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.
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
