Thank you Sebastian for taking a look and rwb tag.

On 2024-11-08 15:48:53 [+0530], Shrikanth Hegde wrote:

quoted

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/ (local)
[2]: https://lore.kernel.org/all/1a973dda-c79e-4d95-935b-e4b93eb077b8@linux.ibm.com/ (local)

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@linutronix.de>

quoted

Signed-off-by: Shrikanth Hegde <redacted>
---

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@CONFIG_PREEMPT@CONFIG_PREEMPTION@ change ?

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

Sebastian