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Subject: Re: [f2fs-dev] [PATCH] f2fs: remove a waiter for checkpoint
 completion
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>,
        <kernel-team@android.com>, Eric Biggers <ebiggers@kernel.org>
References: <20200803172825.4077289-1-jaegeuk@kernel.org>
 <9638d2c5-cfd0-359f-187a-8e23bc6d822d@huawei.com>
 <20200804010412.GA866340@google.com>
 <98ac9355-bb6c-5109-da73-4ab7cdbbf8d5@huawei.com>
 <20200804024317.GA884736@google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <6d84bfea-13a3-af84-16ad-5d0c7eedeb67@huawei.com>
Date:   Tue, 4 Aug 2020 11:00:01 +0800
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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@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.
>>
>> 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@kernel.org>
> 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@kernel.org>
> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Better,

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

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);
>