From: Sebastian Andrzej Siewior Date: Sun, 16 Aug 2015 14:27:50 +0200 Subject: dump stack: don't disable preemption during trace Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.4/patches-4.4.1-rt5.tar.xz I see here large latencies during a stack dump on x86. The preempt_disable() and get_cpu() should forbid moving the task to another CPU during a stack dump and avoiding two stack traces in parallel on the same CPU. However a stack trace from a second CPU may still happen in parallel. Also nesting is allowed so a stack trace happens in process-context and we may have another one from IRQ context. With migrate disable we keep this code preemptible and allow a second backtrace on the same CPU by another task. Signed-off-by: Sebastian Andrzej Siewior --- arch/x86/kernel/dumpstack_32.c | 4 ++-- arch/x86/kernel/dumpstack_64.c | 8 ++++---- lib/dump_stack.c | 4 ++-- 3 files changed, 8 insertions(+), 8 deletions(-) --- a/arch/x86/kernel/dumpstack_32.c +++ b/arch/x86/kernel/dumpstack_32.c @@ -42,7 +42,7 @@ void dump_trace(struct task_struct *task unsigned long *stack, unsigned long bp, const struct stacktrace_ops *ops, void *data) { - const unsigned cpu = get_cpu(); + const unsigned cpu = get_cpu_light(); int graph = 0; u32 *prev_esp; @@ -86,7 +86,7 @@ void dump_trace(struct task_struct *task break; touch_nmi_watchdog(); } - put_cpu(); + put_cpu_light(); } EXPORT_SYMBOL(dump_trace); --- a/arch/x86/kernel/dumpstack_64.c +++ b/arch/x86/kernel/dumpstack_64.c @@ -152,7 +152,7 @@ void dump_trace(struct task_struct *task unsigned long *stack, unsigned long bp, const struct stacktrace_ops *ops, void *data) { - const unsigned cpu = get_cpu(); + const unsigned cpu = get_cpu_light(); struct thread_info *tinfo; unsigned long *irq_stack = (unsigned long *)per_cpu(irq_stack_ptr, cpu); unsigned long dummy; @@ -241,7 +241,7 @@ void dump_trace(struct task_struct *task * This handles the process stack: */ bp = ops->walk_stack(tinfo, stack, bp, ops, data, NULL, &graph); - put_cpu(); + put_cpu_light(); } EXPORT_SYMBOL(dump_trace); @@ -255,7 +255,7 @@ show_stack_log_lvl(struct task_struct *t int cpu; int i; - preempt_disable(); + migrate_disable(); cpu = smp_processor_id(); irq_stack_end = (unsigned long *)(per_cpu(irq_stack_ptr, cpu)); @@ -291,7 +291,7 @@ show_stack_log_lvl(struct task_struct *t pr_cont(" %016lx", *stack++); touch_nmi_watchdog(); } - preempt_enable(); + migrate_enable(); pr_cont("\n"); show_trace_log_lvl(task, regs, sp, bp, log_lvl); --- a/lib/dump_stack.c +++ b/lib/dump_stack.c @@ -33,7 +33,7 @@ asmlinkage __visible void dump_stack(voi * Permit this cpu to perform nested stack dumps while serialising * against other CPUs */ - preempt_disable(); + migrate_disable(); retry: cpu = smp_processor_id(); @@ -52,7 +52,7 @@ asmlinkage __visible void dump_stack(voi if (!was_locked) atomic_set(&dump_lock, -1); - preempt_enable(); + migrate_enable(); } #else asmlinkage __visible void dump_stack(void)