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

[Peter Zijlstra]

On Thu, Apr 06, 2023 at 09:56:07AM -0700, Ankur Arora wrote:

> 
> Right, so:
> 
> CONFIG_PREEMPTION: raw_foo()
> CONFIG_PREEMPTION && (preempt_dynamic_mode == preempt_dynamic_full): raw_foo()
> 
> This is NOP if CONFIG_PREEMPTION && preempt_dynamic_mode != preempt_dynamic_full.
> 
> > 	if (!A)
> > 		raw_foo();
> 
> So I would call raw_foo() for the CONFIG_PREEMPTION=n case.
> 
> > What you really care about is the CONFIG_PREEMPTION=n case, but that you
> > don't actually handle.
> 
> I don't see that. The CONFIG_PREEMPTION=n (or its dynamic version)
> is being handled here.

Bah, I overlooked we have multiple definitions of the
preempt_model_foo() things. For some reason I thought all that was only
for DYNAMIC_PREEMPT.

> > 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.
> 
> I don't disagree. There's a quite a lot of static/dynamic config options
> here and adding this clause didn't improve things.
> 
> I think just going with a static call here for the allow-resched case
> might have been cleaner. Alternately I'll see if it can be cleanly
> folded in with the irqentry_exit_cond_resched() logic.

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
 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();
 
 		/* Covers both tracing and lockdep */
 		trace_hardirqs_on();