Re: [PATCH] cfq: priority boost on meta/prio marked IO

[Jeff Moyer <jmoyer@redhat.com>]

Jens Axboe <axboe@fb.com> writes:

> At Facebook, we have a number of cases where people use ionice to set a
> lower priority, then end up having tasks stuck for a long time because
> eg meta data updates from an idle priority tasks is blocking out higher
> priority processes. It's bad enough that it will trigger the softlockup
> warning.
>
> This patch adds code to CFQ that bumps the priority class and data for
> an idle task, if is doing IO marked as PRIO or META. With this, we no
> longer see the softlockups.
>
> Signed-off-by: Jens Axboe <axboe@fb.com>
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 32a283eb7274..3cfd67d006fb 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1781,6 +1781,11 @@ get_rq:
>  		rw_flags |= REQ_SYNC;
>  
>  	/*
> +	 * Add in META/PRIO flags, if set, before we get to the IO scheduler
> +	 */
> +	rw_flags |= (bio->bi_rw & (REQ_META | REQ_PRIO));
> +
> +	/*

This needs a docbook update.  It now reads:

 * @rw_flags: RW and SYNC flags

so whatever flags we're adding should be specified, I guess.

Speaking of which, after much waffling, I think I've decided it would be
cleaner to limit the priority boost to REQ_PRIO requests only.

Other than that, I think this looks fine.

Cheers,
Jeff