Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

Tim Chen
2021-04-09T17:59:22Z

On 4/9/21 8:26 AM, Vincent Guittot wrote:

>>>>
>>>> I was expecting idle load balancer to be rate limited to 60 Hz, which
>>>
>>> Why 60Hz ?
>>>
>>
>> My thinking is we will trigger load balance only after rq->next_balance.
>>
>> void trigger_load_balance(struct rq *rq)
>> {
>> /* Don't need to rebalance while attached to NULL domain */
>> if (unlikely(on_null_domain(rq)))
>> return;
>>
>> if (time_after_eq(jiffies, rq->next_balance))
>> raise_softirq(SCHED_SOFTIRQ);
>>
>> nohz_balancer_kick(rq);
>> }
>>
>> And it seems like next_balance is set to be 60 Hz
>>
>> static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>> {
>> int continue_balancing = 1;
>> int cpu = rq->cpu;
>> int busy = idle != CPU_IDLE && !sched_idle_cpu(cpu);
>> unsigned long interval;
>> struct sched_domain *sd;
>> /* Earliest time when we have to do rebalance again */
>> unsigned long next_balance = jiffies + 60*HZ;
>
> This doesn't mean 60 Hz period but 60*HZ with HZ being the number of
> jiffies per second. We init next_balance with now + 60 sec to make
> sure it's far later than the next balance of the sched_domains
>
> Then, update_next_balance() keeps track of 1st balance to happen next time
>

Thanks for pointing out my misread of the code. In this case the
balance frequency should be lower than I thought as balance should be 60 sec
apart in theory.

>> Here's a snapshot of the trace. However I didn't have the current task in my trace.
>> You can tell the frequency that update_blocked_averages is called on
>> cpu 2 by the jiffies value. They are quite close together (1 to 3 jiffies apart).
>> When I have a chance to get on the machine, I'll take another look
>> at the current task and whether we got to trigger_load_balance() from scheduler_tick().
>>
>>
>> 3.505 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb731
>> 4.505 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb732
>> 6.484 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb733
>> 6.506 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb734
>> 9.503 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb737
>> 11.504 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb739
>> 11.602 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
>> 11.624 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 11.642 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 11.645 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 11.977 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 12.003 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 12.015 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 12.043 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 12.567 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb73a
>> 13.856 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73b
>> 13.910 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>> 14.003 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>> 14.159 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>> 14.203 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>> 14.223 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>> 14.301 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>> 14.504 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb73c
>> 14.637 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73c
>> 14.666 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.059 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.083 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.100 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.103 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.150 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.227 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.248 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.311 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>> 15.503 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb73d
>> 16.140 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73d
>> 16.185 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73d
>> 16.224 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73d
>> 16.340 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73d
>> 16.384 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73d
>> 16.503 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb73e
>> 16.993 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73e
>> 17.504 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb73f
>> 17.630 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73f
>> 17.830 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73f
>> 18.015 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73f
>> 18.031 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73f
>> 18.036 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73f
>> 18.040 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73f
>> 18.502 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb740
>>
>
> I don't know exactly what you track with "next_balance=" in

It is the rq->next_balance value as we enter the newidle_balance function.

> probe:newidle_balance but it always starts with the same value
> 0x1004fb76c in the future to finish with a value 0x1004fb731 in the
> past.

This indeed is odd as the next_balance should move forward and not backward.

> This would mean that a load balance is needed during the next
> tick which explains why we can see then the
> probe:update_blocked_averages for each tick.

Will try to debug and find out why the next_balance has gone backwards
next time I get access to the test system.

>
> Also could you check if the tick is stopped when idle. When the
> predicted idle time is short and the next wake is expected to happen
> before the next tick, the tick is not stopped.
>

Will do.

Tim