Re: [f2fs-dev] [PATCH] f2fs: fix unnecessary periodic wakeup of discard thread when dev is busy

[Chao Yu <chao@kernel.org>]

On 2018/9/2 18:34, Sahitya Tummala wrote:
> On Sun, Sep 02, 2018 at 04:52:40PM +0800, Chao Yu wrote:
>> On 2018/8/31 17:39, Sahitya Tummala wrote:
>>> When dev is busy, discard thread wake up timeout can be aligned with the
>>> exact time that it needs to wait for dev to come out of busy. This helps
>>> to avoid unnecessary periodic wakeups and thus save some power.
>>>
>>> Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
>>> ---
>>>  fs/f2fs/segment.c | 8 +++++++-
>>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index 8bcbb50..df14030 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -1379,6 +1379,8 @@ static int issue_discard_thread(void *data)
>>>  	struct discard_policy dpolicy;
>>>  	unsigned int wait_ms = DEF_MIN_DISCARD_ISSUE_TIME;
>>>  	int issued;
>>> +	unsigned long interval = sbi->interval_time[REQ_TIME] * HZ;
>>> +	long delta;
>>>  
>>>  	set_freezable();
>>>  
>>> @@ -1410,7 +1412,11 @@ static int issue_discard_thread(void *data)
>>>  			__wait_all_discard_cmd(sbi, &dpolicy);
>>>  			wait_ms = dpolicy.min_interval;
>>>  		} else if (issued == -1){
>>> -			wait_ms = dpolicy.mid_interval;
>>> +			delta = (sbi->last_time[REQ_TIME] + interval) - jiffies;
>>
>> I agree that we need to consider power consumption. One more consideration is
>> that discard thread may need different submission frequency comparing to garbage
>> collection thread, maybe a little fast, would it be better to split
>> sbi->interval_time[REQ_TIME] according to gc/discard type.
>>
>> How do you think?
>>
>> Thanks,
>>
> 
> Thanks for the review.
> 
> You mean when GC type is urgent? I see that for that case, the discard policy is

Actually, I mean splitting sbi->interval_time[REQ_TIME] into:
- sbi->interval_time[GC_TIM] which can be used for GC thread.
- sbi->interval_time[DISCARD_TIME] which can be used for Discard thread.

Then we can configure sbi->interval_time[DISCARD_TIME] independently, and set
more suitable interval value for discard thread, since discard thread may need
to wake to submit discards more frequently.

I guess if we can accept above idea, it can be sent as another patch, so anyway,
I'm okay with your change. :)

Reviewed-by: Chao Yu <yuchao0@huawei.com>

Thanks,

> changed to DPOLICY_FORCE, which sets dpolicy->io_aware as false and hence,
> cannot fall into this (issued == -1) case at all.
> 
>>> +			if (delta > 0)
>>> +				wait_ms = jiffies_to_msecs(delta);
>>> +			else
>>> +				wait_ms = dpolicy.mid_interval;
>>>  		} else {
>>>  			wait_ms = dpolicy.max_interval;
>>>  		}
>>>
>