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

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 22/01/2021 20:10, Joel Fernandes wrote:
> Hi Vincent,
> 
> Thanks for reply. Please see the replies below:
> 
> On Fri, Jan 22, 2021 at 05:56:22PM +0100, Vincent Guittot wrote:
>> On Fri, 22 Jan 2021 at 16:46, Joel Fernandes (Google)
>> <joel@joelfernandes.org> wrote:
>>>
>>> On an octacore ARM64 device running ChromeOS Linux kernel v5.4, I found
>>> that there are a lot of calls to update_blocked_averages(). This causes
>>> the schedule loop to slow down to taking upto 500 micro seconds at
>>> times (due to newidle load balance). I have also seen this manifest in
>>> the periodic balancer.
>>>
>>> Closer look shows that the problem is caused by the following
>>> ingredients:
>>> 1. If the system has a lot of inactive CGroups (thanks Dietmar for
>>> suggesting to inspect /proc/sched_debug for this), this can make
>>> __update_blocked_fair() take a long time.
>>
>> Inactive cgroups are removed from the list so they should not impact
>> the duration
> 
> I meant blocked CGroups. According to this code, a cfs_rq can be partially
> decayed and not have any tasks running on it but its load needs to be
> decayed, correct? That's what I meant by 'inactive'. I can reword it to
> 'blocked'.
> 
>                   * There can be a lot of idle CPU cgroups.  Don't let fully
>                   * decayed cfs_rqs linger on the list.
>                   */
>                  if (cfs_rq_is_decayed(cfs_rq))
>                          list_del_leaf_cfs_rq(cfs_rq);
> 
>>> 2. The device has a lot of CPUs in a cluster which causes schedutil in a
>>> shared frequency domain configuration to be slower than usual. (the load
>>
>> What do you mean exactly by it causes schedutil to be slower than usual ?
> 
> sugov_next_freq_shared() is order number of CPUs in the a cluster. This
> system is a 6+2 system with 6 CPUs in a cluster. schedutil shared policy
> frequency update needs to go through utilization of other CPUs in the
> cluster. I believe this could be adding to the problem but is not really
> needed to optimize if we can rate limit the calls to update_blocked_averages
> to begin with.
> 
>>> average updates also try to update the frequency in schedutil).
>>>
>>> 3. The CPU is running at a low frequency causing the scheduler/schedutil
>>> code paths to take longer than when running at a high CPU frequency.
>>
>> Low frequency usually means low utilization so it should happen that much.
> 
> It happens a lot as can be seen with schbench. It is super easy to reproduce.
> 
> schedule() can result in new idle balance with the CFS pick call happening
> often. Here is a function graph trace.  The tracer shows
> update_blocked_averages taking a lot of time.
> 
>      sugov:0-2454  [002]  2657.992570: funcgraph_entry:                   |  load_balance() {
>      sugov:0-2454  [002]  2657.992577: funcgraph_entry:                   |    update_group_capacity() {
>      sugov:0-2454  [002]  2657.992580: funcgraph_entry:        2.656 us   |      __msecs_to_jiffies();
>      sugov:0-2454  [002]  2657.992585: funcgraph_entry:        2.447 us   |      _raw_spin_lock_irqsave();
>      sugov:0-2454  [002]  2657.992591: funcgraph_entry:        2.552 us   |      _raw_spin_unlock_irqrestore();
>      sugov:0-2454  [002]  2657.992595: funcgraph_exit:       + 17.448 us  |    }
>      sugov:0-2454  [002]  2657.992597: funcgraph_entry:        1.875 us   |    update_nohz_stats();
>      sugov:0-2454  [002]  2657.992601: funcgraph_entry:        1.667 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992605: funcgraph_entry:                   |    update_nohz_stats() {
>      sugov:0-2454  [002]  2657.992608: funcgraph_entry:      + 33.333 us  |      update_blocked_averages();
>      sugov:0-2454  [002]  2657.992643: funcgraph_exit:       + 38.073 us  |    }
>      sugov:0-2454  [002]  2657.992645: funcgraph_entry:        1.770 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992649: funcgraph_entry:                   |    update_nohz_stats() {
>      sugov:0-2454  [002]  2657.992651: funcgraph_entry:      + 41.823 us  |      update_blocked_averages();
>      sugov:0-2454  [002]  2657.992694: funcgraph_exit:       + 45.729 us  |    }
>      sugov:0-2454  [002]  2657.992696: funcgraph_entry:        1.823 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992700: funcgraph_entry:                   |    update_nohz_stats() {
>      sugov:0-2454  [002]  2657.992702: funcgraph_entry:      + 35.312 us  |      update_blocked_averages();
>      sugov:0-2454  [002]  2657.992740: funcgraph_exit:       + 39.792 us  |    }
>      sugov:0-2454  [002]  2657.992742: funcgraph_entry:        1.771 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992746: funcgraph_entry:                   |    update_nohz_stats() {
>      sugov:0-2454  [002]  2657.992748: funcgraph_entry:      + 33.438 us  |      update_blocked_averages();
>      sugov:0-2454  [002]  2657.992783: funcgraph_exit:       + 37.500 us  |    }
>      sugov:0-2454  [002]  2657.992785: funcgraph_entry:        1.771 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992790: funcgraph_entry:                   |    update_nohz_stats() {
>      sugov:0-2454  [002]  2657.992792: funcgraph_entry:      + 45.521 us  |      update_blocked_averages();
>      sugov:0-2454  [002]  2657.992839: funcgraph_exit:       + 49.323 us  |    }
>      sugov:0-2454  [002]  2657.992842: funcgraph_entry:        1.823 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992847: funcgraph_entry:                   |    update_nohz_stats() {
>      sugov:0-2454  [002]  2657.992850: funcgraph_entry:      + 67.187 us  |      update_blocked_averages();
>      sugov:0-2454  [002]  2657.992919: funcgraph_exit:       + 72.031 us  |    }
>      sugov:0-2454  [002]  2657.992921: funcgraph_entry:        2.760 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992926: funcgraph_entry:                   |    update_nohz_stats() {
>      sugov:0-2454  [002]  2657.992928: funcgraph_entry:      + 61.146 us  |      update_blocked_averages();
>      sugov:0-2454  [002]  2657.992992: funcgraph_exit:       + 65.886 us  |    }
>      sugov:0-2454  [002]  2657.992994: funcgraph_entry:        1.771 us   |    idle_cpu();
>      sugov:0-2454  [002]  2657.992998: funcgraph_exit:       ! 430.209 us |  }
>      sugov:0-2454  [002]  2657.993006: bprint:               trace_long: wtf: lb: 432916
>      sugov:0-2454  [002]  2657.993017: bprint:               trace_long: wtf: newidle_balance: 501458
> 
> 
>>> The fix is simply rate limit the calls to update_blocked_averages to 20
>>> times per second. It appears that updating the blocked average less
>>> often is sufficient. Currently I see about 200 calls per second
>>
>> Would be good to explain why updating less often is sufficient ?
> 
> I don't know this code that well, intuitively it seems to me updating blocked
> averages at such a high rate seems pointless. But I defer to your expertise
> on that. Why do you feel an update is needed at least HZ times per second?
> What about system with HZ=1000 or 300, that seems to be an insane rate of
> updating (not to mention all the complexity of going through the leaf cgroup
> list and doing the frequency updates).

