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

From: Tim Chen
Date: Thu Apr 08 2021 - 19:05:27 EST

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On 4/8/21 7:51 AM, Vincent Guittot wrote:

>> 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
>
> At least , we have removed the useless call to update_blocked_averages
> in newidle_balance when we will not perform any newly idle load
> balance
>
>> 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
>
> The call of update_blocked_averages mainly comes from SCHED_SOFTIRQ.
> And as a result, not from the new path
> do_idle()->nohz_run_idle_balance() which has been added by this patch
> to defer the call to update_nohz_stats() after newlyidle_balance and
> before entering idle.
>
>> | | |
>> | | |--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
>
> 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;

>> 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
>
> 2 things can trigger a SCHED_SOFTIRQ/run_rebalance_domains:
> - the need for an update of blocked load which should not happen more
> than once every 32ms which means a rate of around 30Hz
> - the need for a load balance of a sched_domain. The min interval for
> a sched_domain is its weight when the CPU is idle which is usually few
> jiffies
>
> The only idea that I have for now is that we spend less time in
> newidle_balance which changes the dynamic of your system.
>
> In your trace, could you check if update_blocked_averages is called
> during the tick ? and Is the current task idle task ?

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

Thanks for taking a look.

Tim

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Previous message: Darrick J. Wong: "Re: [PATCH v4 7/7] fs/xfs: Add dedupe support for fsdax"
In reply to: Vincent Guittot: "Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ"
Next in thread: Vincent Guittot: "Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ"
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