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powerpc: enable dynamic preemption

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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.

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

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>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250210184334.567383-2-sshegde@linux.ibm.com
This commit is contained in:
Shrikanth Hegde 2025-02-11 00:13:34 +05:30 • committed by Madhavan Srinivasan
parent 28affd477b
commit 6ad7751537
4 changed files with 23 additions and 2 deletions
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1
arch/powerpc/Kconfig
View file

@ -277,6 +277,7 @@ 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

16
arch/powerpc/include/asm/preempt.h Normal file
View file

@ -0,0 +1,16 @@
/* 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)
#include <linux/jump_label.h>
DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
#define need_irq_preemption() \
(static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
#else
#define need_irq_preemption() (IS_ENABLED(CONFIG_PREEMPTION))
#endif
#endif /* __ASM_POWERPC_PREEMPT_H */

6
arch/powerpc/kernel/interrupt.c
View file

@ -25,6 +25,10 @@
unsigned long global_dbcr0[NR_CPUS];
#endif
#if defined(CONFIG_PREEMPT_DYNAMIC)
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 (need_irq_preemption()) {
/* Return to preemptible kernel context */
if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
if (preempt_count() == 0)

2
arch/powerpc/lib/vmx-helper.c
View file

@ -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 (need_irq_preemption() && need_resched())
set_dec(1);
return 0;
}