[PATCH 2/3] powerpc: support dynamic preemption

[Shrikanth Hegde]

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://lore.kernel.org/all/1a973dda-c79e-4d95-935b- 
>> e4b93eb077b8 at linux.ibm.com/
>>
>> Signed-off-by: Shrikanth Hegde <sshegde at linux.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.

I hope this answers the question on patch 1 as well. Otherwise i have to 
declare that in one of other arch/asm/.h file.

> 
>> +
>>   /*
>>    * local irqs must be disabled. Returns false if the caller must re- 
>> enable
>>    * them, check for new work, and try again.
>> @@ -396,7 +400,7 @@ notrace unsigned long 
>> interrupt_exit_kernel_prepare(struct pt_regs *regs)
>>           /* Returning to a kernel context with local irqs enabled. */
>>           WARN_ON_ONCE(!(regs->msr & MSR_EE));
>>   again:
>> -        if (IS_ENABLED(CONFIG_PREEMPTION)) {
>> +        if (preempt_model_preemptible()) {
>>               /* Return to preemptible kernel context */
>>               if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
>>                   if (preempt_count() == 0)
>> diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx- 
>> helper.c
>> index 58ed6bd613a6..7b069c832ce2 100644
>> --- a/arch/powerpc/lib/vmx-helper.c
>> +++ b/arch/powerpc/lib/vmx-helper.c
>> @@ -45,7 +45,7 @@ int exit_vmx_usercopy(void)
>>        * set and we are preemptible. The hack here is to schedule a
>>        * decrementer to fire here and reschedule for us if necessary.
>>        */
>> -    if (IS_ENABLED(CONFIG_PREEMPTION) && need_resched())
>> +    if (preempt_model_preemptible() && need_resched())
>>           set_dec(1);
>>       return 0;
>>   }