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SubjectRe: [RFC] x86: perf swallows all NMIs when registered with a user
On 07/22/2010 02:51 PM, Don Zickus wrote:
> Hi,
>
> When debugging a problem with Yinghai, I noticed that when the perf event
> subsystem has a user (in this case the new generic nmi_watchdog), it just
> blindly swallows all the NMIs in the system.
>
> This causes issues for people like Yinghai, who want to use an external
> nmi button to generate a panic, or other big companies that like to
> registered the nmi handlers at a lower priority to be a catch-all for NMI
> problems or also it will start masking any unknown nmi problems that would
> have cropped up due to broken firmware or such.
>
> The problem is spelled out in the comment in
> arch/x86/kernel/cpu/perf_event.c::perf_event_nmi_handler
>
> perf_event_nmi_handler(struct notifier_block *self,
> unsigned long cmd, void *__args)
> {
> struct die_args *args = __args;
> struct pt_regs *regs;
> static int eat_nmis = 0;
>
> if (!atomic_read(&active_events))
> return NOTIFY_DONE;
>
> switch (cmd) {
> case DIE_NMI:
> case DIE_NMI_IPI:
> break;
>
> default:
> return NOTIFY_DONE;
> }
>
> regs = args->regs;
>
> apic_write(APIC_LVTPC, APIC_DM_NMI);
> /*
> * Can't rely on the handled return value to say it was our NMI,
> * two
> * events could trigger 'simultaneously' raising two back-to-back
> * NMIs.
> *
> * If the first NMI handles both, the latter will be empty and
> * daze
> * the CPU.
> */
> x86_pmu.handle_irq(regs);
>
> return NOTIFY_STOP;
> }
>
> In the normal case, there is no perf user, so the function returns with
> NOTIFY_DONE right away. But with the new nmi_watchdog, which is a user of
> the perf subsystem, it catches DIE_NMI, executes x86_pmu.handle_irq, and
> finally returns NOTIFY_STOP.
>
> The comment above describes the problem well, but as a result no other
> NMIs can get through.
>
> I looked at the code and thought I could modify the handle_irq to only
> handle one PMU at a time, with the thought that there is probably another
> NMI waiting for the other PMUs. This would handle the problem nicely.
>
> But I believe the code is structured such that an event can occupy more
> than one PMU in complex cases and as a result would probably break things
> because the event would be in limbo until all the NMIs happened to
> disable it?? I am not familiar enough with how perf works to know if that
> case is correct or not.
>
> So I hacked up some stupid code to start a conversation that just keeps
> track of how many NMIs are supposed to happen based on the number of PMUs
> handled. Then on future NMIs those are 'eaten' until the count is zero
> again.
>
> Like I said this patch is just something to start a conversation. I
> tested it, but could not do anything complicated enough such that more
> than one PMU was handled during one NMI call.
>
> Comments?
>
> Cheers,
> Don
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index f2da20f..df6255c 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1154,7 +1156,7 @@ static int x86_pmu_handle_irq(struct pt_regs *regs)
> /*
> * event overflow
> */
> - handled = 1;
> + handled += 1;
> data.period = event->hw.last_period;
>
> if (!x86_perf_event_set_period(event))
> @@ -1206,6 +1210,7 @@ perf_event_nmi_handler(struct notifier_block *self,
> {
> struct die_args *args = __args;
> struct pt_regs *regs;
> + static int eat_nmis = 0;
>
> if (!atomic_read(&active_events))
> return NOTIFY_DONE;
> @@ -1229,9 +1234,13 @@ perf_event_nmi_handler(struct notifier_block *self,
> * If the first NMI handles both, the latter will be empty and daze
> * the CPU.
> */
> - x86_pmu.handle_irq(regs);
> + eat_nmis += x86_pmu.handle_irq(regs);
> + if (eat_nmis) {
> + eat_nmis--;
> + return NOTIFY_STOP;
> + }
>
> - return NOTIFY_STOP;
> + return NOTIFY_DONE;
> }
>
> static __read_mostly struct notifier_block perf_event_nmi_notifier = {

cool, with your patch and following patch i can use nmi button now with CONFIG_LOCKUP_DETECTOR defined

[PATCH] x86,nmi: move unknown_nmi_panic to traps.c

So we use it even LOCKUP_DETECTOR is defined.

need Don's patch...

