[PATCH RT 3/7] dump stack: dont disable preemption during trace

From: Steven Rostedt
Date: Fri Feb 26 2016 - 17:22:42 EST

3.4.110-rt139-rc1 stable review patch.
If anyone has any objections, please let me know.


From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

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 <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
arch/x86/kernel/dumpstack_64.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index e8206060a0a8..bebc49d03531 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -114,7 +114,7 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
unsigned long *stack, unsigned long bp,
const struct stacktrace_ops *ops, void *data)
- const unsigned cpu = get_cpu();
+ const unsigned cpu = get_cpu_light();
unsigned long *irq_stack_end =
(unsigned long *)per_cpu(irq_stack_ptr, cpu);
unsigned used = 0;
@@ -191,7 +191,7 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
* This handles the process stack:
bp = ops->walk_stack(tinfo, stack, bp, ops, data, NULL, &graph);
- put_cpu();
+ put_cpu_light();

@@ -205,7 +205,7 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
int cpu;
int i;

- preempt_disable();
+ migrate_disable();
cpu = smp_processor_id();

irq_stack_end = (unsigned long *)(per_cpu(irq_stack_ptr, cpu));
@@ -238,7 +238,7 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
printk(KERN_CONT " %016lx", *stack++);
- preempt_enable();
+ migrate_enable();

printk(KERN_CONT "\n");
show_trace_log_lvl(task, regs, sp, bp, log_lvl);