Messages in this thread | | | Date | Tue, 19 Mar 2019 12:33:54 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH] sched: Do not re-read h_load_next during hierarchical load calculation |
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On Tue, Mar 19, 2019 at 01:06:09PM +0100, Peter Zijlstra wrote: > > --- > > kernel/sched/fair.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 310d0637fe4b..34aeb40e69d2 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -7726,7 +7726,7 @@ static void update_cfs_rq_h_load(struct cfs_rq *cfs_rq) > > cfs_rq->last_h_load_update = now; > > } > > > > - while ((se = cfs_rq->h_load_next) != NULL) { > > + while ((se = READ_ONCE(cfs_rq->h_load_next)) != NULL) { > > load = cfs_rq->h_load; > > load = div64_ul(load * se->avg.load_avg, > > cfs_rq_load_avg(cfs_rq) + 1); > > Where there is a READ_ONCE there should also be a corresponding > WRITE_ONCE(). Otherwise the compiler can still screw us over by doing > store-tearing. > > So something like the below. But looking at this, we probably also want > ONCE treatment on cfs_rq->h_load itself, but that's another patch. > > And I think we can do something with cfs_rq->last_h_load_update. >
Ok, I hadn't taken into account the possibility of store tearing but you're right, it is a possibility depending on the architecture and your patch is better. While there are some potential issues around h_load, I had treated it as data that is approximate and only considered the potential NULL dereference to be relevant.
I'll send a v2 shortly.
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