Re: [PATCH 2/3] f2fs: schedule in between two continous batch discards

[Jaegeuk Kim <jaegeuk@kernel.org>]

On Fri, Aug 26, 2016 at 08:50:50AM +0800, Chao Yu wrote:
> Hi Jaegeuk,
> 
> On 2016/8/26 0:57, Jaegeuk Kim wrote:
> > Hi Chao,
> > 
> > On Thu, Aug 25, 2016 at 05:22:29PM +0800, Chao Yu wrote:
> >> Hi Jaegeuk,
> >>
> >> On 2016/8/24 0:53, Jaegeuk Kim wrote:
> >>> Hi Chao,
> >>>
> >>> On Sun, Aug 21, 2016 at 11:21:30PM +0800, Chao Yu wrote:
> >>>> From: Chao Yu <yuchao0@huawei.com>
> >>>>
> >>>> In batch discard approach of fstrim will grab/release gc_mutex lock
> >>>> repeatly, it makes contention of the lock becoming more intensive.
> >>>>
> >>>> So after one batch discards were issued in checkpoint and the lock
> >>>> was released, it's better to do schedule() to increase opportunity
> >>>> of grabbing gc_mutex lock for other competitors.
> >>>>
> >>>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
> >>>> ---
> >>>>  fs/f2fs/segment.c | 2 ++
> >>>>  1 file changed, 2 insertions(+)
> >>>>
> >>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> >>>> index 020767c..d0f74eb 100644
> >>>> --- a/fs/f2fs/segment.c
> >>>> +++ b/fs/f2fs/segment.c
> >>>> @@ -1305,6 +1305,8 @@ int f2fs_trim_fs(struct f2fs_sb_info *sbi, struct fstrim_range *range)
> >>>>  		mutex_unlock(&sbi->gc_mutex);
> >>>>  		if (err)
> >>>>  			break;
> >>>> +
> >>>> +		schedule();
> >>>
> >>> Hmm, if other thread is already waiting for gc_mutex, we don't need this here.
> >>> In order to avoid long latency, wouldn't it be enough to reduce the batch size?
> >>
> >> Hmm, when fstrim call mutex_unlock we will pop one blocked locker from FIFO list
> >> of mutex lock, and wake it up, then fstrimer will try to lock gc_mutex for next
> >> batch trim, so the popped locker and fstrimer will make a new competition in
> >> gc_mutex.
> > 
> > Before trying to grab gc_mutex by fstrim again, there are already blocked tasks
> > waiting for gc_mutex. Hence the next one should be selectec by FIFO, no?
> 
> The next one which is going to be waked up is selected by FIFO, but the waked
> one is still needs to be race with other mutex lock grabber.
> 
> So there is no such guarantee that the waked one must get the lock.

Okay, I'll merge this. :)

Thanks,

> 
> Thanks,
> 
> > 
> > Thanks,
> > 
> >> If fstrimer is running in a big core, and popped locker is running in
> >> a small core, we can't guarantee popped locker can win the race, and for the
> >> most of time, fstrimer will win. So in order to reduce starvation of other
> >> gc_mutext locker, it's better to do schedule() here.
> >>
> >> Thanks,
> >>
> >>>
> >>> Thanks,
> >>>
> >>>>  	}
> >>>>  out:
> >>>>  	range->len = F2FS_BLK_TO_BYTES(cpc.trimmed);
> >>>> -- 
> >>>> 2.7.2
> >>>
> >>> .
> >>>
> > 
> > .
> > 

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