Re: [f2fs-dev] [PATCH] f2fs: remove a waiter for checkpoint completion

[Chao Yu]

On 2020/8/4 10:43, Jaegeuk Kim wrote:
> On 08/04, Chao Yu wrote:
> > On 2020/8/4 9:04, Jaegeuk Kim wrote:
> > > 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@xxxxxxxxxx>
> > > > > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > > > > ---
> > > > >     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.
> >
> > Yup, in previous implementation, last end_io wakes up checkpoint() waiter, we
> > didn't waste any time there.
> >
> > > 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.
> >
> > Then io_schedule_timeout() in this loop may give CPU time slice to other thread
> > until scheduler reselect checkpoint(), that would cause longer latency?
>
> Hmm, how about this then?
>
> > From 4956afa1cedc019cabf6e8bff7bc48d3bcf7a3f5 Mon Sep 17 00:00:00 2001
> From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> Date: Mon, 3 Aug 2020 19:37:12 -0700
> Subject: [PATCH] f2fs: prepare a waiter before entering io_schedule
>
> This is to avoid sleep() in the waiter thread.
>
> [   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@xxxxxxxxxx>
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>

Better,

Reviewed-by: Chao Yu <yuchao0@xxxxxxxxxx>

Thanks,

> ---
>   fs/f2fs/checkpoint.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index 99c8061da55b9..ff807e14c8911 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -1258,8 +1258,6 @@ 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);
> -
>   		if (!get_pages(sbi, type))
>   			break;
>   
> @@ -1271,6 +1269,8 @@ 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);
> +
> +		prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);
>   		io_schedule_timeout(DEFAULT_IO_TIMEOUT);
>   	}
>   	finish_wait(&sbi->cp_wait, &wait);