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/20241007074609.447006...@infradead.org/
> [2]:
> https://lore.kernel.org/all/1a973dda-c79e-4d95-935b-e4b93eb07...@linux.ibm.com/
The lazy bits are only in tip.
Reviewed-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
> Signed-off-by: Shrikanth Hegde <sshe...@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
