On Thu, Apr 11, 2013 at 02:35:12PM -0400, Theodore Ts'o wrote: > On Thu, Apr 11, 2013 at 06:04:02PM +0100, Mel Gorman wrote: > > > If we're stalling on lock_buffer(), that implies that buffer was being > > > written, and for some reason it was taking a very long time to > > > complete. > > > > > > > Yes. > > > > > It might be worthwhile to put a timestamp in struct dm_crypt_io, and > > > record the time when a particular I/O encryption/decryption is getting > > > queued to the kcryptd workqueues, and when they finally squirt out. > > > > > > > That somewhat assumes that dm_crypt was at fault which is not unreasonable > > but I was skeptical as the workload on dm_crypt was opening a maildir > > and mostly reads. > > Hmm... well, I've reviewed all of the places in the ext4 and jbd2 > layer where we call lock_buffer(), and with one exception[1] we're not > holding the the bh locked any longer than necessary. There are a few > places where we grab a spinlock or two before we can do what we need > to do and then release the lock'ed buffer head, but the only time we > hold the bh locked for long periods of time is when we submit metadata > blocks for I/O. > Yeah, ok. This is not the answer I was hoping for but it's the answer I expected. > Could you code which checks the hold time of lock_buffer(), measuing > from when the lock is successfully grabbed, to see if you can see if I > missed some code path in ext4 or jbd2 where the bh is locked and then > there is some call to some function which needs to block for some > random reason? > > What I'd suggest is putting a timestamp in buffer_head > structure, which is set by lock_buffer once it is successfully grabbed > the lock, and then in unlock_buffer(), if it is held for more than a > second or some such, to dump out the stack trace. > I can do that but the results might lack meaning. What I could do instead is use a variation of the page owner tracking patch (current iteration at https://lkml.org/lkml/2012/12/7/487) to record a stack trace in lock_buffer and dump it from jbd2/transaction.c if it stalls for too long. I'll report if I find something useful. > Because at this point, either I'm missing something or I'm beginning > to suspect that your hard drive (or maybe something the block layer?) > is simply taking a long time to service an I/O request. It could be because the drive is a piece of crap. -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
