Re: [PATCH] f2fs: avoid hungtask problem caused by f2fs_balance_fs

[Chao Yu <chao@kernel.org>]

Hi Yunlei,

On 2016/2/26 15:13, Yunlei He wrote:
> I came across a hungtask problem as below:
> 
> INFO: task 962 blocked for more than 120 s.
> 
> Call trace:
> [<ffffffc000085a24>] __switch_to+0x74/0x8c
> [<ffffffc000dd1434>] __schedule+0x3b4/0x8f4
> [<ffffffc000dd1998>] schedule+0x24/0x68
> [<ffffffc000dd1e80>] schedule_preempt_disabled+0x10/0x24
> [<ffffffc000dd04b4>] __mutex_lock_slowpath+0x154/0x26c
> [<ffffffc000dd0610>] mutex_lock+0x44/0x64
> [<ffffffc0003471d0>] f2fs_balance_fs+0x78/0x94
> [<ffffffc00032ca00>] f2fs_create+0x30/0x1d4
> [<ffffffc0001a5e40>] vfs_create+0xc8/0xf8
> [<ffffffc0001a7a34>] do_last.isra.43+0x608/0xc44
> [<ffffffc0001a8118>] path_openat.isra.44+0xa8/0x4b0
> [<ffffffc0001a92dc>] do_filp_open+0x2c/0x98
> [<ffffffc000198da0>] do_sys_open+0x104/0x1c8
> [<ffffffc000198e98>] SyS_openat+0xc/0x18
> 
> Besides, there are also lots of other operations were blocked
> by gc_mutex like here.
> 
> The comments said we should do GC or end up with checkpoint, if there
> are so many dirty dir/node pages without enough free segments. But should
> all theads wait for gc if the condition is not satisfied? Maybe if someone
> is gcing, they can by pass it.

I don't think so, once there is one thread goes into GC flow, if we don't block
all other foreground gc threads here, these concurrent threads may generate more
dirty node/dentry pages, which can easily lead sync_node_pages in GC or
checkpoint runs out of free sections.

Thanks,

> 
> Signed-off-by: Yunlei He <heyunlei@huawei.com>
> ---
>  fs/f2fs/segment.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 6f16b39..48ce7ab 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -345,8 +345,8 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
>  	 * We should do GC or end up with checkpoint, if there are so many dirty
>  	 * dir/node pages without enough free segments.
>  	 */
> -	if (has_not_enough_free_secs(sbi, 0)) {
> -		mutex_lock(&sbi->gc_mutex);
> +	if (has_not_enough_free_secs(sbi, 0) &&
> +					mutex_trylock(&sbi->gc_mutex)) {
>  		f2fs_gc(sbi, false);
>  	}
>  }
> 

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