Messages in this thread | | | Date | Fri, 13 Mar 2020 16:38:46 +0530 | From | Sahitya Tummala <> | Subject | Re: [PATCH] f2fs: fix long latency due to discard during umount |
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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@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)) { > >> > >> 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; > >>> > >
-- -- 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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