Messages in this thread | | | Date | Tue, 2 Aug 2016 13:24:17 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4.4 043/146] sched/fair: Fix cfs_rq avg tracking underflow |
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On Mon, Jul 25, 2016 at 03:12:24PM -0700, bsegall@google.com wrote: > > @@ -2690,15 +2707,15 @@ static inline int update_cfs_rq_load_avg > > > > if (atomic_long_read(&cfs_rq->removed_load_avg)) { > > s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0); > > - sa->load_avg = max_t(long, sa->load_avg - r, 0); > > - sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0); > > + sub_positive(&sa->load_avg, r); > > + sub_positive(&sa->load_sum, r * LOAD_AVG_MAX); > > removed = 1; > > } > > > > if (atomic_long_read(&cfs_rq->removed_util_avg)) { > > long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0); > > - sa->util_avg = max_t(long, sa->util_avg - r, 0); > > - sa->util_sum = max_t(s32, sa->util_sum - r * LOAD_AVG_MAX, 0); > > + sub_positive(&sa->util_avg, r); > > + sub_positive(&sa->util_sum, r * LOAD_AVG_MAX); > > } > > > > decayed = __update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
> I missed this the first time around, and I have no problem with this > backport, but it's not remotely obvious that update_cfs_rq_h_load() is > intended to be something approaching racing-safe when not under > rq->lock. (And given only somewhat-adversarial compilers I agree that it > probably won't do any worse than skip updates, though I certainly won't > swear to it)
Right, this is somewhat 'new' because we now use the avg load value for the load-balancer, which is entirely unserialized.
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