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Subject[PATCH] sysrq: freeze other CPUs during sysrq-t
From: Gary  Shi <garyu.shi@xxxxxxxxxx>
When I read sysrq-t call trace, I find the collected call trace for
some cases is not a still snapshot, but a moving one. After checking
show_state src, I realize other cpus are not frozen when one cpu is
doing show_state.

This moving call traces make debugging much more difficult, or even
impossible for some cases, since during sysrq-t, some tasks have been
switched in/out, threads can jump from kernel space to user space(or
vice versa), or just keep running in kernel space. This make us lose
the trace of some important data, like which thread is
holding a lock when lots of threads are blocked on the lock.

In order to get a still image of sysrq-t call trace, I'd like to
suggest freezing all other cpus.

Another benefit is that the call trace for tasks in "R" can be safely
printed out since the cpus is frozen now.

I posted the above message several months ago. Looks no one follows
this thread; so I did the patches for this issue; it has been tested
on my pc and works fine. The patches are against 2.6.24.2 for x86 64
kernel. I wanted to merge the patches to the latest 2.6 tree, but x86
smp.c has a lot changes recently, which made the merge not easy.

I borrowed some code from diskdump since dumping vmcore faces
the similar situation like sysrq-t. So credits for the writers of
diskdump which is released under GPL.

Any comments about the patches?

Thanx
-gys

pls cc me when you reply since I haven't subscribed to the mailing list.
---
Signed-off-by: Gary Shi <garyu.shi@xxxxxxxxxxxx>
---
arch/x86/kernel/smp_64.c | 55 +++++++++++++++++++++++++++++++++++++++
include/asm-x86/sysrq-t.h | 9 ++++++
include/linux/smp.h | 3 ++
kernel/sched.c | 62 +++++++++++++++++++++++++++++++++++++++++++-
4 files changed, 127 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/smp_64.c b/arch/x86/kernel/smp_64.c
index 03fa6ed..af0fab3 100644
--- a/arch/x86/kernel/smp_64.c
+++ b/arch/x86/kernel/smp_64.c
@@ -300,6 +300,7 @@ void smp_send_reschedule(int cpu)
* static memory requirements. It also looks cleaner.
*/
static DEFINE_SPINLOCK(call_lock);
+static int call_lock_locked_by_freezer;

struct call_data_struct {
void (*func) (void *info);
@@ -531,3 +532,57 @@ asmlinkage void smp_call_function_interrupt(void)
}
}

+/*
+ * sysrq_t version of smp_call_function to avoid deadlock in call_lock
+ */
+void sysrq_t_smp_call_function (void (*func) (void *info), void *info)
+{
+ static struct call_data_struct dumpdata;
+ static int dumping_cpu = -1;
+ int waitcount = 0;
+
+ call_lock_locked_by_freezer = 1;
+
+ dumping_cpu = smp_processor_id();
+
+ /*
+ *Enable irq to avoid the deadlock of call_lock
+ *since the local irq is disabled in __handle_sysrq
+ */
+ local_irq_enable();
+
+ /*
+ * Try to get call_lock or wait for 2 second to let ipis settle down.
+ * If we can't get call_lock, then no one else can get it either;
+ * so its safe to issue ipi now.
+ * And at worst if some cpus are locked up, we just go ahead.
+ */
+ while (!spin_trylock(&call_lock)) {
+ if (waitcount++ > 2000) {
+ call_lock_locked_by_freezer = 0;
+ break;
+ }
+ udelay(1000);
+ barrier();
+ }
+
+ local_irq_disable();
+ dumpdata.func = func;
+ dumpdata.info = info;
+ dumpdata.wait = 0; /* not used */
+ atomic_set(&dumpdata.started, 0); /* not used */
+ atomic_set(&dumpdata.finished, 0); /* not used */
+
+ call_data = &dumpdata;
+ wmb();
+ send_IPI_allbutself(CALL_FUNCTION_VECTOR);
+}
+EXPORT_SYMBOL(sysrq_t_smp_call_function);
+
+void sysrq_t_unlock_call_lock(void)
+{
+ if (call_lock_locked_by_freezer)
+ spin_unlock(&call_lock);
+
+}
+EXPORT_SYMBOL(sysrq_t_unlock_call_lock);
diff --git a/include/asm-x86/sysrq-t.h b/include/asm-x86/sysrq-t.h
new file mode 100644
index 0000000..a523023
--- /dev/null
+++ b/include/asm-x86/sysrq-t.h
@@ -0,0 +1,9 @@
+
+#define platform_freeze_cpu() \
+{ \
+ local_irq_disable(); \
+ while (freezer) \
+ cpu_relax(); \
+ local_irq_enable(); \
+}
+
diff --git a/include/linux/smp.h b/include/linux/smp.h
index c25e66b..41aeb54 100644
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -57,6 +57,9 @@ int smp_call_function(void(*func)(void *info), void
*info, int retry, int wait);
int smp_call_function_single(int cpuid, void (*func) (void *info), void *info,
int retry, int wait);

