Shrikanth Hegde <sshe...@linux.ibm.com> writes:
> On 11/9/24 22:24, Shrikanth Hegde wrote: >> >> On 11/9/24 00:36, Ankur Arora wrote: >>> >>> Shrikanth Hegde <sshe...@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/20241007074609.447006...@infradead.org/ >>>> [2]: https://lore.kernel.org/all/1a973dda-c79e-4d95-935b- >>>> e4b93eb07...@linux.ibm.com/ >>>> >>>> Signed-off-by: Shrikanth Hegde <sshe...@linux.ibm.com> >>> >>> Looks good. Reviewed-by: <ankur.a.ar...@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. The generic kvm entry/exit is gated by CONFIG_KVM_XFER_TO_GUEST_WORK. >> 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. Oh yeah. Missed that it was calling cond_resched(). > So, i need to spend more time on this and figure out, will send the patches > after that. > >>> >>> Ankur >> -- ankur
