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SubjectRe: [PATCH] f2fs: fix long latency due to discard during umount
On 03/13, Sahitya Tummala wrote:
> On Thu, Mar 12, 2020 at 10:02:42AM -0700, Jaegeuk Kim wrote:
> > 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
>
> Yes, i will update it.
>
> >
> > > + __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().
>
> Yes, that is not changed and it still happens for the failed request
> that is re-queued here too when it gets submitted again later.
>
> I am requeuing the discard request failed with -EAGAIN error back to
> dcc->pend_list[] from wait_list. It will call submit_bio() for this request
> and also move to wait_list when it calls __submit_discard_cmd() again next
> time. Please let me know if I am missing anything?

This patch has no problem, but I'm thinking that __submit_discard_cmd() needs
to return with any values by assumption where the waiting list should have
submitted commands.

>
> Thanks,
>
> >
> > > + }
> > >
> > > 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.
>
> --
> --
> 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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