Re: [PATCH 1/1] sched/fair: ratelimit update to tg->load_avg

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 8/24/23 09:03, Vincent Guittot wrote:
> On Thu, 24 Aug 2023 at 14:55, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> On 8/24/23 04:01, Aaron Lu wrote:
>>> On Wed, Aug 23, 2023 at 10:05:31AM -0400, Mathieu Desnoyers wrote:
>>>> On 8/23/23 02:08, Aaron Lu wrote:
>>>>> When using sysbench to benchmark Postgres in a single docker instance
>>>>> with sysbench's nr_threads set to nr_cpu, it is observed there are times
>>>>> update_cfs_group() and update_load_avg() shows noticeable overhead on
>>>>> a 2sockets/112core/224cpu Intel Sapphire Rapids(SPR):
>>>>>
>>>>>        13.75%    13.74%  [kernel.vmlinux]           [k] update_cfs_group
>>>>>        10.63%    10.04%  [kernel.vmlinux]           [k] update_load_avg
>>>>>
>>>>> Annotate shows the cycles are mostly spent on accessing tg->load_avg
>>>>> with update_load_avg() being the write side and update_cfs_group() being
>>>>> the read side. tg->load_avg is per task group and when different tasks
>>>>> of the same taskgroup running on different CPUs frequently access
>>>>> tg->load_avg, it can be heavily contended.
>>>>>
>>>>> E.g. when running postgres_sysbench on a 2sockets/112cores/224cpus Intel
>>>>> Sappire Rapids, during a 5s window, the wakeup number is 14millions and
>>>>> migration number is 11millions and with each migration, the task's load
>>>>> will transfer from src cfs_rq to target cfs_rq and each change involves
>>>>> an update to tg->load_avg. Since the workload can trigger as many wakeups
>>>>> and migrations, the access(both read and write) to tg->load_avg can be
>>>>> unbound. As a result, the two mentioned functions showed noticeable
>>>>> overhead. With netperf/nr_client=nr_cpu/UDP_RR, the problem is worse:
>>>>> during a 5s window, wakeup number is 21millions and migration number is
>>>>> 14millions; update_cfs_group() costs ~25% and update_load_avg() costs ~16%.
>>>>>
>>>>> Reduce the overhead by limiting updates to tg->load_avg to at most once
>>>>> per ms. After this change, the cost of accessing tg->load_avg is greatly
>>>>> reduced and performance improved. Detailed test results below.
>>>>
>>>> By applying your patch on top of my patchset at:
>>>>
>>>> https://lore.kernel.org/lkml/20230822113133.643238-1-mathieu.desnoyers@efficios.com/
>>>>
>>>> The combined hackbench results look very promising:
>>>>
>>>> (hackbench -g 32 -f 20 --threads --pipe -l 480000 -s 100)
>>>> (192 cores AMD EPYC 9654 96-Core Processor (over 2 sockets), with hyperthreading)
>>>>
>>>> Baseline:                                       49s
>>>> With L2-ttwu-queue-skip:                        34s (30% speedup)
>>>> With L2-ttwu-queue-skip + ratelimit-load-avg:   26s (46% speedup)
>>>>
>>>> Feel free to apply my:
>>>>
>>>> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>>> Tested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>>
>>> Thanks a lot for running this and reviewing the patch.
>>> I'll add your number and tag in the changelog when sending a new
>>> version.
>>
>> Now that I come to think of it, I have comment: why use
>> sched_clock_cpu() rather than just read the jiffies value ? AFAIR,
>> sched_clock can be slower than needed when read from a "remote" cpu on
>> architectures that have an unsynchronized tsc.
>>
>> Considering that you only need a time reference more or less accurate at
>> the millisecond level, I suspect that jiffies is what you are looking
>> for here. This is what the NUMA balance code and rseq mm_cid use to
>> execute work every N milliseconds.
> 
> tick can 4ms or even 10ms which means a rate limit up between 10ms to
> 20ms in the latter case

Fair enough, so just to confirm: is the 1ms a target period which has 
been empirically determined to be optimal (lower having too much 
overhead, and higher not being precise enough) ?

