Messages in this thread | | | Subject | Re: [PATCH -next v5 4/8] blk-throttle: fix io hung due to config updates | From | Yu Kuai <> | Date | Thu, 23 Jun 2022 20:27:11 +0800 |
| |
Hi,
在 2022/06/23 1:26, Michal Koutný 写道: > (Apologies for taking so long before answering.) > > On Sat, May 28, 2022 at 02:43:26PM +0800, Yu Kuai <yukuai3@huawei.com> wrote: >> Some simple test: >> 1) >> cd /sys/fs/cgroup/blkio/ >> echo $$ > cgroup.procs >> echo "8:0 2048" > blkio.throttle.write_bps_device >> { >> sleep 2 >> echo "8:0 1024" > blkio.throttle.write_bps_device >> } & >> dd if=/dev/zero of=/dev/sda bs=8k count=1 oflag=direct >> >> 2) >> cd /sys/fs/cgroup/blkio/ >> echo $$ > cgroup.procs >> echo "8:0 1024" > blkio.throttle.write_bps_device >> { >> sleep 4 >> echo "8:0 2048" > blkio.throttle.write_bps_device >> } & >> dd if=/dev/zero of=/dev/sda bs=8k count=1 oflag=direct >> >> test results: io finish time >> before this patch with this patch >> 1) 10s 6s >> 2) 8s 6s > > I agree these are consistent and correct times. > > And the new implementation won't make it worse (in terms of delaying a > bio) than configuring minimal limits from the beginning, AFACT. > >> @@ -801,7 +836,8 @@ static bool tg_with_in_iops_limit(struct throtl_grp *tg, struct bio *bio, >> >> /* Round up to the next throttle slice, wait time must be nonzero */ >> jiffy_elapsed_rnd = roundup(jiffy_elapsed + 1, tg->td->throtl_slice); >> - io_allowed = calculate_io_allowed(iops_limit, jiffy_elapsed_rnd); >> + io_allowed = calculate_io_allowed(iops_limit, jiffy_elapsed_rnd) + >> + tg->io_skipped[rw]; >> if (tg->io_disp[rw] + 1 <= io_allowed) { >> if (wait) >> *wait = 0; >> @@ -838,7 +874,8 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio, >> jiffy_elapsed_rnd = tg->td->throtl_slice; >> >> jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, tg->td->throtl_slice); >> - bytes_allowed = calculate_bytes_allowed(bps_limit, jiffy_elapsed_rnd); >> + bytes_allowed = calculate_bytes_allowed(bps_limit, jiffy_elapsed_rnd) + >> + tg->bytes_skipped[rw]; >> if (tg->bytes_disp[rw] + bio_size <= bytes_allowed) { >> if (wait) >> *wait = 0; >> > > Here we may allow to dispatch a bio above current slice's > calculate_bytes_allowed() if bytes_skipped is already >0.
Hi, I don't expect that to happen. For example, if a bio is still throttled, then old slice is keeped with proper 'bytes_skipped', then new wait time is caculated based on (bio_size - bytes_skipped).
After the bio is dispatched(I assum that other bios can't preempt), if new slice is started, then 'bytes_skipped' is cleared, there should be no problem; If old slice is extended, note that we only wait for 'bio_size - bytes_skipped' bytes, while 'bio_size' bytes is added to 'tg->bytes_disp'. I think this will make sure new bio won't be dispatched above slice.
What do you think? > > bytes_disp + bio_size <= calculate_bytes_allowed() + bytes_skipped > > Then on the next update > >> [shuffle] >> +static void __tg_update_skipped(struct throtl_grp *tg, bool rw) >> +{ >> + unsigned long jiffy_elapsed = jiffies - tg->slice_start[rw]; >> + u64 bps_limit = tg_bps_limit(tg, rw); >> + u32 iops_limit = tg_iops_limit(tg, rw); >> + >> + if (bps_limit != U64_MAX) >> + tg->bytes_skipped[rw] += >> + calculate_bytes_allowed(bps_limit, jiffy_elapsed) - >> + tg->bytes_disp[rw]; >> + if (iops_limit != UINT_MAX) >> + tg->io_skipped[rw] += >> + calculate_io_allowed(iops_limit, jiffy_elapsed) - >> + tg->io_disp[rw]; >> +} > > the difference(s) here could be negative. bytes_skipped should be > reduced to account for the additionally dispatched bio. > This is all unsigned so negative numbers underflow, however, we add them > again to the unsigned, so thanks to modular arithmetics the result is > correctly updated bytes_skipped. > > Maybe add a comment about this (unsigned) intention?
Of course I can do that. > > (But can this happen? The discussed bio would have to outrun another bio > (the one which defined the current slice_end) but since blk-throttle > uses queues (FIFO) everywhere this shouldn't really happen. But it's > good to know this works as intended.) I can also mention that in comment. > > This patch can have > Reviewed-by: Michal Koutný <mkoutny@suse.com> >
Thanks for the review! Kuai
| |