Re: [PATCH] powerpc: Add preempt lazy support

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 11/9/24 22:24, Shrikanth Hegde wrote:
> 
> 
> On 11/9/24 00:36, Ankur Arora wrote:
>>
>> Shrikanth Hegde <sshegde@linux.ibm.com> writes:
>>
>>> 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/
>>>
>>> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
>>
>> Looks good. Reviewed-by: <ankur.a.arora@oracle.com>
> 
> Thank you Ankur for taking a look and rwb tag.
> 
>>
>> However, I just checked and powerpc does not have
>> CONFIG_KVM_XFER_TO_GUEST_WORK. Do you need this additional patch
>> for handling the lazy bit at KVM guest entry?
> 

It doesn't use the generic kvm entry/exit either AFAIK. I need to 
understand more of this kvm maze. There are quite a lot of combinations.

> will take a look. Thanks for the pointers.
> 
>>
>> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
>> index f14329989e9a..7bdf7015bb65 100644
>> --- a/arch/powerpc/kvm/powerpc.c
>> +++ b/arch/powerpc/kvm/powerpc.c
>> @@ -84,7 +84,8 @@ int kvmppc_prepare_to_enter(struct kvm_vcpu *vcpu)
>>          hard_irq_disable();
>>
>>          while (true) {
>> -               if (need_resched()) {
>> +               unsigned long tf = read_thread_flags();
>> +               if (tf & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) {
>>                          local_irq_enable();
>>                          cond_resched();
>>                          hard_irq_disable();
>>

This is not going help since, with LAZY, cond_resched is nop. So it 
doesn't call to schedule. Same is true with preempt=full. I need to 
figure out if kvm stuff was tested for preempt=full.

Instead of cond_resched this needs to use schedule instead. Need to test 
it out and also see other places for kvm.
So, i need to spend more time on this and figure out, will send the 
patches after that.

>>
>> Ankur
>