On Wed, Oct 09, 2019 at 02:21:01PM +1100, Dave Chinner wrote: > From: Dave Chinner <dchinner@xxxxxxxxxx> > > 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 > 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@xxxxxxxxxx> Looks ok, Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> --D > --- > 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 >
