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    Subject[PATCHv5 00/15] kill unnecessary bdi wakeups + cleanups

    here is v5 of the patch series which clean-ups bdi threads and substantially
    lessens amount of unnecessary kernel wake-ups, which is very important on
    battery-powered devices.

    This patch-set is also available at:
    git:// flushers_v5

    Changes since v4
    Most patches are intact. Only patches N9, N10 and N14 were changed. And patch
    N15 was added. Also, now I tested the patch-set much better.

    1. Fix a bug found while testing: in the forker thread, when we create a bdi
    task and then assign it to 'bdi->wb.task', we need to take the
    'bdi->work_lock'. Otherwise we can lose a wake-up. Changed this in patch
    2. Add patch 15 which fixes a warning and cleans up 'bdi_register()'.
    3. Use "switch" in the main forker thread loop. Patch N9 was changed and now it
    also introduces the "switch".
    4. Stick with Christoph's version of tracepoint names and positions -> changed
    patch N14.
    5. Add a couple of "Reviewed-by" tags.

    Changes since v3:
    Tested more internally and found problems in bdi shutdown path. Namely, I forgot
    to move the code which wakes up processes waiting on BDI_pending bit to the
    forker thread. I also spotted few other things. Only patch N10 was changed.

    1. Removed "Reviewed-by: Christoph Hellwig <>" tag from patch 10
    "writeback: move bdi threads exiting logic to the forker thread" in this
    patch-set, because it was changed substantialy, and needs new review.
    2. Move 'clear_bit()' and 'wake_up_bit()' bit to bdi forker
    3. Add 'smp_wmb()' and 'smp_rmb()' in bdi forker and 'bdi_wb_shutdown()'
    4. In 'bdi_wb_shutdown()' first remove the bdi from 'bdi_list', and then wait
    on the 'BDI_pending' bit, not vise-versa.

    Changes since v2:
    Basically, added patches 12, 13, 14. Other patches are almost identical to v2,
    but I fixed few warnings about mixing tabs and spaces were fixed there. Also,
    tweaked them as described in item 6 below.
    1. Delay bdi thread wake-up as suggested by Nick Piggin, see patch N12
    2. Do not invoke init_timer unnecessarily - spotted and fixed an imperfection,
    see patch N13
    3. Introduce tracepoints to the code path which wakes up bdi threads for
    periodic write-back, suggested by Dave Chinner, see patch N14
    4. Fix few warnings
    5. Add more "Reviewed-by" tags
    6. Amend patches so that we would check the optimistic case first

    Changes since v1
    Basically, address all requests from Christoph except of 2.
    1. Drop "[PATCH 01/16] writeback: do not self-wakeup"
    2. Add all "Reviewed-by"
    3. Rebase to the latest "linux-2.6-block / for-2.6.36"
    4. Re-order patches so that the independent ones would go first and could
    be picked independently
    5. Do not remove comment about "temporary measure" in the forker thread.
    6. Drop "[PATCH 01/13] writeback: remove redundant list initialization"
    because one of later patches will kill whole function, so this small
    patch is pointless
    7. Merge "[PATCH 03/13] writeback: clean-up the warning about non-registered
    bdi" with the patch which adds bdi threads wake-ups to
    9. Do not remove bdis from the bdi_list
    8. Drop "[PATCH 09/13] writeback: add to bdi_list in the forker thread"
    because we do not remove bdis from the bdi_list anymore
    10. Use less local variables which are not strictly needed

    The following Christoph's requests were *not* addressed:

    1. Restructure the loop in bdi forker, because we have to drop spinlock
    before forking a thread, see my answer here:
    2. Get rid of 'BDI_pending' and use a per-bdi mutex. We cannot easily
    use a per-bdi mutex, because we would have to take it while holding
    the 'bdi_lock' spinlock. We could turn 'bdi_lock' into a mutex, though,
    and avoid dropping it before the task is created. This would eliminate
    the need in the 'BDI_pending' flag. I can do this change, if needed.


    Each block device has corresponding "flusher" thread, which is usually seen as
    "flusher-x:y" in your 'ps' output. Flusher threads are responsible for
    background write-back and are used in various kernel code paths like memory
    reclamation as well as the periodic background write-out.

    The flusher threads wake up every 5 seconds and check whether they have to
    write anything back or not. In idle systems with good dynamic power-management
    this means that they force the system to wake up from deep sleep, find out that
    there is nothing to do, and waste power. This hurts small battery-powered
    devices, e.g., linux-based phones.

    Idle bdi thread wake-ups do not last forever: the threads kill themselves if
    nothing useful has been done for 5 minutes.

    However, there is the bdi forker thread, seen as 'bdi-default' in your 'ps'
    output. This thread also wakes up every 5 seconds and checks whether it has to
    fork a bdi flusher thread, in case there is dirty data on the bdi, but bdi
    thread was killed. This thread never kills itself, and disturbs the system all
    the time. Again, this is bad for battery-powered devices.


    This patch-set makes bdi threads and the forker thread wake-up only if there is
    job to do, otherwise they just sleep. The main idea is to wake-up the needed
    thread when adding dirty data to the bdi.

    To implement this:

    1. I address various race conditions in the current bdi code.
    2. I move the killing logic from bdi threads to the forker thread, so that we
    would have one central place where we make decisions about killing inactive
    bdi threads. The reason I do this is because otherwise it is difficult to
    kill inactive threads - they never wake-up, so would never kill themselves.
    There are other technical reasons, too.
    3. I add a small piece of code to '__mark_inode_dirt()' which wakes up the bdi
    thread when dirty inodes arrive.
    4. There are also clean-up patches and nicification patches which I found to be
    good for better code readability.
    5. Some patches are just preparations which make the following real patches
    simpler and easier to review.
    6. Some patches are just simplifications of current code.

    With this patch-set bdi threads wake up considerably less.

    v1 can be found here:
    v2 can be found here:
    v3 can be found here:
    v4 can be found here:


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