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

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Date:   Thu, 25 Aug 2016 09:57:16 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     Chao Yu <chao@...nel.org>, linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] f2fs: schedule in between two continous batch
 discards

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@...wei.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@...wei.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?

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
> > 
> > .
> >