lkml.org 
[lkml]   [2010]   [Nov]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 3/5] writeback: stop background/kupdate works from livelocking other works
    On Tue, 09 Nov 2010 07:09:19 +0800
    Wu Fengguang <fengguang.wu@intel.com> wrote:
    >

    I find the description to be somewhat incomplete...

    > From: Jan Kara <jack@suse.cz>
    >
    > Background writeback are easily livelockable (from a definition of their
    > target).

    *why* is background writeback easily livelockable? Under which
    circumstances does this happen and how does it come about?

    > This is inconvenient because it can make sync(1) stall forever waiting
    > on its queued work to be finished.

    Again, why? Because there are works queued from the flusher thread,
    but that thread is stuck in a livelocked state in <unspecified code
    location> so it is unable to service the other works? But the pocess
    which called sync() will as a last resort itself perform all the
    required IO, will it not? If so, how can it livelock?

    > Generally, when a flusher thread has
    > some work queued, someone submitted the work to achieve a goal more specific
    > than what background writeback does. So it makes sense to give it a priority
    > over a generic page cleaning.
    >
    > Thus we interrupt background writeback if there is some other work to do. We
    > return to the background writeback after completing all the queued work.
    >
    > Signed-off-by: Jan Kara <jack@suse.cz>
    > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    > ---
    > fs/fs-writeback.c | 9 +++++++++
    > 1 file changed, 9 insertions(+)
    >
    > --- linux-next.orig/fs/fs-writeback.c 2010-11-07 21:56:42.000000000 +0800
    > +++ linux-next/fs/fs-writeback.c 2010-11-07 22:00:51.000000000 +0800
    > @@ -651,6 +651,15 @@ static long wb_writeback(struct bdi_writ
    > break;
    >
    > /*
    > + * Background writeout and kupdate-style writeback are
    > + * easily livelockable. Stop them if there is other work
    > + * to do so that e.g. sync can proceed.
    > + */
    > + if ((work->for_background || work->for_kupdate) &&
    > + !list_empty(&wb->bdi->work_list))
    > + break;
    > +
    > + /*
    > * For background writeout, stop when we are below the
    > * background dirty threshold
    > */

    So... what prevents higher priority works (eg, sync(1)) from
    livelocking or seriously retarding background or kudate writeout?



    \
     
     \ /
      Last update: 2010-11-09 22:17    [W:0.025 / U:0.284 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site