Messages in this thread | | | Date | Mon, 16 Mar 2020 09:22:33 +0530 | From | Sahitya Tummala <> | Subject | Re: [PATCH] f2fs: fix long latency due to discard during umount |
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Hi Chao,
On Mon, Mar 16, 2020 at 08:52:25AM +0800, Chao Yu wrote: > On 2020/3/13 19:08, Sahitya Tummala wrote: > > 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 > > I guess the case is there are multiple bios related to one dc and partially callback > of bio is called asynchronously and the other is called synchronously, so the race > condition could happen.
You are right. Let me review that case and try to fix it.
Thanks,
> > Thanks, > > > 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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