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Date:   Mon, 3 Aug 2020 18:04:12 -0700
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <yuchao0@huawei.com>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com,
        Eric Biggers <ebiggers@kernel.org>
Subject: Re: [f2fs-dev] [PATCH] f2fs: remove a waiter for checkpoint
 completion
Message-ID: <20200804010412.GA866340@google.com>
References: <20200803172825.4077289-1-jaegeuk@kernel.org>
 <9638d2c5-cfd0-359f-187a-8e23bc6d822d@huawei.com>
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On 08/04, Chao Yu wrote:
> On 2020/8/4 1:28, Jaegeuk Kim wrote:
> > It doesn't need to wait for checkpoint being completed triggered by end_io.
> > 
> > [   20.157753] ------------[ cut here ]------------
> > [   20.158393] do not call blocking ops when !TASK_RUNNING; state=2 set at [<0000000096354225>] prepare_to_wait+0xcd/0x430
> > [   20.159858] WARNING: CPU: 1 PID: 1152 at kernel/sched/core.c:7142 __might_sleep+0x149/0x1a0
> > ...
> > [   20.176110]  __submit_merged_write_cond+0x191/0x310
> > [   20.176739]  f2fs_submit_merged_write+0x18/0x20
> > [   20.177323]  f2fs_wait_on_all_pages+0x269/0x2d0
> > [   20.177899]  ? block_operations+0x980/0x980
> > [   20.178441]  ? __kasan_check_read+0x11/0x20
> > [   20.178975]  ? finish_wait+0x260/0x260
> > [   20.179488]  ? percpu_counter_set+0x147/0x230
> > [   20.180049]  do_checkpoint+0x1757/0x2a50
> > [   20.180558]  f2fs_write_checkpoint+0x840/0xaf0
> > [   20.181126]  f2fs_sync_fs+0x287/0x4a0
> > 
> > Reported-by: Eric Biggers <ebiggers@kernel.org>
> > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> > ---
> >   fs/f2fs/checkpoint.c | 6 +-----
> >   fs/f2fs/data.c       | 4 ----
> >   fs/f2fs/f2fs.h       | 1 -
> >   fs/f2fs/super.c      | 1 -
> >   4 files changed, 1 insertion(+), 11 deletions(-)
> > 
> > diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> > index 99c8061da55b9..2bdddc725e677 100644
> > --- a/fs/f2fs/checkpoint.c
> > +++ b/fs/f2fs/checkpoint.c
> > @@ -1255,11 +1255,7 @@ static void unblock_operations(struct f2fs_sb_info *sbi)
> >   void f2fs_wait_on_all_pages(struct f2fs_sb_info *sbi, int type)
> >   {
> > -	DEFINE_WAIT(wait);
> > -
> >   	for (;;) {
> > -		prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);
> 
> Wouldn't that case high cpu usage before io end?

This is a critical path to wait for IO completion in checkpoint, which would be
better to wait for it to avoid long latency to continue filesystem operations.
Moreover, I expect io_schedule_timeout() can mitigate such the CPU overhead and
actually we don't need to make there-in context switches as well.

> 
> > -
> >   		if (!get_pages(sbi, type))
> >   			break;
> > @@ -1271,9 +1267,9 @@ void f2fs_wait_on_all_pages(struct f2fs_sb_info *sbi, int type)
> >   							FS_CP_META_IO);
> >   		else if (type == F2FS_WB_CP_DATA)
> >   			f2fs_submit_merged_write(sbi, DATA);
> > +
> >   		io_schedule_timeout(DEFAULT_IO_TIMEOUT);
> >   	}
> > -	finish_wait(&sbi->cp_wait, &wait);
> >   }
> >   static void update_ckpt_flags(struct f2fs_sb_info *sbi, struct cp_control *cpc)
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index c1b676be67b9a..588d2871bbe69 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -383,10 +383,6 @@ static void f2fs_write_end_io(struct bio *bio)
> >   		clear_cold_data(page);
> >   		end_page_writeback(page);
> >   	}
> > -	if (!get_pages(sbi, F2FS_WB_CP_DATA) &&
> > -				wq_has_sleeper(&sbi->cp_wait))
> > -		wake_up(&sbi->cp_wait);
> > -
> >   	bio_put(bio);
> >   }
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 02811ce15059b..a036539363030 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -1437,7 +1437,6 @@ struct f2fs_sb_info {
> >   	struct rw_semaphore cp_rwsem;		/* blocking FS operations */
> >   	struct rw_semaphore node_write;		/* locking node writes */
> >   	struct rw_semaphore node_change;	/* locking node change */
> > -	wait_queue_head_t cp_wait;
> >   	unsigned long last_time[MAX_TIME];	/* to store time in jiffies */
> >   	long interval_time[MAX_TIME];		/* to store thresholds */
> > diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> > index 83bf9a02f83f2..5f4d9abc079d6 100644
> > --- a/fs/f2fs/super.c
> > +++ b/fs/f2fs/super.c
> > @@ -3540,7 +3540,6 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
> >   	init_rwsem(&sbi->cp_rwsem);
> >   	init_rwsem(&sbi->quota_sem);
> > -	init_waitqueue_head(&sbi->cp_wait);
> >   	init_sb_info(sbi);
> >   	err = init_percpu_info(sbi);
> >