On Wed, May 19, 2021 at 09:59:29AM +0200, Carlos Maiolino wrote: > On Wed, May 19, 2021 at 11:19:20AM +1000, Dave Chinner wrote: > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > Because this happens at high thread counts on high IOPS devices > > doing mixed read/write AIO-DIO to a single file at about a million > > iops: > > > > 64.09% 0.21% [kernel] [k] io_submit_one > > - 63.87% io_submit_one > > - 44.33% aio_write > > - 42.70% xfs_file_write_iter > > - 41.32% xfs_file_dio_write_aligned > > - 25.51% xfs_file_write_checks > > - 21.60% _raw_spin_lock > > - 21.59% do_raw_spin_lock > > - 19.70% __pv_queued_spin_lock_slowpath > > > > This also happens of the IO completion IO path: > > > > 22.89% 0.69% [kernel] [k] xfs_dio_write_end_io > > - 22.49% xfs_dio_write_end_io > > - 21.79% _raw_spin_lock > > - 20.97% do_raw_spin_lock > > - 20.10% __pv_queued_spin_lock_slowpath ▒ > > > > IOWs, fio is burning ~14 whole CPUs on this spin lock. > > > > So, do an unlocked check against inode size first, then if we are > > at/beyond EOF, take the spinlock and recheck. This makes the > > spinlock disappear from the overwrite fastpath. > > > > I'd like to report that fixing this makes things go faster. > > maybe you meant this does not make things go faster? Yes, that is what this statement means. That is, I'd -like- to report that things went faster, but reality doesn't care about what I'd -like- to have happen, as the next sentence explained... :( > > It > > doesn't - it just exposes the the XFS_ILOCK as the next severe > > contention point doing extent mapping lookups, and that now burns > > all the 14 CPUs this spinlock was burning. > > > > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx> > > The patch looks good, and the comments about why it's safe to not take the > spinlock (specially why the EOF can't be moved back) is much welcomed. > > Feel free to add: > Reviewed-by: Carlos Maiolino <cmaiolino@xxxxxxxxxx> thanks! -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
