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

From: Sahitya Tummala
Date: Fri Mar 13 2020 - 07:09:10 EST


On Fri, Mar 13, 2020 at 02:30:55PM +0800, Chao Yu wrote:
> On 2020/3/13 11:39, Sahitya Tummala wrote:
> > On Fri, Mar 13, 2020 at 10:20:04AM +0800, Chao Yu wrote:
> >> 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.
> >
> > Sure, will update it.
> >
> >>
> >>> + 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.
> >
> > In that case dc->bio_ref will reflect it and until it becomes 0, the dc->state
> > will not be updated to D_DONE in f2fs_submit_discard_endio()?
>
> __submit_discard_cmd()
> lock()
> dc->state = D_SUBMIT;
> dc->bio_ref++;
> unlock()
> ...
> submit_bio()
> f2fs_submit_discard_endio()
> dc->error = -EAGAIN;
> lock()
> dc->bio_ref--;
>
> dc->state = D_PREP;
>
> dc->state = D_DONE;
> unlock()
>
> So finally, dc's state is D_DONE, and it's in wait list, then will be relocated
> to pending list.

In case of queue full, f2fs_submit_discard_endio() will not be called
asynchronously. It will be called in the context of submit_bio() itself.
So by the time, submit_bio returns dc->error will be -EAGAIN and dc->state
will be D_DONE.

submit_bio()
->blk_mq_make_request
->blk_mq_get_request()
->bio_wouldblock_error() (called due to queue full)
->bio_endio()

Thanks,
>
> >
> > Thanks,
> >
> >>
> >> 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;
> >>>
> >

--
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Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.