+extern void sysrq_t_smp_call_function(void(*func)(void *info), void *info);
+extern void sysrq_t_unlock_call_lock(void);
+
/*
* Call a function on all processors
*/
diff --git a/kernel/sched.c b/kernel/sched.c
index e76b11c..fe3a963 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -66,6 +66,7 @@

#include <asm/tlb.h>
#include <asm/irq_regs.h>
+#include <asm/sysrq-t.h>

/*
* Scheduler clock - returns current time in nanosec units.
@@ -4920,8 +4921,62 @@ static void show_task(struct task_struct *p)
printk(KERN_CONT "%5lu %5d %6d\n", free,
task_pid_nr(p), task_pid_nr(p->real_parent));

- if (state != TASK_RUNNING)
- show_stack(p, NULL);
+ show_stack(p, NULL);
+}
+
+static struct task_struct *sysrq_tasks[NR_CPUS];
+
+#define sysrq_t_mdelay(n) \
+({ \
+ unsigned long __ms = (n); \
+ while (__ms--) { \
+ udelay(1000); \
+ barrier(); \
+ touch_nmi_watchdog(); \
+ } \
+})
+
+static int freezer;
+
+#if CONFIG_SMP
+static void freeze_cpu(void *dummy)
+{
+ unsigned int cpu = smp_processor_id();
+ sysrq_tasks[cpu] = current;
+ platform_freeze_cpu();
+}
+#else
+#define freeze_cpu(void *dummy) do { } while (0)
+#endif
+
+
+static void freeze_other_cpus_temporarily(void)
+{
+#if CONFIG_SMP
+ int i;
+ freezer = 1;
+
+ sysrq_t_smp_call_function(freeze_cpu, NULL);
+
+ /* wait for 3 seconds to give more time for other cpus' freezing */
+ sysrq_t_mdelay(3000);
+ printk(KERN_INFO "CPUs frozen: ");
+ for (i = 0; i < NR_CPUS; i++) {
+ if (sysrq_tasks[i] != NULL)
+ printk(KERN_INFO "#%d(pid: %d) ",
+ i, sysrq_tasks[i]->pid);
+
+ }
+ printk("\n");
+ printk(KERN_INFO "CPU#%d is executing sysrq-t(pid: %d).\n",
+ smp_processor_id(), current->pid);
+#endif
+}
+
+static void defreeze_other_cpus(void)
+{
+ freezer = 0;
+ sysrq_t_unlock_call_lock();
}

void show_state_filter(unsigned long state_filter)
@@ -4936,6 +4991,7 @@ void show_state_filter(unsigned long state_filter)
" task PC stack pid father\n");
#endif
read_lock(&tasklist_lock);
+ freeze_other_cpus_temporarily();
do_each_thread(g, p) {
/*
* reset the NMI-timeout, listing all files on a slow
@@ -4957,8 +5013,10 @@ void show_state_filter(unsigned long state_filter)
*/
if (state_filter == -1)
debug_show_all_locks();
+ defreeze_other_cpus();
}

+
void __cpuinit init_idle_bootup_task(struct task_struct *idle)
{
idle->sched_class = &idle_sched_class;

---

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