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Message-ID: <cab82676-27fd-b4e1-2cd8-3d8d26b44aa0@linux.vnet.ibm.com>
Date: Wed, 26 Jul 2023 23:10:10 +0530
From: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Juri Lelli <juri.lelli@...hat.com>,
Swapnil Sapkal <Swapnil.Sapkal@....com>,
Aaron Lu <aaron.lu@...el.com>, x86@...nel.org,
Peter Zijlstra <peterz@...radead.org>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 1/1] sched: Extend cpu idle state for 1ms
On 7/26/23 7:37 PM, Mathieu Desnoyers wrote:
> On 7/26/23 04:04, Shrikanth Hegde wrote:
>>
>>
>> On 7/26/23 1:00 AM, Mathieu Desnoyers wrote:
>>> Allow select_task_rq to consider a cpu as idle for 1ms after that cpu
>>> has exited the idle loop.
>>>
>>> This speeds up the following hackbench workload on a 192 cores AMD EPYC
>>> 9654 96-Core Processor (over 2 sockets):
>>>
>>> hackbench -g 32 -f 20 --threads --pipe -l 480000 -s 100
>>>
>>> from 49s to 34s. (30% speedup)
>>>
>>> My working hypothesis for why this helps is: queuing more than a single
>>> task on the runqueue of a cpu which just exited idle rather than
>>> spreading work over other idle cpus helps power efficiency on systems
>>> with large number of cores.
>>>
>>> This was developed as part of the investigation into a weird regression
>>> reported by AMD where adding a raw spinlock in the scheduler context
>>> switch accelerated hackbench.
Do you have SMT here? What is the system utilization when you are running
this workload?
>>>
>>> It turned out that changing this raw spinlock for a loop of 10000x
>>> cpu_relax within do_idle() had similar benefits.
>>>
>>> This patch achieve a similar effect without the busy-waiting by
>>> introducing a runqueue state sampling the sched_clock() when exiting
>>> idle, which allows select_task_rq to consider "as idle" a cpu which has
>>> recently exited idle.
>>>
>>> This patch should be considered "food for thoughts", and I would be glad
>>> to hear feedback on whether it causes regressions on _other_ workloads,
>>> and whether it helps with the hackbench workload on large Intel system
>>> as well.
>>>
>>> Link:
>>> https://lore.kernel.org/r/09e0f469-a3f7-62ef-75a1-e64cec2dcfc5@amd.com
>>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Cc: Valentin Schneider <vschneid@...hat.com>
>>> Cc: Steven Rostedt <rostedt@...dmis.org>
>>> Cc: Ben Segall <bsegall@...gle.com>
>>> Cc: Mel Gorman <mgorman@...e.de>
>>> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
>>> Cc: Vincent Guittot <vincent.guittot@...aro.org>
>>> Cc: Juri Lelli <juri.lelli@...hat.com>
>>> Cc: Swapnil Sapkal <Swapnil.Sapkal@....com>
>>> Cc: Aaron Lu <aaron.lu@...el.com>
>>> Cc: x86@...nel.org
>>> ---
>>> kernel/sched/core.c | 4 ++++
>>> kernel/sched/sched.h | 3 +++
>>> 2 files changed, 7 insertions(+)
>>>
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index a68d1276bab0..d40e3a0a5ced 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -6769,6 +6769,7 @@ void __sched schedule_idle(void)
>>> * TASK_RUNNING state.
>>> */
>>> WARN_ON_ONCE(current->__state);
>>> + WRITE_ONCE(this_rq()->idle_end_time, sched_clock());
>>> do {
>>> __schedule(SM_NONE);
>>> } while (need_resched());
>>> @@ -7300,6 +7301,9 @@ int idle_cpu(int cpu)
>>> {
>>> struct rq *rq = cpu_rq(cpu);
>>> + if (sched_clock() < READ_ONCE(rq->idle_end_time) +
>>> IDLE_CPU_DELAY_NS)
>>
>>
>> Wouldn't this hurt the latency badly? Specially on a loaded system with
>> a workload that does a lot of wakeup.
>
> Good point !
>
> Can you try your benchmark replacing the if () statement above by:
>
> + if (sched_clock() < READ_ONCE(rq->idle_end_time) +
> IDLE_CPU_DELAY_NS &&
> + READ_ONCE(rq->nr_running) <= 4)
> + return 1;
Tried with this change. I think it does help in reducing latency compared to
earlier specially till 95th percentile.
