Re: [PATCH 1/1] Add check for dirty_writeback_interval inbdi_wakeup_thread_delayed

[Raghavendra D Prabhu]

* On Mon, Apr 18, 2011 at 10:19:12AM +0300, Artem Bityutskiy <Artem.Bityutskiy@xxxxxxxxx> wrote:
> On Sun, 2011-04-17 at 21:53 +0530, Raghavendra D Prabhu wrote:
> > In the function bdi_wakeup_thread_delayed, no checks are performed on
> > dirty_writeback_interval unlike other places and timeout is being set to
> > zero as result, thus defeating the purpose. So, I have changed it to be
> > passed default value of interval which is 500 centiseconds, when it is
> > set to zero.
> > I have also verified this and tested it.

> > Signed-off-by: Raghavendra D Prabhu <rprabhu@xxxxxxxxxxx>
>
> If  dirty_writeback_interval then the periodic write-back has to be
> disabled. Which means we should rather do something like this:
>
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 0d9a036..f38722c 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -334,10 +334,12 @@ static void wakeup_timer_fn(unsigned long data)
>  */
> void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
> {
> -       unsigned long timeout;
> +       if (dirty_writeback_interval) {
> +               unsigned long timeout;
>
> -       timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> -       mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
> +               timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> +               mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
> +       }
> }
>
> I do not see why you use 500 centisecs instead - I think this is wrong.
>
> > ---
> >   mm/backing-dev.c |    5 ++++-
> >   1 files changed, 4 insertions(+), 1 deletions(-)

> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index befc875..d06533c 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -336,7 +336,10 @@ void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
> >   {
> >   	unsigned long timeout;

> > -	timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> > +	if (dirty_writeback_interval)
> > +		timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> > +	else
> > +		timeout = msecs_to_jiffies(5000);
> >   	mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
> >   }
Hi,

I have set it to 500 centisecs as that is the default value of
dirty_writeback_interval. I used this logic for following reason: the
purpose for which dirty_writeback_interval is set to 0 is to disable
periodic writeback
(http://tomoyo.sourceforge.jp/cgi-bin/lxr/source/fs/fs-writeback.c#L818)
, whereas here (in bdi_wakeup_thread_delayed) it is being used for a
different purpose -- to delay the bdi wakeup in order to reduce context
switches for  dirty inode writeback.
Regarding the change you made: in
that case won't it end up disabling the timer altogether ? which
shouldn't happen given the original purpose of defining
dirty_writeback_interval to zero.

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