linux-kernel - Re: [PATCH 2/3] powerpc: support dynamic preemption

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Message-ID: <cd3f01d0-fd34-4a5c-ac45-5d2dd2349f8a@csgroup.eu>
Date: Wed, 27 Nov 2024 07:28:23 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: npiggin@...il.com, maddy@...ux.ibm.com, bigeasy@...utronix.de,
 ankur.a.arora@...cle.com, linux-kernel@...r.kernel.org,
 mark.rutland@....com, vschneid@...hat.com, peterz@...radead.org,
 mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/3] powerpc: support dynamic preemption



Le 26/11/2024 à 12:15, Shrikanth Hegde a écrit :
> 
> 
> On 11/26/24 16:18, Christophe Leroy wrote:
>>
>>
> 
> Hi Christophe, Thanks for taking a look at this.
> 
>> Le 25/11/2024 à 05:22, Shrikanth Hegde a écrit :
>>> Once the lazy preemption is supported, it would be desirable to change
>>> the preemption models at runtime. So this change adds support for 
>>> dynamic
>>> preemption using DYNAMIC_KEY.
>>>
>>> In irq-exit to kernel path, use preempt_model_preemptible for decision.
>>> Other way would be using static key based decision. Keeping it
>>> simpler since key based change didn't show performance improvement.
>>>
>>> Tested lightly on Power10 LPAR. Performance numbers indicate that,
>>> preempt=none(no dynamic) and preempt=none(dynamic) are similar.
>>> Only hackbench pipe shows a regression. There is slight overhead of code
>>> check if it is preemptible kernel. hackbench pipe is prone to such
>>> patterns[1]
>>>
>>> cat /sys/kernel/debug/sched/preempt
>>> (none) voluntary full lazy
>>> perf stat -e probe:__cond_resched -a sleep 1
>>>   Performance counter stats for 'system wide':
>>>               1,253      probe:__cond_resched
>>>
>>> echo full > /sys/kernel/debug/sched/preempt
>>> cat /sys/kernel/debug/sched/preempt
>>> none voluntary (full) lazy
>>> perf stat -e probe:__cond_resched -a sleep 1
>>>   Performance counter stats for 'system wide':
>>>                   0      probe:__cond_resched
>>>
>>> echo lazy > /sys/kernel/debug/sched/preempt
>>> cat /sys/kernel/debug/sched/preempt
>>> none voluntary full (lazy)
>>> perf stat -e probe:__cond_resched -a sleep 1
>>>   Performance counter stats for 'system wide':
>>>                   0      probe:__cond_resched
>>>
>>> [1]: 
>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2F1a973dda-c79e-4d95-935b-&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cf0474c2567834b69dfd908dd0e0bb554%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638682165690258507%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Gcw6nRSPkp78lGEkG8NX04KWW%2FjCZm0oA%2BTGTjpUZUc%3D&reserved=0 e4b93eb077b8@...ux.ibm.com/
>>>
>>> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
>>> ---
>>>   arch/powerpc/Kconfig               | 1 +
>>>   arch/powerpc/include/asm/preempt.h | 1 +
>>>   arch/powerpc/kernel/interrupt.c    | 6 +++++-
>>>   arch/powerpc/lib/vmx-helper.c      | 2 +-
>>>   4 files changed, 8 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>>> index 6d6bbd93abab..01c58f5258c9 100644
>>> --- a/arch/powerpc/Kconfig
>>> +++ b/arch/powerpc/Kconfig
>>> @@ -270,6 +270,7 @@ config PPC
>>>       select HAVE_PERF_EVENTS_NMI        if PPC64
>>>       select HAVE_PERF_REGS
>>>       select HAVE_PERF_USER_STACK_DUMP
>>> +    select HAVE_PREEMPT_DYNAMIC_KEY
>>>       select HAVE_RETHOOK            if KPROBES
>>>       select HAVE_REGS_AND_STACK_ACCESS_API
>>>       select HAVE_RELIABLE_STACKTRACE
>>> diff --git a/arch/powerpc/include/asm/preempt.h b/arch/powerpc/ 
>>> include/asm/preempt.h
>>> index 51f8f3881523..c0a19ff3f78c 100644
>>> --- a/arch/powerpc/include/asm/preempt.h
>>> +++ b/arch/powerpc/include/asm/preempt.h
>>> @@ -84,6 +84,7 @@ extern asmlinkage void preempt_schedule_notrace(void);
>>>   #if defined(CONFIG_PREEMPT_DYNAMIC) && 
>>> defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
>>> +DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
>>>   void dynamic_preempt_schedule(void);
>>>   void dynamic_preempt_schedule_notrace(void);
>>>   #define __preempt_schedule()        dynamic_preempt_schedule()
>>> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/ 
>>> interrupt.c
>>> index 8f4acc55407b..0fb01019d7e0 100644
>>> --- a/arch/powerpc/kernel/interrupt.c
>>> +++ b/arch/powerpc/kernel/interrupt.c
>>> @@ -38,6 +38,10 @@ static inline bool exit_must_hard_disable(void)
>>>   }
>>>   #endif
>>> +#ifdef CONFIG_PREEMPT_DYNAMIC
>>> +DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
>>> +#endif
>>
>> Why is that needed at all ? It isn't used.
> 
> This is needed else compilation fails.
> 
> It has be defined by arch if it doesn't use kernel infra of entry/exit.
> So if an arch does enable, CONFIG_HAVE_PREEMPT_DYNAMIC_KEY it has to be 
> define this key has well. The generic sched/core enables this flag.
> 
> This was one of the point I was requesting answer for. Either to use 
> preempt_model_preemptible or define macros based on this key. Other 
> archs are doing the later and hence the generic code enables this key.
> 
> It can be done in either way. if we do the later way, then this variable 
> will be used as well.
> 

Ah right, I did a grep on sk_dynamic_irqentry_exit_cond_resched but 
indeed it is triggered by static_key_enable(&sk_dynamic_##f.key)

Christophe