Messages in this thread Patch in this message | | | Subject | Re: [PATCH] sched/fair: don't assign runtime for throttled cfs_rq | From | Valentin Schneider <> | Date | Thu, 15 Aug 2019 17:36:43 +0100 |
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On 14/08/2019 19:00, Liangyan wrote: > do_sched_cfs_period_timer() will refill cfs_b runtime and call > distribute_cfs_runtime() to unthrottle cfs_rq, sometimes cfs_b->runtime > will allocate all quota to one cfs_rq incorrectly.
> This will cause other cfs_rq can't get runtime and will be throttled. > We find that one throttled cfs_rq has non-negative > cfs_rq->runtime_remaining and cause an unexpetced cast from s64 to u64 > in snippet: distribute_cfs_runtime() { > runtime = -cfs_rq->runtime_remaining + 1; }.
> This cast will cause that runtime will be a large number and > cfs_b->runtime will be subtracted to be zero at last. >
I'm a complete CFS bandwidth noob but let me give this a try...
-Wconversion does pick this up (turning this thing on made me understand why it's not on by default)
kernel/sched/fair.c: In function ‘distribute_cfs_runtime’: kernel/sched/fair.c:4633:13: warning: conversion to ‘u64’ {aka ‘long long unsigned int’} from ‘s64’ {aka ‘long long int’} may change the sign of the result [-Wsign-conversion] runtime = -cfs_rq->runtime_remaining + 1; ^ kernel/sched/fair.c:4638:29: warning: conversion to ‘long long unsigned int’ from ‘s64’ {aka ‘long long int’} may change the sign of the result [-Wsign-conversion] cfs_rq->runtime_remaining += runtime; ^~
The thing is we have a !cfs_rq_throttled() check just before the snippet you're calling out, so AFAICT cfs_rq->runtime_remaining has to be <= 0 there (otherwise this cfs_rq wouldn't be throttled).
I doubt you can get this to fire, but just to be sure... -----8<----- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index bbd90adabe2a..836948a3ae23 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4630,6 +4630,8 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining) if (!cfs_rq_throttled(cfs_rq)) goto next; + WARN_ON(cfs_rq->runtime_remaining > 0); + runtime = -cfs_rq->runtime_remaining + 1; if (runtime > remaining) runtime = remaining; ----->8----- Other than those signed/unsigned shenanigans, I only see one other scenario that leads to a cfs_rq getting allocated all the remaining runtime: its .runtime_remaining just has to be greater or equal (in absolute value) than the remaining runtime.
If that's happening consistently, I suppose that could be due to long delays between update_curr_fair() calls, but I can't think right why that would happen.
> This commit prevents cfs_rq to be assgined new runtime if it has been > throttled to avoid the above incorrect type cast. > > Signed-off-by: Liangyan <liangyan.peng@linux.alibaba.com> > --- > kernel/sched/fair.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 81fd8a2a605b..b14d67d28138 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4068,6 +4068,8 @@ static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec) > if (likely(cfs_rq->runtime_remaining > 0)) > return; > > + if (cfs_rq->throttled) > + return; > /* > * if we're unable to extend our runtime we resched so that the active > * hierarchy can be throttled >
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