Re: [PATCH 1/1] Add check for dirty_writeback_interval in bdi_wakeup_thread_delayed

Raghavendra D Prabhu
2011-04-18T09:16:49Z

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* On Mon, Apr 18, 2011 at 10:19:12AM +0300, Artem Bityutskiy <Artem.Bityutskiy@nokia.com> 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@wnohang.net>
>
>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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