I assume this is what you're seeing on your device. This is on tip sched/core
but should be close to your kernel. I glanced over the diffs in fair.c between
chromeos-5.4 and tip sched/core and didn't spot any changes in this area.

I ran on a hikey620 w/o CONFIG_SCHED_MC to mimic the 8 CPUs (8 sched groups
(sg)) in the MC domain (the only sched domain). 

Since nohz.has_blocked=1 in your newidle_balance() calls,
load_balance() -> update_sd_lb_stats() sets LBF_NOHZ_STATS and calls
update_sg_lb_stats() for each of the 8 sg's.

Since LBF_NOHZ_STATS is set, update_sg_lb_stats() calls
update_nohz_stats(..., false) per cpu in sg.

And for a lot of these 8 sg's, i.e. 8 CPUs, update_blocked_averages()
is called since none of the 3 bail-out conditions:

 (1) !rq->has_blocked_load
 (2) !cpumask_test_cpu(cpu, nohz.idle_cpus_mask) 
 (3) force && !time_after(jiffies, rq->last_blocked_load_update_tick))

trigger.

We advance nohz.next_blocked by msecs_to_jiffies(LOAD_AVG_PERIOD) (32ms,
8 jiffies w/ HZ=250) but we advance 'rq->last_blocked_load_update_tick
only to jiffies' in update_blocked_load_status().


[005] 7370.188469: bprint: nohz_balance_enter_idle: CPU5 nohz.has_blocked=1
...
[005] 7370.210068: bprint: pick_next_task_fair: CPU5
[005] 7370.210079: bprint: update_sd_lb_stats: CPU5 nohz.has_blocked=1 -> set LBF_NOHZ_STATS
[005] 7370.210082: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=5 weight=1)
[005] 7370.210085: bprint: update_nohz_stats: CPU5 cpu=5 not in nohz.idle_cpus_mask -> bail
[005] 7370.210088: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=6 weight=1)
[005] 7370.210091: bprint: update_nohz_stats: CPU5 cpu=6 force=0 jiffies-last_blocked_load_update_tick=1 (jiffies-nohz.next_blocked=-7) -> update_blocked_averages()
[005] 7370.210112: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=7 weight=1)
[005] 7370.210116: bprint: update_nohz_stats: CPU5 cpu=7 force=0 jiffies-last_blocked_load_update_tick=1 (jiffies-nohz.next_blocked=-7) -> update_blocked_averages()
[005] 7370.210134: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=0 weight=1)
[005] 7370.210137: bprint: update_nohz_stats: CPU5 cpu=0 force=0 jiffies-last_blocked_load_update_tick=1 (jiffies-nohz.next_blocked=-7) -> update_blocked_averages()
[005] 7370.210156: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=1 weight=1)
[005] 7370.210159: bprint: update_nohz_stats: CPU5 cpu=1 not in nohz.idle_cpus_mask -> bail
[005] 7370.210162: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=2 weight=1)
[005] 7370.210165: bprint: update_nohz_stats: CPU5 cpu=2 force=0 jiffies-last_blocked_load_update_tick=1 (jiffies-nohz.next_blocked=-7) -> update_blocked_averages()
[005] 7370.210183: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=3 weight=1)
[005] 7370.210186: bprint: update_nohz_stats: CPU5 cpu=3 force=0 jiffies-last_blocked_load_update_tick=1 (jiffies-nohz.next_blocked=-7) -> update_blocked_averages()
[005] 7370.210205: bprint: update_sd_lb_stats: CPU5 dst_cpu=5 sg=(first_cpu=4 weight=1)
[005] 7370.210207: bprint: update_nohz_stats: CPU5 cpu=4 not in nohz.idle_cpus_mask -> bail
...
[005] 7370.444704: bprint: _nohz_idle_balance: CPU5 nohz.has_blocked=0


If I understood you correctly, you want to avoid these frequent calls
to update_blocked_averages() here to further avoid invoking sched_util
via update_blocked_averages() -> cpufreq_update_util() (since 'decayed'
is set) very often in your setup.
Since you have up to 6 CPUs in a frequency domain, this could be more
costly than usual.