Messages in this thread Patch in this message | | | Date | Thu, 18 Apr 2024 02:34:53 +0800 | From | Chen Yu <> | Subject | Re: [RFC PATCH] sched/eevdf: Return leftmost entity in pick_eevdf() if no eligible entity is found |
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On 2024-04-09 at 11:21:04 +0200, Peter Zijlstra wrote: > On Mon, Apr 08, 2024 at 09:11:39PM +0800, Chen Yu wrote: > > On 2024-04-08 at 13:58:33 +0200, Peter Zijlstra wrote: > > > On Thu, Feb 29, 2024 at 05:00:18PM +0800, Abel Wu wrote: > > > > > > > > According to the log, vruntime is 18435852013561943404, the > > > > > cfs_rq->min_vruntime is 763383370431, the load is 629 + 2048 = 2677, > > > > > thus: > > > > > s64 delta = (s64)(18435852013561943404 - 763383370431) = -10892823530978643 > > > > > delta * 2677 = 7733399554989275921 > > > > > that is to say, the multiply result overflow the s64, which turns the > > > > > negative value into a positive value, thus eligible check fails. > > > > > > > > Indeed. > > > > > > From the data presented it looks like min_vruntime is wrong and needs > > > update. If you can readily reproduce this, dump the vruntime of all > > > tasks on the runqueue and see if min_vruntime is indeed correct. > > > > > > > This was the dump of all the entities on the tree, from left to right, > > Oh, my bad, I thought it was the pick path. > > > and also from top down in middle order traverse, when this issue happens: > > > > [ 514.461242][ T8390] cfs_rq avg_vruntime:386638640128 avg_load:2048 cfs_rq->min_vruntime:763383370431 > > [ 514.535935][ T8390] current on_rq se 0xc5851400, deadline:18435852013562231446 > > min_vruntime:18437121115753667698 vruntime:18435852013561943404, load:629 > > > > > > [ 514.536772][ T8390] Traverse rb-tree from left to right > > [ 514.537138][ T8390] se 0xec1234e0 deadline:763384870431 min_vruntime:763383370431 vruntime:763383370431 non-eligible <-- leftmost se > > [ 514.537835][ T8390] se 0xec4fcf20 deadline:763762447228 min_vruntime:763760947228 vruntime:763760947228 non-eligible > > > > [ 514.538539][ T8390] Traverse rb-tree from topdown > > [ 514.538877][ T8390] middle se 0xec1234e0 deadline:763384870431 min_vruntime:763383370431 vruntime:763383370431 non-eligible <-- root se > > [ 514.539605][ T8390] middle se 0xec4fcf20 deadline:763762447228 min_vruntime:763760947228 vruntime:763760947228 non-eligible > > > > The tree looks like: > > > > se (0xec1234e0) > > | > > | > > ----> se (0xec4fcf20) > > > > > > The root se 0xec1234e0 is also the leftmost se, its min_vruntime and > > vruntime are both 763383370431, which is aligned with > > cfs_rq->min_vruntime. It seems that the cfs_rq's min_vruntime gets > > updated correctly, because it is monotonic increasing. > > Right. > > > My guess is that, for some reason, one newly forked se in a newly > > created task group, in the rb-tree has not been picked for a long > > time(maybe not eligible). Its vruntime stopped at the negative > > value(near (unsigned long)(-(1LL << 20)) for a long time, its vruntime > > is long behind the cfs_rq->vruntime, thus the overflow happens. > > I'll have to do the math again, but that's something in the order of not > picking a task in about a day, that would be 'bad' :-) > > Is there any sane way to reproduce this, and how often does it happen?
After adding some ftrace in place_entity() and pick_eevdf(), with the help from Yujie in lkp, the issue was reproduced today. The reason why se's vruntime is very small seems to be related to task group's reweight_entity():
vlag = (s64)(avruntime - se->vruntime); vlag = div_s64(vlag * old_weight, weight); se->vruntime = avruntime - vlag;
The vlag above is not limited by neither 2*se->slice nor TICK_NSEC, if the new weight is very small, which is very likely, then the vlag could be very large, results in a very small vruntime.
The followings are the details why I think above could bring problems:
Here is the debug log printed by place_entity():
[ 397.597268]cfs_rq:0xe75f7100 cfs_rq.avg_vruntime:-1111846207333767 cfs_rq.min_vruntime:810640668779 avg_vruntime():686982466017 curr(0xc59f4f20 rb_producer weight:15 vruntime:1447773196654 sum_exec_ns:187707021870 ctx(0 73) leftmost(0xeacb6e00 vruntime:332464705486 sum_exec_ns:78776125437 load:677) ..
[ 397.877251]cfs_rq:0xe75f7100 cfs_rq.avg_vruntime:-759390883821798 cfs_rq.min_vruntime:810640668779 avg_vruntime(): 689577229374 curr(0xc59f4f20 rb_producer weight:15 vruntime:1453640907998 sum_ns:187792974673 ctx(0 73) leftmost(0xeacb6e00 vruntime:-59752941080010 sum_ns:78776125437 load:4)
The leftmost se is a task group, its vruntime reduces from 332464705486 to -59752941080010, because its load reduced from 677 to 4 due to update_cfs_group() on the tree entities.
Back to reweight_entity(): vlag = avruntime - se->vruntime = 689577229374 - 332464705486 = 357112523888; vlag = vlag * old_weight / weight = 357112523888 * 677 / 4 = 60441294668044; se->vruntime = avruntime - vlag = -59751717438670;
the new se vruntime -59751717438670 is close to what we printed -59752941080010, consider that the avg_vruntime() vary.
Then later this leftmost se has changed its load back and forth, and when the load is 2, the vuntime has reached a dangerous threshold to trigger the s64 overflow in eligible check:
[ 398.011991]cfs_rq:0xe75f7100 cfs_rq.avg_vruntime:-11875977385353427 cfs_rq.min_vruntime:810640668779 cfs_rq.avg_load:96985 leftmost(0xeacb6e00 vruntime:18446623907344963655 load:2)
vruntime_eligible() {
key = se.vruntime - cfs_rq.min_vruntime = -120977005256740; key * avg_load overflow s64... }
As a result the leftmost one can not be picked, and NULL is returned.
One workaround patch I'm thinking of, if this analysis is in the right direction, maybe I can have a test later:
thanks, Chenyu
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 6e0968fb9ba8..7ab26cdc3487 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3965,8 +3965,13 @@ static void reweight_eevdf(struct cfs_rq *cfs_rq, struct sched_entity *se, * = V - vl' */ if (avruntime != se->vruntime) { + s64 limit; + vlag = (s64)(avruntime - se->vruntime); vlag = div_s64(vlag * old_weight, weight); + /* TBD: using old weight or new weight? */ + limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se); + vlag = clamp(lag, -limit, limit); se->vruntime = avruntime - vlag; } -- 2.25.1
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