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SubjectRe: [PATCH] f2fs: fix long latency due to discard during umount
Hi Chao,

On Thu, Mar 26, 2020 at 05:00:18PM +0800, Chao Yu wrote:
> Hi Sahitya,
>
> On 2020/3/18 12:44, 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>
> > ---
> > v2:
> > - Handle the case where a dc can have multiple bios associated with it
> >
> > fs/f2fs/f2fs.h | 1 +
> > fs/f2fs/segment.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++++-------
> > 2 files changed, 74 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 12a197e..67b8dcc 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -340,6 +340,7 @@ struct discard_cmd_control {
> > struct list_head pend_list[MAX_PLIST_NUM];/* store pending entries */
> > struct list_head wait_list; /* store on-flushing entries */
> > struct list_head fstrim_list; /* in-flight discard from fstrim */
> > + struct list_head retry_list; /* list of cmds to retry */
> > wait_queue_head_t discard_wait_queue; /* waiting queue for wake-up */
> > unsigned int discard_wake; /* to wake up discard thread */
> > struct mutex cmd_lock;
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index fb3e531..4162c76 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -1029,13 +1029,16 @@ static void f2fs_submit_discard_endio(struct bio *bio)
> > struct discard_cmd *dc = (struct discard_cmd *)bio->bi_private;
> > unsigned long flags;
> >
> > - dc->error = blk_status_to_errno(bio->bi_status);
> > -
> > spin_lock_irqsave(&dc->lock, flags);
> > + if (!dc->error)
> > + dc->error = blk_status_to_errno(bio->bi_status);
> > +
> > dc->bio_ref--;
> > - if (!dc->bio_ref && dc->state == D_SUBMIT) {
> > - dc->state = D_DONE;
> > - complete_all(&dc->wait);
> > + if (!dc->bio_ref) {
> > + if (dc->error || dc->state == D_SUBMIT) {
> > + dc->state = D_DONE;
> > + complete_all(&dc->wait);
> > + }
> > }
> > spin_unlock_irqrestore(&dc->lock, flags);
> > bio_put(bio);
> > @@ -1124,10 +1127,13 @@ 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;
> > - block_t lstart, start, len, total_len;
> > + int flag;
> > + block_t lstart, start, len, total_len, orig_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;
> >
> > @@ -1139,7 +1145,7 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
> > lstart = dc->lstart;
> > start = dc->start;
> > len = dc->len;
> > - total_len = len;
> > + orig_len = total_len = len;
> >
> > dc->len = 0;
> >
> > @@ -1203,6 +1209,14 @@ 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) {
> > + if (dc->error == -EAGAIN) {
> > + dc->len = orig_len;
> > + list_move(&dc->list, &dcc->retry_list);
> > + err = dc->error;
>
> I encounter lots of dmesg, which should be printed by __remove_discard_cmd()
>
> F2FS-fs (dm-0): Issue discard(23552, 23552, 2) failed, ret: -11
>
> This should happen only if we didn't handle all discard in 5 seconds during
> umount.
>
> So I doubt we failed to move dc to retry_list, because after submit_bio(),
> end_io() is not called synchronously as the bio was just pluged?
>
This can happen if a discard cmd has multiple bios and at least 1 bio is
already submitted and when submitting other bios, we encounter -EAGAIN.
In this case, this dc will be moved to retry_list and will be moved back
to dcc->pend_list only if the dc->bio_ref becomes 0 within 5 sec timeout.
If it doesn't become zero, then it will be left in retry_list itself, which
will be later removed from retry_list. Before removing from retry_list we
will however ensure that submitted bio is done i.e., dc->bio_ref is 0, but
dc->error will be -EAGAIN as this dc could not be requeued/retried.

So this is expected, where it only means that this dc could not be
submitted/retried again within timeout. I think we can ignore
this -EAGAIN error in __remove_discard_cmd().