6.5-rc3 6.5-rc3+RFC_Patch 6.5-rc3_RFC_Patch
+ nr<4
4 Groups
50.0th: 18.00 18.50 18.50
75.0th: 21.50 26.00 23.50
90.0th: 56.00 940.50 501.00
95.0th: 678.00 1896.00 1392.00
99.0th: 2484.00 3756.00 3708.00
99.5th: 3224.00 4616.00 5088.00
99.9th: 4960.00 6824.00 8068.00
8 Groups
50.0th: 23.50 25.50 23.00
75.0th: 30.50 421.50 30.50
90.0th: 443.50 1722.00 741.00
95.0th: 1410.00 2736.00 1670.00
99.0th: 3942.00 5496.00 4032.00
99.5th: 5232.00 7016.00 5064.00
99.9th: 7996.00 8896.00 8012.00
16 Groups
50.0th: 33.50 41.50 32.50
75.0th: 49.00 752.00 47.00
90.0th: 1067.50 2332.00 994.50
95.0th: 2093.00 3468.00 2117.00
99.0th: 5048.00 6728.00 5568.00
99.5th: 6760.00 7624.00 6960.00
99.9th: 8592.00 9504.00 11104.00
32 Groups
50.0th: 60.00 79.00 53.00
75.0th: 456.50 1712.00 209.50
90.0th: 2788.00 3996.00 2752.00
95.0th: 4544.00 5768.00 5024.00
99.0th: 8444.00 9104.00 10352.00
99.5th: 9168.00 9808.00 12720.00
99.9th: 11984.00 12448.00 17624.00
>
> It speeds up the hackbench test-case even more here. It's now 30s, and
> it should
> improve tail latency.
>
> Thanks,
>
> Mathieu
>
>
>>
>> ran schbench on a 50% loaded system with stress-ng. (there could be a
>> better benchmark to measure latency)
>> I see that latency takes a hit. specially tail latencies.full log
>> below with different schbench groups.
>>
>> 6.5-rc3 6.5-rc3+this patch
>>
>> Groups: 1
>> 50.0th: 14.0 13.0
>> 75.0th: 16.0 16.0
>> 90.0th: 19.5 20.0
>> 95.0th: 53.0 226.0
>> 99.0th: 1969.0 2165.0
>> 99.5th: 2912.0 2648.0
>> 99.9th: 4680.0 4142.0
>>
>> Groups: 2
>> 50.0th: 15.5 15.5
>> 75.0th: 18.0 19.5
>> 90.0th: 25.5 497.0
>> 95.0th: 323.0 1384.0
>> 99.0th: 2055.0 3144.0
>> 99.5th: 2972.0 4014.0
>> 99.9th: 6026.0 6560.0
>>
>> Groups: 4
>> 50.0th: 18.0 18.5
>> 75.0th: 21.5 26.0
>> 90.0th: 56.0 940.5
>> 95.0th: 678.0 1896.0
>> 99.0th: 2484.0 3756.0
>> 99.5th: 3224.0 4616.0
>> 99.9th: 4960.0 6824.0
>>
>> Groups: 8
>> 50.0th: 23.5 25.5
>> 75.0th: 30.5 421.5
>> 90.0th: 443.5 1722.0
>> 95.0th: 1410.0 2736.0
>> 99.0th: 3942.0 5496.0
>> 99.5th: 5232.0 7016.0
>> 99.9th: 7996.0 8896.0
>>
>> Groups: 16
>> 50.0th: 33.5 41.5
>> 75.0th: 49.0 752.0
>> 90.0th: 1067.5 2332.0
>> 95.0th: 2093.0 3468.0
>> 99.0th: 5048.0 6728.0
>> 99.5th: 6760.0 7624.0
>> 99.9th: 8592.0 9504.0
>>
>> Groups: 32
>> 50.0th: 60.0 79.0
>> 75.0th: 456.5 1712.0
>> 90.0th: 2788.0 3996.0
>> 95.0th: 4544.0 5768.0
>> 99.0th: 8444.0 9104.0
>> 99.5th: 9168.0 9808.0
>> 99.9th: 11984.0 12448.0
>>
>>
>>> + return 1;
>>> +
>>> if (rq->curr != rq->idle)
>>> return 0;
>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>> index 81ac605b9cd5..8932e198a33a 100644
>>> --- a/kernel/sched/sched.h
>>> +++ b/kernel/sched/sched.h
>>> @@ -97,6 +97,8 @@
>>> # define SCHED_WARN_ON(x) ({ (void)(x), 0; })
>>> #endif
>>> +#define IDLE_CPU_DELAY_NS 1000000 /* 1ms */
>>> +
>>> struct rq;
>>> struct cpuidle_state;
>>> @@ -1010,6 +1012,7 @@ struct rq {
>>> struct task_struct __rcu *curr;
>>> struct task_struct *idle;
>>> + u64 idle_end_time;
There is clock_idle already in the rq. Can that be used for the same?
>>> struct task_struct *stop;
>>> unsigned long next_balance;
>>> struct mm_struct *prev_mm;
>
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