Re: [PATCH v2 3/4] fs/dcache: Enable automatic pruning of negative dentries

[Waiman Long <longman@redhat.com>]

On 07/21/2017 03:30 PM, James Bottomley wrote:
> On Fri, 2017-07-21 at 09:43 -0400, Waiman Long wrote:
>> Having a limit for the number of negative dentries does have an
>> undesirable side effect that no new negative dentries will be allowed
>> when the limit is reached. This will have performance implication
>> for some types of workloads.
> This really seems like a significant problem: negative dentries should
> be released in strict lru order because the chances are no-one cares
> about the least recently used one, but they may care about having the
> most recently created one.

This should not happen under normal circumstances as the asynchronous
shrinker should be able to keep enough free negative dentry available in
the pool that direct negative dentry killing will rarely happen.

> [...]
>> @@ -323,6 +329,16 @@ static void __neg_dentry_inc(struct dentry
>> *dentry)
>>  	 */
>>  	if (!cnt)
>>  		dentry->d_flags |= DCACHE_KILL_NEGATIVE;
>> +
>> +	/*
>> +	 * Initiate negative dentry pruning if free pool has less
>> than
>> +	 * 1/4 of its initial value.
>> +	 */
>> +	if (READ_ONCE(ndblk.nfree) < neg_dentry_nfree_init/4) {
>> +		WRITE_ONCE(ndblk.prune_sb, dentry->d_sb);
>> +		schedule_delayed_work(&prune_neg_dentry_work,
>> +				      NEG_PRUNING_DELAY);
>> +	}
> So here, why not run the negative dentry shrinker synchronously to see
> if we can shrink the cache and avoid killing the current negative
> dentry.  If there are context problems doing that, we should at least
> make the effort to track down the least recently used negative dentry
> and mark that for killing instead.

Only one CPU will be calling the asynchronous shrinker. So its effect on
the overall performance of the system should be negligible.

Allowing all CPUs to potentially do synchronous shrinking can cause a
lot of lock and cacheline contention. I will look further to see if
there is opportunity to do some optimistic synchronous shrinking. If
that fails because of a contended lock, for example, we will need to
fall back to killing the dentry. That should only happen under the worst
case situation, like when a malicious process is running.

Cheers,
Longman