diff options
| author | Tejun Heo <tj@kernel.org> | 2021-12-13 14:14:43 -1000 |
|---|---|---|
| committer | Jens Axboe <axboe@kernel.dk> | 2021-12-14 06:58:15 -0700 |
| commit | edaa26334c117a584add6053f48d63a988d25a6e (patch) | |
| tree | 6c8f74d3eaebd0148c6e39e4d9263eccc51780e6 /block | |
| parent | 5eff363838654790f67f4bd564c5782967f67bcc (diff) | |
iocost: Fix divide-by-zero on donation from low hweight cgroup
The donation calculation logic assumes that the donor has non-zero
after-donation hweight, so the lowest active hweight a donating cgroup can
have is 2 so that it can donate 1 while keeping the other 1 for itself.
Earlier, we only donated from cgroups with sizable surpluses so this
condition was always true. However, with the precise donation algorithm
implemented, f1de2439ec43 ("blk-iocost: revamp donation amount
determination") made the donation amount calculation exact enabling even low
hweight cgroups to donate.
This means that in rare occasions, a cgroup with active hweight of 1 can
enter donation calculation triggering the following warning and then a
divide-by-zero oops.
WARNING: CPU: 4 PID: 0 at block/blk-iocost.c:1928 transfer_surpluses.cold+0x0/0x53 [884/94867]
...
RIP: 0010:transfer_surpluses.cold+0x0/0x53
Code: 92 ff 48 c7 c7 28 d1 ab b5 65 48 8b 34 25 00 ae 01 00 48 81 c6 90 06 00 00 e8 8b 3f fe ff 48 c7 c0 ea ff ff ff e9 95 ff 92 ff <0f> 0b 48 c7 c7 30 da ab b5 e8 71 3f fe ff 4c 89 e8 4d 85 ed 74 0
4
...
Call Trace:
<IRQ>
ioc_timer_fn+0x1043/0x1390
call_timer_fn+0xa1/0x2c0
__run_timers.part.0+0x1ec/0x2e0
run_timer_softirq+0x35/0x70
...
iocg: invalid donation weights in /a/b: active=1 donating=1 after=0
Fix it by excluding cgroups w/ active hweight < 2 from donating. Excluding
these extreme low hweight donations shouldn't affect work conservation in
any meaningful way.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: f1de2439ec43 ("blk-iocost: revamp donation amount determination")
Cc: stable@vger.kernel.org # v5.10+
Link: https://lore.kernel.org/r/Ybfh86iSvpWKxhVM@slm.duckdns.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block')
| -rw-r--r-- | block/blk-iocost.c | 9 |
1 files changed, 8 insertions, 1 deletions
diff --git a/block/blk-iocost.c b/block/blk-iocost.c index a5b37cc65b17..769b64394298 100644 --- a/block/blk-iocost.c +++ b/block/blk-iocost.c @@ -2311,7 +2311,14 @@ static void ioc_timer_fn(struct timer_list *timer) hwm = current_hweight_max(iocg); new_hwi = hweight_after_donation(iocg, old_hwi, hwm, usage, &now); - if (new_hwi < hwm) { + /* + * Donation calculation assumes hweight_after_donation + * to be positive, a condition that a donor w/ hwa < 2 + * can't meet. Don't bother with donation if hwa is + * below 2. It's not gonna make a meaningful difference + * anyway. + */ + if (new_hwi < hwm && hwa >= 2) { iocg->hweight_donating = hwa; iocg->hweight_after_donation = new_hwi; list_add(&iocg->surplus_list, &surpluses); |