On Wed, Dec 14, 2016 at 06:39:24PM +0100, Michal Hocko wrote: > On Wed 14-12-16 08:48:27, Paul E. McKenney wrote: > > On Wed, Dec 14, 2016 at 05:15:41PM +0100, Michal Hocko wrote: > > > On Wed 14-12-16 03:06:09, Paul E. McKenney wrote: > > > > On Wed, Dec 14, 2016 at 10:54:25AM +0100, Michal Hocko wrote: > > > > > On Tue 13-12-16 07:14:08, Paul E. McKenney wrote: > > > > > > Just FYI for the moment... > > > > > > > > > > > > So even with the slowed-down checking, making cond_resched() do what > > > > > > cond_resched_rcu_qs() does results in a smallish but quite measurable > > > > > > degradation according to 0day. > > > > > > > > > > So if I understand those results properly, the reason seems to be the > > > > > increased involuntary context switches, right? Or am I misreading the > > > > > data? > > > > > I am looking at your "sched,rcu: Make cond_resched() provide RCU > > > > > quiescent state" in linux-next and I am wondering whether rcu_all_qs has > > > > > to be called unconditionally and not only when should_resched failed few > > > > > times? I guess you have discussed that with Peter already but do not > > > > > remember the outcome. > > > > > > > > My first thought is to wait for the grace period to age further before > > > > checking, the idea being to avoid increasing cond_resched() overhead > > > > any further. But if that doesn't work, then yes, I may have to look at > > > > adding more checks to cond_resched(). > > > > > > This might be really naive but would something like the following work? > > > The overhead should be pretty much negligible, I guess. Ideally the pcp > > > variable could be set somewhere from check_cpu_stall() but I couldn't > > > wrap my head around that code to see how exactly. > > > > My concern (perhaps misplaced) with this approach is that there are > > quite a few tight loops containing cond_resched(). So I would still > > need to throttle the resulting grace-period acceleration to keep the > > context switches down to a dull roar. > > Yes, I see your point. Something based on the stall timeout would be > much better of course. I just failed to come up with something that > would make sense. This was more my lack of familiarity with the code so > I hope you will be more successful ;) Well, here is my current shot at this. And so do I. ;-) So now it ignores cond_resched_rcu_qs() until at least jiffies_till_sched_qs jiffies have elapsed since the start of the grace period. The jiffies_till_sched_qs variable defaults to HZ/20, which should slow the checks down by about a factor of seven. Plus I don't see a problem with changing the default to (say) HZ/10 if needed. Thoughts? Thanx, Paul ------------------------------------------------------------------------ commit 7acd02c9e62fb21e7466e7a99fc21bf6ed6cc3cf Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Date: Tue Jan 3 16:49:46 2017 -0800 squash! rcu: Check cond_resched_rcu_qs() state less often to reduce GP overhead Now polling only after jiffies_till_sched_qs jiffies have elapsed. diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 083cb8a6299c..0369e0e0fe00 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1274,7 +1274,9 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp, static int rcu_implicit_dynticks_qs(struct rcu_data *rdp, bool *isidle, unsigned long *maxj) { + unsigned long jtsq; int *rcrmp; + unsigned long rjtsc; struct rcu_node *rnp; /* @@ -1291,6 +1293,17 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp, return 1; } + /* Compute and saturate jiffies_till_sched_qs. */ + jtsq = jiffies_till_sched_qs; + rjtsc = rcu_jiffies_till_stall_check(); + if (jtsq > rjtsc / 2) { + WRITE_ONCE(jiffies_till_sched_qs, rjtsc); + jtsq = rjtsc / 2; + } else if (jtsq < 1) { + WRITE_ONCE(jiffies_till_sched_qs, 1); + jtsq = 1; + } + /* * Has this CPU encountered a cond_resched_rcu_qs() since the * beginning of the grace period? For this to be the case, @@ -1298,7 +1311,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp, * might not be the case for nohz_full CPUs looping in the kernel. */ rnp = rdp->mynode; - if (READ_ONCE(rdp->rcu_qs_ctr_snap) != per_cpu(rcu_qs_ctr, rdp->cpu) && + if (time_after(jiffies, rdp->rsp->gp_start + jtsq) && + READ_ONCE(rdp->rcu_qs_ctr_snap) != per_cpu(rcu_qs_ctr, rdp->cpu) && READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) { trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc")); return 1; @@ -1333,9 +1347,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp, * warning delay. */ rcrmp = &per_cpu(rcu_sched_qs_mask, rdp->cpu); - if (ULONG_CMP_GE(jiffies, - rdp->rsp->gp_start + jiffies_till_sched_qs) || - ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) { + if (time_after(jiffies, rdp->rsp->gp_start + jtsq) || + time_after(jiffies, rdp->rsp->jiffies_resched)) { if (!(READ_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) { WRITE_ONCE(rdp->cond_resched_completed, READ_ONCE(rdp->mynode->completed)); -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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