[PATCH v2 0/1] powerpc: Enable dynamic preemption

[PATCH v2 0/1] powerpc: Enable dynamic preemption

[Shrikanth Hegde]

Now that preempt=lazy patches[1] are in powerpc-next tree, sending out the
patch to support dynamic preemption based on DYNAMIC_KEY. I am not sure
when it was to be sent out exactly. So here it is.

Depends on [1] to be applied first, if not in the tree yet.

Once the arch supports static inline calls, it would be needed to
evaluate to see if that gives better performance. 

v1->v2:
- Instead of copying asm-generic preempt.h content include it in
  arch/asm preempt.h. (Christophe Leroy)
- Merge the patches into one patch (Christophe Leroy)

v1: https://lore.kernel.org/all/20241125042212.1522315-1-sshegde@linux.ibm.com/
[1]: https://lore.kernel.org/all/173572211264.1875638.9927288574435880962.b4-ty@linux.ibm.com/

Shrikanth Hegde (1):
  powerpc: Enable dynamic preemption

 arch/powerpc/Kconfig               |  1 +
 arch/powerpc/include/asm/preempt.h | 11 +++++++++++
 arch/powerpc/kernel/interrupt.c    |  6 +++++-
 arch/powerpc/kernel/traps.c        |  6 +++++-
 arch/powerpc/lib/vmx-helper.c      |  2 +-
 5 files changed, 23 insertions(+), 3 deletions(-)
 create mode 100644 arch/powerpc/include/asm/preempt.h

-- 
2.39.3

[PATCH v2 1/1] powerpc: Enable dynamic preemption

[Shrikanth Hegde]

Once the lazy preemption is supported, it would be desirable to change
the preemption models at runtime. So add 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.

Also print the right preemption model in __die. 

::Tested lightly on Power10 LPAR
Performance numbers indicate that, preempt=none(no dynamic) and 
preempt=none(dynamic) are similar.

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


Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com> 
---
 arch/powerpc/Kconfig               |  1 +
 arch/powerpc/include/asm/preempt.h | 11 +++++++++++
 arch/powerpc/kernel/interrupt.c    |  6 +++++-
 arch/powerpc/kernel/traps.c        |  6 +++++-
 arch/powerpc/lib/vmx-helper.c      |  2 +-
 5 files changed, 23 insertions(+), 3 deletions(-)
 create mode 100644 arch/powerpc/include/asm/preempt.h

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index db9f7b2d07bf..b14218344e74 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -272,6 +272,7 @@ 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
new file mode 100644
index 000000000000..9d2da4846874
--- /dev/null
+++ b/arch/powerpc/include/asm/preempt.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_POWERPC_PREEMPT_H
+#define __ASM_POWERPC_PREEMPT_H
+
+#include <asm-generic/preempt.h>
+
+#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
+DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+#endif
+
+#endif /* __ASM_POWERPC_PREEMPT_H */
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 8f4acc55407b..8e2400ba208c 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -25,6 +25,10 @@
 unsigned long global_dbcr0[NR_CPUS];
 #endif
 
+#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
+DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+#endif
+
 #ifdef CONFIG_PPC_BOOK3S_64
 DEFINE_STATIC_KEY_FALSE(interrupt_exit_not_reentrant);
 static inline bool exit_must_hard_disable(void)
@@ -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/kernel/traps.c b/arch/powerpc/kernel/traps.c
index edf5cabe5dfd..2556fa8ec019 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -266,7 +266,11 @@ static int __die(const char *str, struct pt_regs *regs, long err)
 	printk("%s PAGE_SIZE=%luK%s%s%s%s%s%s %s\n",
 	       IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ? "LE" : "BE",
 	       PAGE_SIZE / 1024, get_mmu_str(),
-	       IS_ENABLED(CONFIG_PREEMPT) ? " PREEMPT" : "",
+	       preempt_model_none()      ? "none" :
+	       preempt_model_voluntary() ? "voluntary" :
+	       preempt_model_full()      ? "full" :
+	       preempt_model_lazy()      ? "lazy" :
+	       "",
 	       IS_ENABLED(CONFIG_SMP) ? " SMP" : "",
 	       IS_ENABLED(CONFIG_SMP) ? (" NR_CPUS=" __stringify(NR_CPUS)) : "",
 	       debug_pagealloc_enabled() ? " DEBUG_PAGEALLOC" : "",
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

Re: [PATCH v2 1/1] powerpc: Enable dynamic preemption

[kernel test robot]

Hi Shrikanth,

kernel test robot noticed the following build errors:

