linux-kernel - Re: [PATCH 1/1] Add check for dirty_writeback_interval in bdi_wakeup_thread

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Date:	Mon, 18 Apr 2011 14:46:09 +0530
From:	Raghavendra D Prabhu <rprabhu@...hang.net>
To:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
Cc:	linux-mm@...ck.org, Jens Axboe <jaxboe@...ionio.com>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Add check for dirty_writeback_interval in
 bdi_wakeup_thread_delayed

* On Mon, Apr 18, 2011 at 10:19:12AM +0300, Artem Bityutskiy <Artem.Bityutskiy@...ia.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@...hang.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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