Thanks,

Mathieu

> 
>>
>> Thanks,
>>
>> Mathieu
>>
>>>
>>> Regards,
>>> Aaron
>>>
>>>>>
>>>>> ==============================
>>>>> postgres_sysbench on SPR:
>>>>> 25%
>>>>> base:   42382±19.8%
>>>>> patch:  50174±9.5%  (noise)
>>>>>
>>>>> 50%
>>>>> base:   67626±1.3%
>>>>> patch:  67365±3.1%  (noise)
>>>>>
>>>>> 75%
>>>>> base:   100216±1.2%
>>>>> patch:  112470±0.1% +12.2%
>>>>>
>>>>> 100%
>>>>> base:    93671±0.4%
>>>>> patch:  113563±0.2% +21.2%
>>>>>
>>>>> ==============================
>>>>> hackbench on ICL:
>>>>> group=1
>>>>> base:    114912±5.2%
>>>>> patch:   117857±2.5%  (noise)
>>>>>
>>>>> group=4
>>>>> base:    359902±1.6%
>>>>> patch:   361685±2.7%  (noise)
>>>>>
>>>>> group=8
>>>>> base:    461070±0.8%
>>>>> patch:   491713±0.3% +6.6%
>>>>>
>>>>> group=16
>>>>> base:    309032±5.0%
>>>>> patch:   378337±1.3% +22.4%
>>>>>
>>>>> =============================
>>>>> hackbench on SPR:
>>>>> group=1
>>>>> base:    100768±2.9%
>>>>> patch:   103134±2.9%  (noise)
>>>>>
>>>>> group=4
>>>>> base:    413830±12.5%
>>>>> patch:   378660±16.6% (noise)
>>>>>
>>>>> group=8
>>>>> base:    436124±0.6%
>>>>> patch:   490787±3.2% +12.5%
>>>>>
>>>>> group=16
>>>>> base:    457730±3.2%
>>>>> patch:   680452±1.3% +48.8%
>>>>>
>>>>> ============================
>>>>> netperf/udp_rr on ICL
>>>>> 25%
>>>>> base:    114413±0.1%
>>>>> patch:   115111±0.0% +0.6%
>>>>>
>>>>> 50%
>>>>> base:    86803±0.5%
>>>>> patch:   86611±0.0%  (noise)
>>>>>
>>>>> 75%
>>>>> base:    35959±5.3%
>>>>> patch:   49801±0.6% +38.5%
>>>>>
>>>>> 100%
>>>>> base:    61951±6.4%
>>>>> patch:   70224±0.8% +13.4%
>>>>>
>>>>> ===========================
>>>>> netperf/udp_rr on SPR
>>>>> 25%
>>>>> base:   104954±1.3%
>>>>> patch:  107312±2.8%  (noise)
>>>>>
>>>>> 50%
>>>>> base:    55394±4.6%
>>>>> patch:   54940±7.4%  (noise)
>>>>>
>>>>> 75%
>>>>> base:    13779±3.1%
>>>>> patch:   36105±1.1% +162%
>>>>>
>>>>> 100%
>>>>> base:     9703±3.7%
>>>>> patch:   28011±0.2% +189%
>>>>>
>>>>> ==============================================
>>>>> netperf/tcp_stream on ICL (all in noise range)
>>>>> 25%
>>>>> base:    43092±0.1%
>>>>> patch:   42891±0.5%
>>>>>
>>>>> 50%
>>>>> base:    19278±14.9%
>>>>> patch:   22369±7.2%
>>>>>
>>>>> 75%
>>>>> base:    16822±3.0%
>>>>> patch:   17086±2.3%
>>>>>
>>>>> 100%
>>>>> base:    18216±0.6%
>>>>> patch:   18078±2.9%
>>>>>
>>>>> ===============================================
>>>>> netperf/tcp_stream on SPR (all in noise range)
>>>>> 25%
>>>>> base:    34491±0.3%
>>>>> patch:   34886±0.5%
>>>>>
>>>>> 50%
>>>>> base:    19278±14.9%
>>>>> patch:   22369±7.2%
>>>>>
>>>>> 75%
>>>>> base:    16822±3.0%
>>>>> patch:   17086±2.3%
>>>>>
>>>>> 100%
>>>>> base:    18216±0.6%
>>>>> patch:   18078±2.9%
>>>>>
>>>>> Reported-by: Nitin Tekchandani <nitin.tekchandani@intel.com>
>>>>> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
>>>>> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
>>>>> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
>>>>> ---
>>>>>     kernel/sched/fair.c  | 13 ++++++++++++-
>>>>>     kernel/sched/sched.h |  1 +
>>>>>     2 files changed, 13 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index c28206499a3d..a5462d1fcc48 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -3664,7 +3664,8 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>>>>>      */
>>>>>     static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>>>>>     {
>>>>> -   long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
>>>>> +   long delta;
>>>>> +   u64 now;
>>>>>      /*
>>>>>       * No need to update load_avg for root_task_group as it is not used.
>>>>> @@ -3672,9 +3673,19 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>>>>>      if (cfs_rq->tg == &root_task_group)
>>>>>              return;
>>>>> +   /*
>>>>> +    * For migration heavy workload, access to tg->load_avg can be
>>>>> +    * unbound. Limit the update rate to at most once per ms.
>>>>> +    */
>>>>> +   now = sched_clock_cpu(cpu_of(rq_of(cfs_rq)));
>>>>> +   if (now - cfs_rq->last_update_tg_load_avg < NSEC_PER_MSEC)
>>>>> +           return;
>>>>> +
>>>>> +   delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
>>>>>      if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
>>>>>              atomic_long_add(delta, &cfs_rq->tg->load_avg);
>>>>>              cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg;
>>>>> +           cfs_rq->last_update_tg_load_avg = now;
>>>>>      }
>>>>>     }
>>>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>>>> index 6a8b7b9ed089..52ee7027def9 100644
>>>>> --- a/kernel/sched/sched.h
>>>>> +++ b/kernel/sched/sched.h
>>>>> @@ -593,6 +593,7 @@ struct cfs_rq {
>>>>>      } removed;
>>>>>     #ifdef CONFIG_FAIR_GROUP_SCHED
>>>>> +   u64                     last_update_tg_load_avg;
>>>>>      unsigned long           tg_load_avg_contrib;
>>>>>      long                    propagate;
>>>>>      long                    prop_runnable_sum;
>>>>
>>>> --
>>>> Mathieu Desnoyers
>>>> EfficiOS Inc.
>>>> https://www.efficios.com
>>>>
>>
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> https://www.efficios.com
>>

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com