Re: [PATCH v5 2/8] btrfs: only let one thread pre-flush delayed refs in commit

[Nikolay Borisov <nborisov@suse.com>]

On 8.01.21 г. 18:01 ч., David Sterba wrote:
> On Fri, Dec 18, 2020 at 02:24:20PM -0500, Josef Bacik wrote:
>> I've been running a stress test that runs 20 workers in their own
>> subvolume, which are running an fsstress instance with 4 threads per
>> worker, which is 80 total fsstress threads.  In addition to this I'm
>> running balance in the background as well as creating and deleting
>> snapshots.  This test takes around 12 hours to run normally, going
>> slower and slower as the test goes on.
>>
>> The reason for this is because fsstress is running fsync sometimes, and
>> because we're messing with block groups we often fall through to
>> btrfs_commit_transaction, so will often have 20-30 threads all calling
>> btrfs_commit_transaction at the same time.
>>
>> These all get stuck contending on the extent tree while they try to run
>> delayed refs during the initial part of the commit.
>>
>> This is suboptimal, really because the extent tree is a single point of
>> failure we only want one thread acting on that tree at once to reduce
>> lock contention.  Fix this by making the flushing mechanism a bit
>> operation, to make it easy to use test_and_set_bit() in order to make
>> sure only one task does this initial flush.
>>
>> Once we're into the transaction commit we only have one thread doing
>> delayed ref running, it's just this initial pre-flush that is
>> problematic.  With this patch my stress test takes around 90 minutes to
>> run, instead of 12 hours.
>>
>> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>> ---
>>  fs/btrfs/delayed-ref.h | 12 ++++++------
>>  fs/btrfs/transaction.c | 33 ++++++++++++++++-----------------
>>  2 files changed, 22 insertions(+), 23 deletions(-)
>>
>> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
>> index 1c977e6d45dc..6e414785b56f 100644
>> --- a/fs/btrfs/delayed-ref.h
>> +++ b/fs/btrfs/delayed-ref.h
>> @@ -135,6 +135,11 @@ struct btrfs_delayed_data_ref {
>>  	u64 offset;
>>  };
>>  
>> +enum btrfs_delayed_ref_flags {
>> +	/* Used to indicate that we are flushing delayed refs for the commit. */
>> +	BTRFS_DELAYED_REFS_FLUSHING,
>> +};
>> +
>>  struct btrfs_delayed_ref_root {
>>  	/* head ref rbtree */
>>  	struct rb_root_cached href_root;
>> @@ -158,12 +163,7 @@ struct btrfs_delayed_ref_root {
>>  
>>  	u64 pending_csums;
>>  
>> -	/*
>> -	 * set when the tree is flushing before a transaction commit,
>> -	 * used by the throttling code to decide if new updates need
>> -	 * to be run right away
>> -	 */
>> -	int flushing;
>> +	unsigned long flags;
>>  
>>  	u64 run_delayed_start;
>>  
>> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
>> index f51f9e39bcee..ccd37fbe5db1 100644
>> --- a/fs/btrfs/transaction.c
>> +++ b/fs/btrfs/transaction.c
>> @@ -909,9 +909,9 @@ bool btrfs_should_end_transaction(struct btrfs_trans_handle *trans)
>>  {
>>  	struct btrfs_transaction *cur_trans = trans->transaction;
>>  
>> -	smp_mb();
>>  	if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
>> -	    cur_trans->delayed_refs.flushing)
>> +	    test_bit(BTRFS_DELAYED_REFS_FLUSHING,
>> +		     &cur_trans->delayed_refs.flags))
>>  		return true;
>>  
>>  	return should_end_transaction(trans);
>> @@ -2043,23 +2043,22 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
>>  	btrfs_trans_release_metadata(trans);
>>  	trans->block_rsv = NULL;
>>  
>> -	/* make a pass through all the delayed refs we have so far
>> -	 * any runnings procs may add more while we are here
>> -	 */
>> -	ret = btrfs_run_delayed_refs(trans, 0);
>> -	if (ret) {
>> -		btrfs_end_transaction(trans);
>> -		return ret;
>> -	}
>> -
>> -	cur_trans = trans->transaction;
>> -
>>  	/*
>> -	 * set the flushing flag so procs in this transaction have to
>> -	 * start sending their work down.
>> +	 * We only want one transaction commit doing the flushing so we do not
>> +	 * waste a bunch of time on lock contention on the extent root node.
>>  	 */
>> -	cur_trans->delayed_refs.flushing = 1;
>> -	smp_wmb();
> 
> This barrier obviously separates the flushing = 1 and the rest of the
> code, now implemented as test_and_set_bit, which implies full barrier.
> 
> However, hunk in btrfs_should_end_transaction removes the barrier and
> I'm not sure whether this is correct:
> 
> -	smp_mb();
>  	if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
> -	    cur_trans->delayed_refs.flushing)
> +	    test_bit(BTRFS_DELAYED_REFS_FLUSHING,
> +		     &cur_trans->delayed_refs.flags))
>  		return true;
> 
> This is never called under locks so we don't have complete
> synchronization of neither the transaction state nor the flushing bit.
> btrfs_should_end_transaction is merely a hint and not called in critical
> places so we could probably afford to keep it without a barrier, or keep
> it with comment(s).

I think the point is moot in this case, because the test_bit either sees
the flag or it doesn't. It's not possible for the flag to be set AND
should_end_transaction return false that would be gross violation of
program correctness.

>