[auto build test ERROR on powerpc/next]
[cannot apply to powerpc/fixes linus/master v6.13-rc5 next-20241220]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Shrikanth-Hegde/powerpc-Enable-dynamic-preemption/20250103-032131
base:   https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git next
patch link:    https://lore.kernel.org/r/20250102191856.499424-2-sshegde%40linux.ibm.com
patch subject: [PATCH v2 1/1] powerpc: Enable dynamic preemption
config: powerpc-randconfig-r072-20250104 (https://download.01.org/0day-ci/archive/20250104/202501040657.5tSQad1Y-lkp@intel.com/config)
compiler: clang version 20.0.0git (https://github.com/llvm/llvm-project 096551537b2a747a3387726ca618ceeb3950e9bc)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250104/202501040657.5tSQad1Y-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202501040657.5tSQad1Y-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:14:
   In file included from include/linux/sem.h:5:
   In file included from include/uapi/linux/sem.h:5:
   In file included from include/linux/ipc.h:7:
   In file included from include/linux/rhashtable-types.h:12:
   In file included from include/linux/alloc_tag.h:11:
   In file included from include/linux/preempt.h:79:
>> arch/powerpc/include/asm/preempt.h:8:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
       8 | DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
         | ^
         | int
>> arch/powerpc/include/asm/preempt.h:8:25: error: a parameter list without types is only allowed in a function definition
       8 | DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
         |                         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:98:11: warning: array index 3 is past the end of the array (that has type 'unsigned long[2]') [-Warray-bounds]
      98 |                 return (set->sig[3] | set->sig[2] |
         |                         ^        ~
   arch/powerpc/include/uapi/asm/signal.h:18:2: note: array 'sig' declared here
      18 |         unsigned long sig[_NSIG_WORDS];
         |         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:98:25: warning: array index 2 is past the end of the array (that has type 'unsigned long[2]') [-Warray-bounds]
      98 |                 return (set->sig[3] | set->sig[2] |
         |                                       ^        ~
   arch/powerpc/include/uapi/asm/signal.h:18:2: note: array 'sig' declared here
      18 |         unsigned long sig[_NSIG_WORDS];
         |         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:114:11: warning: array index 3 is past the end of the array (that has type 'const unsigned long[2]') [-Warray-bounds]
     114 |                 return  (set1->sig[3] == set2->sig[3]) &&
         |                          ^         ~
   arch/powerpc/include/uapi/asm/signal.h:18:2: note: array 'sig' declared here
      18 |         unsigned long sig[_NSIG_WORDS];
         |         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:114:27: warning: array index 3 is past the end of the array (that has type 'const unsigned long[2]') [-Warray-bounds]
     114 |                 return  (set1->sig[3] == set2->sig[3]) &&
         |                                          ^         ~
   arch/powerpc/include/uapi/asm/signal.h:18:2: note: array 'sig' declared here
      18 |         unsigned long sig[_NSIG_WORDS];
         |         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:115:5: warning: array index 2 is past the end of the array (that has type 'const unsigned long[2]') [-Warray-bounds]
     115 |                         (set1->sig[2] == set2->sig[2]) &&
         |                          ^         ~
   arch/powerpc/include/uapi/asm/signal.h:18:2: note: array 'sig' declared here
      18 |         unsigned long sig[_NSIG_WORDS];
         |         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:115:21: warning: array index 2 is past the end of the array (that has type 'const unsigned long[2]') [-Warray-bounds]
     115 |                         (set1->sig[2] == set2->sig[2]) &&
         |                                          ^         ~
   arch/powerpc/include/uapi/asm/signal.h:18:2: note: array 'sig' declared here
      18 |         unsigned long sig[_NSIG_WORDS];
         |         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:157:1: warning: array index 3 is past the end of the array (that has type 'const unsigned long[2]') [-Warray-bounds]
     157 | _SIG_SET_BINOP(sigorsets, _sig_or)
         | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/signal.h:138:8: note: expanded from macro '_SIG_SET_BINOP'
     138 |                 a3 = a->sig[3]; a2 = a->sig[2];                         \
         |                      ^      ~
   arch/powerpc/include/uapi/asm/signal.h:18:2: note: array 'sig' declared here
      18 |         unsigned long sig[_NSIG_WORDS];
         |         ^
   In file included from arch/powerpc/kernel/asm-offsets.c:12:
   In file included from include/linux/compat.h:17:
   In file included from include/linux/fs.h:33:
   In file included from include/linux/percpu-rwsem.h:7:
   In file included from include/linux/rcuwait.h:6:
   In file included from include/linux/sched/signal.h:6:
   include/linux/signal.h:157:1: warning: array index 2 is past the end of the array (that has type 'const unsigned long[2]') [-Warray-bounds]
     157 | _SIG_SET_BINOP(sigorsets, _sig_or)
         | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/signal.h:138:24: note: expanded from macro '_SIG_SET_BINOP'
     138 |                 a3 = a->sig[3]; a2 = a->sig[2];                         \


vim +/int +8 arch/powerpc/include/asm/preempt.h

     6	
     7	#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
   > 8	DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
     9	#endif
    10	

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