| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <447f756c-9c79-f801-8257-a97cc8256efe@efficios.com>
Date: Wed, 26 Jul 2023 14:56:19 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Shrikanth Hegde <sshegde@...ux.vnet.ibm.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 13:40, Shrikanth Hegde wrote:
>
>
> 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?
Yes, SMT is enabled, which brings the number of logical cpus to 384.
CPU utilization (through htop):
* 6.4.4: 27500%
* 6.4.4 with the extend-idle+nr_running<=4 patch: 30500%
>
>>>>
>>>> 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.
For the records, I also tried with nr_running <= 2 and still had decent performance
(32s with nr_running <= 2 instead of 30s for nr_running <= 4). It did drop with
nr_running <= 1 (40s). nr_running <= 5 was similar to 4, and performances start
degrading with nr_running <= 8 (31s).
So it might be interesting to measure the latency with nr_running <= 2 as well.
Perhaps nr_running <= 2 would be a good compromise between throughput and tail
latency.
>
> 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
[...]
>>>> @@ -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?
Good point! And I'll change my use of "sched_clock()" in idle_cpu() for a
proper "sched_clock_cpu(cpu_of(rq))", which will work better on systems
without constant tsc.
The updated patch:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a68d1276bab0..1c7d5bd2968b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7300,6 +7300,10 @@ int idle_cpu(int cpu)
{
struct rq *rq = cpu_rq(cpu);
+ if (READ_ONCE(rq->nr_running) <= IDLE_CPU_DELAY_MAX_RUNNING &&
+ sched_clock_cpu(cpu_of(rq)) < READ_ONCE(rq->clock_idle) + IDLE_CPU_DELAY_NS)
+ return 1;
+
if (rq->curr != rq->idle)
return 0;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 81ac605b9cd5..57a49a5524f0 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -97,6 +97,9 @@
# define SCHED_WARN_ON(x) ({ (void)(x), 0; })
#endif
+#define IDLE_CPU_DELAY_NS 1000000 /* 1ms */
+#define IDLE_CPU_DELAY_MAX_RUNNING 4
+
struct rq;
struct cpuidle_state;
And using it now brings the hackbench wall time at 28s :)
Thanks,
Mathieu
>
>>>> struct task_struct *stop;
>>>> unsigned long next_balance;
>>>> struct mm_struct *prev_mm;
>>
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
Powered by blists - more mailing lists