[PATCH] powerpc: Add preempt lazy support

[Shrikanth Hegde]

Thank you Sebastian for taking a look and rwb tag.

> On 2024-11-08 15:48:53 [+0530], Shrikanth Hegde wrote:
>> Define preempt lazy bit for Powerpc. Use bit 9 which is free and within
>> 16 bit range of NEED_RESCHED, so compiler can issue single andi.
>>
>> Since Powerpc doesn't use the generic entry/exit, add lazy check at exit
>> to user. CONFIG_PREEMPTION is defined for lazy/full/rt so use it for
>> return to kernel.
>>
>> Ran a few benchmarks and db workload on Power10. Performance is close to
>> preempt=none/voluntary. It is possible that some patterns would
>> differ in lazy[2]. More details of preempt lazy is here [1]
>>
>> Since Powerpc system can have large core count and large memory,
>> preempt lazy is going to be helpful in avoiding soft lockup issues.
>>
>> [1]: https://lore.kernel.org/lkml/20241007074609.447006177@infradead.org/
>> [2]: https://lore.kernel.org/all/1a973dda-c79e-4d95-935b-e4b93eb077b8@linux.ibm.com/
> 
> The lazy bits are only in tip.
>

Hi Michael, I sent it to powerpc tree since all the changes were in 
arch/powerpc. Please let me know if i have send it to tip tree instead.


> Reviewed-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
> 
>> Signed-off-by: Shrikanth Hegde <sshegde at linux.ibm.com>
>> ---
>> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
>> index af62ec974b97..8f4acc55407b 100644
>> --- a/arch/powerpc/kernel/interrupt.c
>> +++ b/arch/powerpc/kernel/interrupt.c
>> @@ -396,7 +396,7 @@ 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_PREEMPT)) {
>> +		if (IS_ENABLED(CONFIG_PREEMPTION)) {
>>   			/* Return to preemptible kernel context */
>>   			if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
>>   				if (preempt_count() == 0)
> 
> Shouldn't exit_vmx_usercopy() get also this
> s at CONFIG_PREEMPT@CONFIG_PREEMPTION@ change ?
> 

I had seen this, but wasn't sure. Will take a look at it.
Thanks for the pointers.

> Sebastian