Re: [PATCH] btrfs: transaction: Commit transaction more frequently for BPF

[WenRuo Qu <wqu@suse.com>]

On 2019/8/6 下午11:23, Nikolay Borisov wrote:
> 
> 
> On 6.08.19 г. 11:22 ч., Qu Wenruo wrote:
>> Btrfs has btrfs_end_transaction_throttle() which could try to commit
>> transaction when needed.
>>
>> However under most cases btrfs_end_transaction_throttle() won't really
>> commit transaction, due to the hard timing requirement.
>>
>> Now introduce a new error injection point, btrfs_need_trans_pressure(),
>> to allow btrfs_should_end_transaction() to return 1 and
>> btrfs_end_transaction_throttle() to fallback to
>> btrfs_commit_transaction().
>>
>> With such more aggressive transaction commit, we can dig deeper into
>> cases like snapshot drop.
>> Now each reference drop of btrfs_drop_snapshot() will lead to a
>> transaction commit, allowing dm-logwrites to catch more details, other
>> than one big transaction dropping everything.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/transaction.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
>> index 248d535bb14d..2e758957126e 100644
>> --- a/fs/btrfs/transaction.c
>> +++ b/fs/btrfs/transaction.c
>> @@ -10,6 +10,7 @@
>>  #include <linux/pagemap.h>
>>  #include <linux/blkdev.h>
>>  #include <linux/uuid.h>
>> +#include <linux/error-injection.h>
>>  #include "ctree.h"
>>  #include "disk-io.h"
>>  #include "transaction.h"
>> @@ -781,10 +782,18 @@ void btrfs_throttle(struct btrfs_fs_info *fs_info)
>>  	wait_current_trans(fs_info);
>>  }
>>  
>> +static noinline bool btrfs_need_trans_pressure(struct btrfs_trans_handle *trans)
>> +{
>> +	return false;
>> +}
>> +ALLOW_ERROR_INJECTION(btrfs_need_trans_pressure, TRUE);
>> +
>>  static int should_end_transaction(struct btrfs_trans_handle *trans)
>>  {
>>  	struct btrfs_fs_info *fs_info = trans->fs_info;
>>  
>> +	if (btrfs_need_trans_pressure(trans))
>> +		return 1;
>>  	if (btrfs_check_space_for_delayed_refs(fs_info))
>>  		return 1;
>>  
>> @@ -845,6 +854,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>>  
>>  	btrfs_trans_release_chunk_metadata(trans);
>>  
>> +	if (throttle && btrfs_need_trans_pressure(trans))
>> +		return btrfs_commit_transaction(trans);
> 
> I'd rather have this gated behind CONFIG_BTRFS_DEBUG.

Then we need both CONFIG_BTRFS_DEBUG and CONFIG_BPF_KPROBE_OVERRIDE to
enable this feature.

And for CONFIG_BTRFS_DEBUG not enabled case, that function would not be
used at all (if you also gate the one in should_end_transaction()).

Not sure if extra hassle is really worthy for such a simple debug feature.

Thanks,
Qu
> 
> 
>>  	if (lock && READ_ONCE(cur_trans->state) == TRANS_STATE_BLOCKED) {
>>  		if (throttle)
>>  			return btrfs_commit_transaction(trans);
>>