[LTP] [RFC] Slow down of LTP tests aiodio_sparse.c and dio_sparse.c in kernel 6.6

[Petr Vorel]

Hi all,


[ Cc Peter and Mike ]
> Hi!

> On Fri 19-07-24 19:43:25, Petr Vorel wrote:
> > LTP AIO DIO tests aiodio_sparse.c [1] and dio_sparse.c [2] (using [3])
> > slowed down on kernel 6.6 on Btrfs and XFS, when run with default
> > parameters. These tests create 100 MB sparse file and write zeros (using
> > libaio or O_DIRECT) while 16 other processes reads the buffer and check
> > only zero is there.

> So the performance of this test is irrelevant because combining buffered
> reads with direct IO writes was always in "better don't do it" territory.
> Definitely not if you care about perfomance.

> > Runtime of this particular setup (i.e. 100 MB file) on Btrfs and XFS on the
> > same system slowed down 9x (6.5: ~1 min 6.6: ~9 min). Ext4 is not affected.
> > (Non default parameter creates much smaller file, thus the change is not that
> > obvious).

> But still it's kind of curious what caused the 9x slow down. So I'd be
> curious to know the result of the bisection :). Thanks for report!

It looks to be the slowdown was introduced by commit 63304558ba5d
("sched/eevdf: Curb wakeup-preemption") [1] from v6.6-rc1.

I also compiled current next (next-20240724), it's also slow  and reverting
commit from it returns the original speed on both Btrfs and XFS.

Kind regards,
Petr

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=63304558ba5dcaaff9e052ee43cfdcc7f9c29e85

> 								Honza