[PATCH] powerpc: Add preempt lazy support

[Shrikanth Hegde]

On 11/9/24 00:36, Ankur Arora wrote:
> 
> Shrikanth Hegde <sshegde at 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 at linux.ibm.com>
> 
> Looks good. Reviewed-by: <ankur.a.arora at 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?

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();
> 
> 
> Ankur