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

Peter Zijlstra
2023-04-05T20:23:24Z

On Sun, Apr 02, 2023 at 10:22:32PM -0700, Ankur Arora wrote:
> Allow threads marked TIF_ALLOW_RESCHED to be rescheduled in irqexit.
>
> This is only necessary under !preempt_model_preemptible() for which
> we reuse the same logic as irqentry_exit_code_resched().
>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
> kernel/entry/common.c | 8 ++++++++
> kernel/sched/core.c | 36 +++++++++++++++++++++---------------
> 2 files changed, 29 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index be61332c66b5..f1005595ebe7 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -390,6 +390,9 @@ void raw_irqentry_exit_cond_resched(void)
> preempt_schedule_irq();
> }
> }
> +
> +void irqentry_exit_allow_resched(void) __alias(raw_irqentry_exit_cond_resched);

Because typing raw_irqentry_exit_cond_resched() was too much effort?

> +
> #ifdef CONFIG_PREEMPT_DYNAMIC
> #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
> DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> @@ -431,6 +434,11 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
> instrumentation_begin();
> if (IS_ENABLED(CONFIG_PREEMPTION))
> irqentry_exit_cond_resched();
> + /*
> + * We care about this clause only in the dynamic !preemptible case.
> + */
> + if (unlikely(!preempt_model_preemptible() && resched_allowed()))
> + irqentry_exit_allow_resched();

This is wrong, if we have dynamic preemption then we have
CONFIG_PREEMPTION and we'll have that irqentry_exit_cond_resched() call.

Basically what you've written above is something like:

static_call(foo); // raw_foo() when A, NOP if !A
if (!A)
raw_foo();

And yeah, you've got the extra resched_allowed() thing in there, but
that doesn't make it any better -- this is all quite horrible.

What you really care about is the CONFIG_PREEMPTION=n case, but that you
don't actually handle.

> /* Covers both tracing and lockdep */
> trace_hardirqs_on();
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0d18c3969f90..11845a91b691 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c

> @@ -8597,28 +8599,32 @@ EXPORT_SYMBOL(__cond_resched_rwlock_write);
> * SC:preempt_schedule
> * SC:preempt_schedule_notrace
> * SC:irqentry_exit_cond_resched
> + * SC:irqentry_exit_allow_resched
> *
> *
> * NONE:
> - * cond_resched <- __cond_resched
> - * might_resched <- RET0
> - * preempt_schedule <- NOP
> - * preempt_schedule_notrace <- NOP
> - * irqentry_exit_cond_resched <- NOP
> + * cond_resched <- __cond_resched
> + * might_resched <- RET0
> + * preempt_schedule <- NOP
> + * preempt_schedule_notrace <- NOP
> + * irqentry_exit_cond_resched <- NOP
> + * irqentry_exit_allow_resched <- irqentry_exit_allow_resched
> *
> * VOLUNTARY:
> - * cond_resched <- __cond_resched
> - * might_resched <- __cond_resched
> - * preempt_schedule <- NOP
> - * preempt_schedule_notrace <- NOP
> - * irqentry_exit_cond_resched <- NOP
> + * cond_resched <- __cond_resched
> + * might_resched <- __cond_resched
> + * preempt_schedule <- NOP
> + * preempt_schedule_notrace <- NOP
> + * irqentry_exit_cond_resched <- NOP
> + * irqentry_exit_allow_resched <- irqentry_exit_allow_resched
> *
> * FULL:
> - * cond_resched <- RET0
> - * might_resched <- RET0
> - * preempt_schedule <- preempt_schedule
> - * preempt_schedule_notrace <- preempt_schedule_notrace
> - * irqentry_exit_cond_resched <- irqentry_exit_cond_resched
> + * cond_resched <- RET0
> + * might_resched <- RET0
> + * preempt_schedule <- preempt_schedule
> + * preempt_schedule_notrace <- preempt_schedule_notrace
> + * irqentry_exit_cond_resched <- irqentry_exit_cond_resched
> + * irqentry_exit_allow_resched <- NOP
> */

This ^ is all complete nonsense.. irqentry_exit_allow_resched() is not
a static call. It's an alias of raw_irqentry_exit_cond_resched which
circumvents the static call entirely.