On Wed, 15 Apr 2015 13:26:51 -0600 Jens Axboe <axboe@xxxxxx> wrote: > > Is there similar impact to direct-io-to-file? It would be nice to fix > > that up also. Many filesystems do something along the lines of > > > > atomic_inc(i_dio_count); > > wibble() > > atomic_dev(i_dio_count); > > __blockdev_direct_IO(...); > > > > and with your patch I think we could change them to > > > > atomic_inc(i_dio_count); > > wibble() > > __blockdev_direct_IO(..., flags|DIO_IGNORE_TRUNCATE); > > atomic_dev(i_dio_count); > > > > which would halve the atomic op load. > > I haven't checked pure file, but without extending, I suspect that we > should see similar benefits there. In any case, it'd make sense doing, > having twice the atomic inc/dec is just a bad idea in general, if we can > get rid of it. > > A quick grep doesn't show this use case, or I'm just blind. Where do you > see that? btrfs_direct_IO() holds i_dio_count across its call to __blockdev_direct_IO() for writes. That makes the dio_bio_count manipulation in do_blockdev_direct_IO() unneeded? ext4 is similar. Reducing from 4 ops to 2 probably won't make as much difference as reducing from 2 to 0 - most of the cost comes from initially grabbing that cacheline from a different CPU. > > But that's piling hack on top of hack. Can we change the > > I'd more view it as reducing the hack, the real hack is the way that we > manually do atomic_inc() on i_dio_count, and then call a magic > inode_dio_done() that decrements it again. It's not very pretty, I'm > just reducing the scope of the hack :-) A magic flag which says "you don't need to do this in that case because I know this inode is special". direct-io already has too much of this :( > > do_blockdev_direct_IO() interface to "caller shall hold i_mutex, or > > increment i_dio_count"? ie: exclusion against truncate is wholly the > > caller's responsibility. That way, this awkward sharing of > > responsibility between caller and callee gets cleaned up and > > DIO_IGNORE_TRUNCATE goes away. > > That would clean it up, at the expense of touching more churn. I'd be > fine with doing it that way. OK, could be done later I suppose. > > inode_dio_begin() would be a good place to assert that i_mutex is held, > > btw. > > How would you check it? If the i_dio_count is bumped, then you'd not > need to hold i_mutex. if (atomic_add_return() == 1) assert() I guess. It was just a thought. Having wandered around the code, I'm not 100% confident that everyone is holding i_mutex - it's not all obviously correct. otoh, the caller doesn't *have* to choose i_mutex for the external exclusion, and perhaps some callers have used something else. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
