Re: [PATCH 8/9] irqentry: define irqentry_exit_allow_resched()

Peter Zijlstra
2023-04-07T10:25:05Z

On Thu, Apr 06, 2023 at 07:29:59PM -0700, Ankur Arora wrote:
> Peter Zijlstra <peterz@infradead.org> writes:

> > Something like the below perhaps?
> >
> > ---
> > include/linux/entry-common.h | 6 ++++++
> > kernel/entry/common.c | 23 +++++++++++++++++++++--
> > 2 files changed, 27 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> > index d95ab85f96ba..0c365dc1f1c2 100644
> > --- a/include/linux/entry-common.h
> > +++ b/include/linux/entry-common.h
> > @@ -415,10 +415,16 @@ irqentry_state_t noinstr irqentry_enter(struct pt_regs *regs);
> > * Conditional reschedule with additional sanity checks.
> > */
> > void raw_irqentry_exit_cond_resched(void);
> > +void irqentry_exit_cond_resched_tif(void);
> > +
> > #ifdef CONFIG_PREEMPT_DYNAMIC
> > #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
> > #define irqentry_exit_cond_resched_dynamic_enabled raw_irqentry_exit_cond_resched
> > +#ifdef TIF_RESCHED_ALLOW
> > +#define irqentry_exit_cond_resched_dynamic_disabled irqentry_exit_cond_resched_tif
> > +#else
> > #define irqentry_exit_cond_resched_dynamic_disabled NULL
> > +#endif
>
> So this is clever. Essentially this would toggle between the two kinds
> for the preempt_model_preemptible()/!preempt_model_preemptible() dynamic
> case. Do I have that right?

Exactly!

> > DECLARE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> > #define irqentry_exit_cond_resched() static_call(irqentry_exit_cond_resched)()
> > #elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
> > diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> > index be61332c66b5..211d118aa672 100644
> > --- a/kernel/entry/common.c
> > +++ b/kernel/entry/common.c
> > @@ -390,6 +390,21 @@ void raw_irqentry_exit_cond_resched(void)
> > preempt_schedule_irq();
> > }
> > }
> > +
> > +void irqentry_exit_cond_resched_tif(void)
> > +{
> > +#ifdef TIF_RESCHED_ALLOW
> > + if (resched_allowed()) {
> > + /* Sanity check RCU and thread stack */
> > + rcu_irq_exit_check_preempt();
> > + if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
> > + WARN_ON_ONCE(!on_thread_stack());
> > + if (need_resched())
> > + preempt_schedule_irq();
> > + }
> > +#endif
> > +}
> > +
> > #ifdef CONFIG_PREEMPT_DYNAMIC
> > #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
> > DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> > @@ -397,8 +412,10 @@ DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> > DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> > void dynamic_irqentry_exit_cond_resched(void)
> > {
> > - if (!static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
> > - return;
> > + if (!static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched)) {
> > + irqentry_exit_cond_resched_tif();
> > + return
> > + }
> > raw_irqentry_exit_cond_resched();
> > }
> > #endif
> > @@ -431,6 +448,8 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
> > instrumentation_begin();
> > if (IS_ENABLED(CONFIG_PREEMPTION))
> > irqentry_exit_cond_resched();
> > + else
> > + irqentry_exit_cond_resched_tif();
>
> And, if we choose between the two resched modes at compile time then this
> would work.

Just so.

> Might massage the names a little but this should work as is.

Yeah, I'm not liking the naming either, perhaps your
irqentry_exit_allow_resched() would've been better, dunno, see what
works.

> Okay if I use your Codeveloped-by/Suggested-by on this patch?

Yep.