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SubjectRe: [PATCH] f2fs: fix long latency due to discard during umount
On 03/12, 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@codeaurora.org>
> ---
> 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)) {
> + dc->state = D_PREP;
> + 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);

--> need to be DEFAULT_IO_TIMEOUT

> + __relocate_discard_cmd(dcc, dc);

It seems we need to submit bio first, and then move dc to wait_list, if there's
no error, in __submit_discard_cmd().

> + }
>
> if (issued >= dpolicy->max_requests)
> break;
> --
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

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