Thanks,

> Thanks,
>
> > + break;
> > + }
> > + }
> >
> > atomic_inc(&dcc->issued_discard);
> >
> > @@ -1463,6 +1477,40 @@ static unsigned int __issue_discard_cmd_orderly(struct f2fs_sb_info *sbi,
> > return issued;
> > }
> >
> > +static bool __should_discard_retry(struct f2fs_sb_info *sbi,
s> > + struct discard_policy *dpolicy)
> > +{
> > + struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
> > + struct discard_cmd *dc, *tmp;
> > + bool retry = false;
> > + unsigned long flags;
> > +
> > + if (dpolicy->type != DPOLICY_UMOUNT)
> > + f2fs_bug_on(sbi, 1);
> > +
> > + mutex_lock(&dcc->cmd_lock);
> > + list_for_each_entry_safe(dc, tmp, &(dcc->retry_list), list) {
> > + if (dpolicy->timeout != 0 &&
> > + f2fs_time_over(sbi, dpolicy->timeout)) {
> > + retry = false;
> > + break;
> > + }
> > +
> > + spin_lock_irqsave(&dc->lock, flags);
> > + if (!dc->bio_ref) {
> > + dc->state = D_PREP;
> > + dc->error = 0;
> > + reinit_completion(&dc->wait);
> > + __relocate_discard_cmd(dcc, dc);
> > + retry = true;
> > + }
> > + spin_unlock_irqrestore(&dc->lock, flags);
> > + }
> > + mutex_unlock(&dcc->cmd_lock);
> > +
> > + return retry;
> > +}
> > +
> > static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> > struct discard_policy *dpolicy)
> > {
> > @@ -1470,12 +1518,13 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> > struct list_head *pend_list;
> > struct discard_cmd *dc, *tmp;
> > struct blk_plug plug;
> > - int i, issued = 0;
> > + int i, err, issued = 0;
> > bool io_interrupted = false;
> >
> > if (dpolicy->timeout != 0)
> > f2fs_update_time(sbi, dpolicy->timeout);
> >
> > +retry:
> > for (i = MAX_PLIST_NUM - 1; i >= 0; i--) {
> > if (dpolicy->timeout != 0 &&
> > f2fs_time_over(sbi, dpolicy->timeout))
> > @@ -1509,7 +1558,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> > break;
> > }
> >
> > - __submit_discard_cmd(sbi, dpolicy, dc, &issued);
> > + err = __submit_discard_cmd(sbi, dpolicy, dc, &issued);
> > + if (err == -EAGAIN)
> > + congestion_wait(BLK_RW_ASYNC,
> > + DEFAULT_IO_TIMEOUT);
> >
> > if (issued >= dpolicy->max_requests)
> > break;
> > @@ -1522,6 +1574,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> > break;
> > }
> >
> > + if (!list_empty(&dcc->retry_list) &&
> > + __should_discard_retry(sbi, dpolicy))
> > + goto retry;
> > +
> > if (!issued && io_interrupted)
> > issued = -1;
> >
> > @@ -1613,6 +1669,12 @@ static unsigned int __wait_discard_cmd_range(struct f2fs_sb_info *sbi,
> > goto next;
> > }
> >
> > + if (dpolicy->type == DPOLICY_UMOUNT &&
> > + !list_empty(&dcc->retry_list)) {
> > + wait_list = &dcc->retry_list;
> > + goto next;
> > + }
> > +
> > return trimmed;
> > }
> >
> > @@ -2051,6 +2113,7 @@ static int create_discard_cmd_control(struct f2fs_sb_info *sbi)
> > for (i = 0; i < MAX_PLIST_NUM; i++)
> > INIT_LIST_HEAD(&dcc->pend_list[i]);
> > INIT_LIST_HEAD(&dcc->wait_list);
> > + INIT_LIST_HEAD(&dcc->retry_list);
> > INIT_LIST_HEAD(&dcc->fstrim_list);
> > mutex_init(&dcc->cmd_lock);
> > atomic_set(&dcc->issued_discard, 0);
> >

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

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