linux-kernel - [PATCH 2/3] powerpc: support dynamic preemption

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Message-ID: <20241125042212.1522315-3-sshegde@linux.ibm.com>
Date: Mon, 25 Nov 2024 09:52:11 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org
Cc: sshegde@...ux.ibm.com, npiggin@...il.com, christophe.leroy@...roup.eu,
        maddy@...ux.ibm.com, bigeasy@...utronix.de, ankur.a.arora@...cle.com,
        linux-kernel@...r.kernel.org, mark.rutland@....com,
        vschneid@...hat.com, peterz@...radead.org
Subject: [PATCH 2/3] powerpc: support dynamic preemption

Once the lazy preemption is supported, it would be desirable to change
the preemption models at runtime. So this change adds support for dynamic
preemption using DYNAMIC_KEY.

In irq-exit to kernel path, use preempt_model_preemptible for decision.
Other way would be using static key based decision. Keeping it
simpler since key based change didn't show performance improvement.

Tested lightly on Power10 LPAR. Performance numbers indicate that,
preempt=none(no dynamic) and preempt=none(dynamic) are similar.
Only hackbench pipe shows a regression. There is slight overhead of code
check if it is preemptible kernel. hackbench pipe is prone to such 
patterns[1]

cat /sys/kernel/debug/sched/preempt
(none) voluntary full lazy
perf stat -e probe:__cond_resched -a sleep 1
 Performance counter stats for 'system wide':
             1,253      probe:__cond_resched

echo full > /sys/kernel/debug/sched/preempt
cat /sys/kernel/debug/sched/preempt
none voluntary (full) lazy
perf stat -e probe:__cond_resched -a sleep 1
 Performance counter stats for 'system wide':
                 0      probe:__cond_resched

echo lazy > /sys/kernel/debug/sched/preempt
cat /sys/kernel/debug/sched/preempt
none voluntary full (lazy)
perf stat -e probe:__cond_resched -a sleep 1
 Performance counter stats for 'system wide':
                 0      probe:__cond_resched

[1]: https://lore.kernel.org/all/1a973dda-c79e-4d95-935b-e4b93eb077b8@linux.ibm.com/

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 arch/powerpc/Kconfig               | 1 +
 arch/powerpc/include/asm/preempt.h | 1 +
 arch/powerpc/kernel/interrupt.c    | 6 +++++-
 arch/powerpc/lib/vmx-helper.c      | 2 +-
 4 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 6d6bbd93abab..01c58f5258c9 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -270,6 +270,7 @@ config PPC
 	select HAVE_PERF_EVENTS_NMI		if PPC64
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_RETHOOK			if KPROBES
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE
diff --git a/arch/powerpc/include/asm/preempt.h b/arch/powerpc/include/asm/preempt.h
index 51f8f3881523..c0a19ff3f78c 100644
--- a/arch/powerpc/include/asm/preempt.h
+++ b/arch/powerpc/include/asm/preempt.h
@@ -84,6 +84,7 @@ extern asmlinkage void preempt_schedule_notrace(void);
 
 #if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
 
+DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
 void dynamic_preempt_schedule(void);
 void dynamic_preempt_schedule_notrace(void);
 #define __preempt_schedule()		dynamic_preempt_schedule()
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 8f4acc55407b..0fb01019d7e0 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -38,6 +38,10 @@ static inline bool exit_must_hard_disable(void)
 }
 #endif
 
+#ifdef CONFIG_PREEMPT_DYNAMIC
+DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+#endif
+
 /*
  * local irqs must be disabled. Returns false if the caller must re-enable
  * them, check for new work, and try again.
@@ -396,7 +400,7 @@ 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_PREEMPTION)) {
+		if (preempt_model_preemptible()) {
 			/* Return to preemptible kernel context */
 			if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
 				if (preempt_count() == 0)
diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c
index 58ed6bd613a6..7b069c832ce2 100644
--- a/arch/powerpc/lib/vmx-helper.c
+++ b/arch/powerpc/lib/vmx-helper.c
@@ -45,7 +45,7 @@ int exit_vmx_usercopy(void)
 	 * set and we are preemptible. The hack here is to schedule a
 	 * decrementer to fire here and reschedule for us if necessary.
 	 */
-	if (IS_ENABLED(CONFIG_PREEMPTION) && need_resched())
+	if (preempt_model_preemptible() && need_resched())
 		set_dec(1);
 	return 0;
 }
-- 
2.39.3