Re: [PATCH] powerpc: Add preempt lazy support

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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

The lazy bits are only in tip.

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> 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 ?

Sebastian