Messages in this thread | | | Subject | Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ | From | Tim Chen <> | Date | Wed, 7 Apr 2021 10:19:41 -0700 |
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On 4/7/21 7:02 AM, Vincent Guittot wrote: > Hi Tim, > > On Wed, 24 Mar 2021 at 17:05, Tim Chen <tim.c.chen@linux.intel.com> wrote: >> >> >> >> On 3/24/21 6:44 AM, Vincent Guittot wrote: >>> Hi Tim, >> >>> >>> IIUC your problem, we call update_blocked_averages() but because of: >>> >>> if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost) { >>> update_next_balance(sd, &next_balance); >>> break; >>> } >>> >>> the for_each_domain loop stops even before running load_balance on the 1st >>> sched domain level which means that update_blocked_averages() was called >>> unnecessarily. >>> >> >> That's right >> >>> And this is even more true with a small sysctl_sched_migration_cost which allows newly >>> idle LB for very small this_rq->avg_idle. We could wonder why you set such a low value >>> for sysctl_sched_migration_cost which is lower than the max_newidle_lb_cost of the >>> smallest domain but that's probably because of task_hot(). >>> >>> if avg_idle is lower than the sd->max_newidle_lb_cost of the 1st sched_domain, we should >>> skip spin_unlock/lock and for_each_domain() loop entirely >>> >>> Maybe something like below: >>> >> >> The patch makes sense. I'll ask our benchmark team to queue this patch for testing. > > Do you have feedback from your benchmark team ? >
Vincent,
Thanks for following up. I just got some data back from the benchmark team. The performance didn't change with your patch. And the overall cpu% of update_blocked_averages also remain at about the same level. My first thought was perhaps this update still didn't catch all the calls to update_blocked_averages
if (this_rq->avg_idle < sysctl_sched_migration_cost || - !READ_ONCE(this_rq->rd->overload)) { + !READ_ONCE(this_rq->rd->overload) || + (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
To experiment, I added one more check on the next_balance to further limit the path to actually do idle load balance with the next_balance time.
if (this_rq->avg_idle < sysctl_sched_migration_cost || - !READ_ONCE(this_rq->rd->overload)) { + time_before(jiffies, this_rq->next_balance) || + !READ_ONCE(this_rq->rd->overload) || + (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
I was suprised to find the overall cpu% consumption of update_blocked_averages and throughput of the benchmark still didn't change much. So I took a peek into the profile and found the update_blocked_averages calls shifted to the idle load balancer. The call to update_locked_averages was reduced in newidle_balance so the patch did what we intended. But the overall rate of calls to update_blocked_averages remain roughly the same, shifting from newidle_balance to run_rebalance_domains.
100.00% (ffffffff810cf070) | ---update_blocked_averages | |--95.47%--run_rebalance_domains | __do_softirq | | | |--94.27%--asm_call_irq_on_stack | | do_softirq_own_stack | | | | | |--93.74%--irq_exit_rcu | | | | | | | |--88.20%--sysvec_apic_timer_interrupt | | | | asm_sysvec_apic_timer_interrupt | | | | | ... | | --4.53%--newidle_balance pick_next_task_fair
I was expecting idle load balancer to be rate limited to 60 Hz, which should be 15 jiffies apart on the test system with CONFIG_HZ_250. When I did a trace on a single CPU, I see that update_blocked_averages are often called between 1 to 4 jiffies apart, which is at a much higher rate than I expected. I haven't taken a closer look yet. But you may have a better idea. I won't have access to the test system and workload till probably next week.
Thanks.
Tim
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