Re: [PATCH] f2fs: fix long latency due to discard during umount

[Chao Yu]

On 2020/3/12 19:14, Sahitya Tummala wrote:
> F2FS already has a default timeout of 5 secs for discards that
> can be issued during umount, but it can take more than the 5 sec
> timeout if the underlying UFS device queue is already full and there
> are no more available free tags to be used. In that case, submit_bio()
> will wait for the already queued discard requests to complete to get
> a free tag, which can potentially take way more than 5 sec.
> 
> Fix this by submitting the discard requests with REQ_NOWAIT
> flags during umount. This will return -EAGAIN for UFS queue/tag full
> scenario without waiting in the context of submit_bio(). The FS can
> then handle these requests by retrying again within the stipulated
> discard timeout period to avoid long latencies.
> 
> Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
> ---
>  fs/f2fs/segment.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index fb3e531..a06bbac 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -1124,10 +1124,13 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>  	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
>  	struct list_head *wait_list = (dpolicy->type == DPOLICY_FSTRIM) ?
>  					&(dcc->fstrim_list) : &(dcc->wait_list);
> -	int flag = dpolicy->sync ? REQ_SYNC : 0;
> +	int flag;
>  	block_t lstart, start, len, total_len;
>  	int err = 0;
>  
> +	flag = dpolicy->sync ? REQ_SYNC : 0;
> +	flag |= dpolicy->type == DPOLICY_UMOUNT ? REQ_NOWAIT : 0;
> +
>  	if (dc->state != D_PREP)
>  		return 0;
>  
> @@ -1203,6 +1206,11 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>  		bio->bi_end_io = f2fs_submit_discard_endio;
>  		bio->bi_opf |= flag;
>  		submit_bio(bio);
> +		if ((flag & REQ_NOWAIT) && (dc->error == -EAGAIN)) {

If we want to update dc->state, we need to cover it with dc->lock.

> +			dc->state = D_PREP;

BTW, one dc can be referenced by multiple bios, so dc->state could be updated to
D_DONE later by f2fs_submit_discard_endio(), however we just relocate it to
pending list... which is inconsistent status.

Thanks,

> +			err = dc->error;
> +			break;
> +		}
>  
>  		atomic_inc(&dcc->issued_discard);
>  
> @@ -1510,6 +1518,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>  			}
>  
>  			__submit_discard_cmd(sbi, dpolicy, dc, &issued);
> +			if (dc->error == -EAGAIN) {
> +				congestion_wait(BLK_RW_ASYNC, HZ/50);
> +				__relocate_discard_cmd(dcc, dc);
> +			}
>  
>  			if (issued >= dpolicy->max_requests)
>  				break;
>