linux-kernel - Re: [PATCH v2] mm: Make logic in bdi_forker

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Message-ID: <CAFPAmTTQeHAd9o9y_SfRbQefovo6ukASHodopMtFLCZ4zL07RQ@mail.gmail.com>
Date:	Fri, 2 Sep 2011 17:41:22 +0530
From:	"kautuk.c @samsung.com" <consul.kautuk@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH v2] mm: Make logic in bdi_forker_thread() straight

Sorry to butt in before Jens' review but i have one small comment:

On Fri, Sep 2, 2011 at 5:01 PM, Jan Kara <jack@...e.cz> wrote:
> The logic in bdi_forker_thread() is unnecessarily convoluted by setting task
> state there and back or calling schedule_timeout() in TASK_RUNNING state. Also
> clearing of BDI_pending bit is placed at the and of global loop and cases of a
> switch which mustn't reach it must call 'continue' instead of 'break' which is
> non-intuitive and thus asking for trouble. So make the logic more obvious.
>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: Wu Fengguang <fengguang.wu@...el.com>
> CC: consul.kautuk@...il.com
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  mm/backing-dev.c |   37 ++++++++++++++++++++-----------------
>  1 files changed, 20 insertions(+), 17 deletions(-)
>
>  This should be the right cleanup. Jens?
>
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index d6edf8d..bdf7d6b 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -359,6 +359,17 @@ static unsigned long bdi_longest_inactive(void)
>        return max(5UL * 60 * HZ, interval);
>  }
>
> +/*
> + * Clear pending bit and wakeup anybody waiting for flusher thread startup
> + * or teardown.
> + */
> +static void bdi_clear_pending(struct backing_dev_info *bdi)
> +{
> +       clear_bit(BDI_pending, &bdi->state);
> +       smp_mb__after_clear_bit();
> +       wake_up_bit(&bdi->state, BDI_pending);
> +}
> +
>  static int bdi_forker_thread(void *ptr)
>  {
>        struct bdi_writeback *me = ptr;
> @@ -390,8 +401,6 @@ static int bdi_forker_thread(void *ptr)
>                }
>
>                spin_lock_bh(&bdi_lock);
> -               set_current_state(TASK_INTERRUPTIBLE);
> -
>                list_for_each_entry(bdi, &bdi_list, bdi_list) {
>                        bool have_dirty_io;
>
> @@ -441,13 +450,8 @@ static int bdi_forker_thread(void *ptr)
>                }
>                spin_unlock_bh(&bdi_lock);
>
> -               /* Keep working if default bdi still has things to do */
> -               if (!list_empty(&me->bdi->work_list))
> -                       __set_current_state(TASK_RUNNING);
> -
>                switch (action) {
>                case FORK_THREAD:
> -                       __set_current_state(TASK_RUNNING);
>                        task = kthread_create(bdi_writeback_thread, &bdi->wb,
>                                              "flush-%s", dev_name(bdi->dev));
>                        if (IS_ERR(task)) {
> @@ -469,14 +473,21 @@ static int bdi_forker_thread(void *ptr)
>                                spin_unlock_bh(&bdi->wb_lock);
>                                wake_up_process(task);
>                        }
> +                       bdi_clear_pending(bdi);
>                        break;
>
>                case KILL_THREAD:
> -                       __set_current_state(TASK_RUNNING);
>                        kthread_stop(task);
> +                       bdi_clear_pending(bdi);
>                        break;
>
>                case NO_ACTION:
> +                       /* Keep working if default bdi still has things to do */

Can we acquire and release the spinlocks as below:
                            spin_lock_bh(&me->bdi->wb_lock) ;

> +                       if (!list_empty(&me->bdi->work_list)) {

                            spin_unlock_bh(&me->bdi->wb_lock) ;

> +                               try_to_freeze();
> +                               break;
> +                       }
> +                       set_current_state(TASK_INTERRUPTIBLE);

                            spin_unlock_bh(&me->bdi->wb_lock) ;

>                        if (!wb_has_dirty_io(me) || !dirty_writeback_interval)
>                                /*
>                                 * There are no dirty data. The only thing we
> @@ -489,16 +500,8 @@ static int bdi_forker_thread(void *ptr)
>                        else
>                                schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
>                        try_to_freeze();
> -                       /* Back to the main loop */
> -                       continue;
> +                       break;
>                }
> -
> -               /*
> -                * Clear pending bit and wakeup anybody waiting to tear us down.
> -                */
> -               clear_bit(BDI_pending, &bdi->state);
> -               smp_mb__after_clear_bit();
> -               wake_up_bit(&bdi->state, BDI_pending);
>        }
>
>        return 0;
> --
> 1.7.1
>
>

That should take care of the problem I initially mentioned due to the
wakeup_timer_fn executing
in parallel on another CPU as the task state will now be protected by
the wb_lock spinlock.
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