrown
>> 
>> 
>>  fs/dcache.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>> 
>> diff --git a/fs/dcache.c b/fs/dcache.c
>> index 92d5140de851..e83f1ac1540c 100644
>> --- a/fs/dcache.c
>> +++ b/fs/dcache.c
>> @@ -104,6 +104,8 @@ static unsigned int d_hash_shift __read_mostly;
>>  
>>  static struct hlist_bl_head *dentry_hashtable __read_mostly;
>>  
>> +static DEFINE_SPINLOCK(s_anon_lock);
>> +
>>  static inline struct hlist_bl_head *d_hash(const struct dentry *parent,
>>  					unsigned int hash)
>>  {
>> @@ -490,10 +492,14 @@ void __d_drop(struct dentry *dentry)
>>  		else
>>  			b = d_hash(dentry->d_parent, dentry->d_name.hash);
>>  
>> +		if (b == &dentry->d_sb->s_anon)
>> +			spin_lock(&s_anon_lock);
>>  		hlist_bl_lock(b);
>>  		__hlist_bl_del(&dentry->d_hash);
>>  		dentry->d_hash.pprev = NULL;
>>  		hlist_bl_unlock(b);
>> +		if (b == &dentry->d_sb->s_anon)
>> +			spin_unlock(&s_anon_lock);
>>  		dentry_rcuwalk_invalidate(dentry);
>>  	}
>>  }
>> @@ -1978,9 +1984,11 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected)
>>  	spin_lock(&tmp->d_lock);
>>  	__d_set_inode_and_type(tmp, inode, add_flags);
>>  	hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry);
>> +	spin_lock(&s_anon_lock);
>>  	hlist_bl_lock(&tmp->d_sb->s_anon);
>>  	hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon);
>>  	hlist_bl_unlock(&tmp->d_sb->s_anon);
>> +	spin_unlock(&s_anon_lock);
>>  	spin_unlock(&tmp->d_lock);
>>  	spin_unlock(&inode->i_lock);
>>  	security_d_instantiate(tmp, inode);
>> -- 
>> 2.7.0
>>

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