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    Subject[PATCHv3 12/14] writeback: optimize periodic bdi thread wakeups
    Date
    From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

    Whe the first inode for a bdi is marked dirty, we wake up the bdi thread which
    should take care of the periodic background write-out. However, the write-out
    will actually start only 'dirty_writeback_interval' centisecs later, so we can
    delay the wake-up.

    This change was requested by Nick Piggin who pointed out that if we delay the
    wake-up, we weed out 2 unnecessary contex switches, which matters because
    '__mark_inode_dirty()' is a hot-path function.

    This patch introduces a new function - 'bdi_wakeup_thread_delayed()', which
    sets up a timer to wake-up the bdi thread and returns. So the wake-up is
    delayed.

    We also delete the timer in bdi threads just before writing-back.

    This patch also moves the 'bdi_wb_init()' function down in the file to avoid
    forward-declaration of 'bdi_wakeup_thread_delayed()'.

    Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
    ---
    fs/fs-writeback.c | 28 +++++--------------
    include/linux/backing-dev.h | 2 +
    mm/backing-dev.c | 64 +++++++++++++++++++++++++++++++++++--------
    3 files changed, 61 insertions(+), 33 deletions(-)

    diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    index 6477782..d8d0a21 100644
    --- a/fs/fs-writeback.c
    +++ b/fs/fs-writeback.c
    @@ -817,6 +817,12 @@ int bdi_writeback_thread(void *data)
    trace_writeback_thread_start(bdi);

    while (!kthread_should_stop()) {
    + /*
    + * Remove own delayed wake-up timer, since we are already awake
    + * and we'll take care of the preriodic write-back.
    + */
    + del_timer(&wb->wakeup_timer);
    +
    pages_written = wb_do_writeback(wb, 0);

    trace_writeback_pages_written(pages_written);
    @@ -875,26 +881,6 @@ void wakeup_flusher_threads(long nr_pages)
    rcu_read_unlock();
    }

    -/*
    - * This function is used when the first inode for this bdi is marked dirty. It
    - * wakes-up the corresponding bdi thread which should then take care of the
    - * periodic background write-out of dirty inodes.
    - */
    -static void wakeup_bdi_thread(struct backing_dev_info *bdi)
    -{
    - spin_lock(&bdi->wb_lock);
    - if (bdi->wb.task)
    - wake_up_process(bdi->wb.task);
    - else
    - /*
    - * When bdi tasks are inactive for long time, they are killed.
    - * In this case we have to wake-up the forker thread which
    - * should create and run the bdi thread.
    - */
    - wake_up_process(default_backing_dev_info.wb.task);
    - spin_unlock(&bdi->wb_lock);
    -}
    -
    static noinline void block_dump___mark_inode_dirty(struct inode *inode)
    {
    if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
    @@ -1026,7 +1012,7 @@ out:
    spin_unlock(&inode_lock);

    if (wakeup_bdi)
    - wakeup_bdi_thread(bdi);
    + bdi_wakeup_thread_delayed(bdi);

    }
    EXPORT_SYMBOL(__mark_inode_dirty);
    diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
    index 71b6223..7628219 100644
    --- a/include/linux/backing-dev.h
    +++ b/include/linux/backing-dev.h
    @@ -52,6 +52,7 @@ struct bdi_writeback {
    unsigned long last_active; /* last time bdi thread was active */

    struct task_struct *task; /* writeback thread */
    + struct timer_list wakeup_timer; /* used for delayed bdi thread wakeup */
    struct list_head b_dirty; /* dirty inodes */
    struct list_head b_io; /* parked for writeback */
    struct list_head b_more_io; /* parked for more writeback */
    @@ -105,6 +106,7 @@ void bdi_start_background_writeback(struct backing_dev_info *bdi);
    int bdi_writeback_thread(void *data);
    int bdi_has_dirty_io(struct backing_dev_info *bdi);
    void bdi_arm_supers_timer(void);
    +void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi);

    extern spinlock_t bdi_lock;
    extern struct list_head bdi_list;
    diff --git a/mm/backing-dev.c b/mm/backing-dev.c
    index ef880d9..cba2255 100644
    --- a/mm/backing-dev.c
    +++ b/mm/backing-dev.c
    @@ -248,17 +248,6 @@ static int __init default_bdi_init(void)
    }
    subsys_initcall(default_bdi_init);

    -static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
    -{
    - memset(wb, 0, sizeof(*wb));
    -
    - wb->bdi = bdi;
    - wb->last_old_flush = jiffies;
    - INIT_LIST_HEAD(&wb->b_dirty);
    - INIT_LIST_HEAD(&wb->b_io);
    - INIT_LIST_HEAD(&wb->b_more_io);
    -}
    -
    int bdi_has_dirty_io(struct backing_dev_info *bdi)
    {
    return wb_has_dirty_io(&bdi->wb);
    @@ -316,6 +305,43 @@ static void sync_supers_timer_fn(unsigned long unused)
    bdi_arm_supers_timer();
    }

    +static void wakeup_timer_fn(unsigned long data)
    +{
    + struct backing_dev_info *bdi = (struct backing_dev_info *)data;
    +
    + spin_lock(&bdi->wb_lock);
    + if (bdi->wb.task) {
    + wake_up_process(bdi->wb.task);
    + } else {
    + /*
    + * When bdi tasks are inactive for long time, they are killed.
    + * In this case we have to wake-up the forker thread which
    + * should create and run the bdi thread.
    + */
    + wake_up_process(default_backing_dev_info.wb.task);
    + }
    + spin_unlock(&bdi->wb_lock);
    +}
    +
    +/*
    + * This function is used when the first inode for this bdi is marked dirty. It
    + * wakes-up the corresponding bdi thread which should then take care of the
    + * periodic background write-out of dirty inodes. Since the write-out would
    + * starts only 'dirty_writeback_interval' centisecs from now anyway, we just
    + * set up a timer which wakes the bdi thread up later.
    + *
    + * Note, we wouldn't bother setting up the timer, but this function is on the
    + * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
    + * by delaying the wake-up.
    + */
    +void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
    +{
    + unsigned long timeout;
    +
    + timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
    + mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
    +}
    +
    /*
    * Calculate the longest interval (jiffies) bdi threads are allowed to be
    * inactive.
    @@ -349,8 +375,10 @@ static int bdi_forker_thread(void *ptr)
    * Temporary measure, we want to make sure we don't see
    * dirty data on the default backing_dev_info
    */
    - if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list))
    + if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) {
    + del_timer(&me->wakeup_timer);
    wb_do_writeback(me, 0);
    + }

    spin_lock_bh(&bdi_lock);
    set_current_state(TASK_INTERRUPTIBLE);
    @@ -577,6 +605,18 @@ void bdi_unregister(struct backing_dev_info *bdi)
    }
    EXPORT_SYMBOL(bdi_unregister);

    +static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
    +{
    + memset(wb, 0, sizeof(*wb));
    +
    + wb->bdi = bdi;
    + wb->last_old_flush = jiffies;
    + INIT_LIST_HEAD(&wb->b_dirty);
    + INIT_LIST_HEAD(&wb->b_io);
    + INIT_LIST_HEAD(&wb->b_more_io);
    + setup_timer(&wb->wakeup_timer, wakeup_timer_fn, (unsigned long)bdi);
    +}
    +
    int bdi_init(struct backing_dev_info *bdi)
    {
    int i, err;
    --
    1.7.1.1


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