Re: [PATCH 03/26] xfs: don't allow log IO to be throttled

Brian Foster
2019-10-11T12:39:33Z

On Wed, Oct 09, 2019 at 02:21:01PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Running metadata intensive workloads, I've been seeing the AIL
> pushing getting stuck on pinned buffers and triggering log forces.
> The log force is taking a long time to run because the log IO is
> getting throttled by wbt_wait() - the block layer writeback
> throttle. It's being throttled because there is a huge amount of
> metadata writeback going on which is filling the request queue.
>
> IOWs, we have a priority inversion problem here.
>
> Mark the log IO bios with REQ_IDLE so they don't get throttled
> by the block layer writeback throttle. When we are forcing the CIL,
> we are likely to need to to tens of log IOs, and they are issued as
> fast as they can be build and IO completed. Hence REQ_IDLE is
> appropriate - it's an indication that more IO will follow shortly.
>
> And because we also set REQ_SYNC, the writeback throttle will no

s/no/now ?

> treat log IO the same way it treats direct IO writes - it will not
> throttle them at all. Hence we solve the priority inversion problem
> caused by the writeback throttle being unable to distinguish between
> high priority log IO and background metadata writeback.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

> fs/xfs/xfs_log.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 6f99d6eae6a4..cf098e19967e 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1751,7 +1751,15 @@ xlog_write_iclog(
> iclog->ic_bio.bi_iter.bi_sector = log->l_logBBstart + bno;
> iclog->ic_bio.bi_end_io = xlog_bio_end_io;
> iclog->ic_bio.bi_private = iclog;
> - iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC | REQ_FUA;
> +
> + /*
> + * We use REQ_SYNC | REQ_IDLE here to tell the block layer the are more
> + * IOs coming immediately after this one. This prevents the block layer
> + * writeback throttle from throttling log writes behind background
> + * metadata writeback and causing priority inversions.
> + */
> + iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC |
> + REQ_IDLE | REQ_FUA;
> if (need_flush)
> iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
>
> --
> 2.23.0.rc1
>