Signed-off-by: Yinghai Lu <yinghai@kernel.org>

---
arch/x86/include/asm/nmi.h | 8 --------
arch/x86/kernel/apic/hw_nmi.c | 1 -
arch/x86/kernel/apic/nmi.c | 27 ---------------------------
arch/x86/kernel/traps.c | 29 ++++++++++++++++++++++++++++-
kernel/sysctl.c | 4 +++-
5 files changed, 31 insertions(+), 38 deletions(-)
Index: linux-2.6/arch/x86/kernel/apic/nmi.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/apic/nmi.c
+++ linux-2.6/arch/x86/kernel/apic/nmi.c
@@ -37,7 +37,6 @@

#include <asm/mach_traps.h>

-int unknown_nmi_panic;
int nmi_watchdog_enabled;

/* For reliability, we're prepared to waste bits here. */
@@ -483,23 +482,6 @@ static void disable_ioapic_nmi_watchdog(
on_each_cpu(stop_apic_nmi_watchdog, NULL, 1);
}

-static int __init setup_unknown_nmi_panic(char *str)
-{
- unknown_nmi_panic = 1;
- return 1;
-}
-__setup("unknown_nmi_panic", setup_unknown_nmi_panic);
-
-static int unknown_nmi_panic_callback(struct pt_regs *regs, int cpu)
-{
- unsigned char reason = get_nmi_reason();
- char buf[64];
-
- sprintf(buf, "NMI received for unknown reason %02x\n", reason);
- die_nmi(buf, regs, 1); /* Always panic here */
- return 0;
-}
-
/*
* proc handler for /proc/sys/kernel/nmi
*/
@@ -540,15 +522,6 @@ int proc_nmi_enabled(struct ctl_table *t

#endif /* CONFIG_SYSCTL */

-int do_nmi_callback(struct pt_regs *regs, int cpu)
-{
-#ifdef CONFIG_SYSCTL
- if (unknown_nmi_panic)
- return unknown_nmi_panic_callback(regs, cpu);
-#endif
- return 0;
-}
-
void arch_trigger_all_cpu_backtrace(void)
{
int i;
Index: linux-2.6/arch/x86/kernel/apic/hw_nmi.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/apic/hw_nmi.c
+++ linux-2.6/arch/x86/kernel/apic/hw_nmi.c
@@ -100,7 +100,6 @@ void acpi_nmi_disable(void) { return; }
#endif
atomic_t nmi_active = ATOMIC_INIT(0); /* oprofile uses this */
EXPORT_SYMBOL(nmi_active);
-int unknown_nmi_panic;
void cpu_nmi_set_wd_enabled(void) { return; }
void stop_apic_nmi_watchdog(void *unused) { return; }
void setup_apic_nmi_watchdog(void *unused) { return; }
Index: linux-2.6/arch/x86/kernel/traps.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/traps.c
+++ linux-2.6/arch/x86/kernel/traps.c
@@ -377,6 +377,33 @@ unknown_nmi_error(unsigned char reason,
printk(KERN_EMERG "Dazed and confused, but trying to continue\n");
}

+#if defined(CONFIG_SYSCTL) && defined(CONFIG_X86_LOCAL_APIC)
+int unknown_nmi_panic;
+static int __init setup_unknown_nmi_panic(char *str)
+{
+ unknown_nmi_panic = 1;
+ return 1;
+}
+__setup("unknown_nmi_panic", setup_unknown_nmi_panic);
+
+static int unknown_nmi_panic_callback(struct pt_regs *regs, int cpu)
+{
+ unsigned char reason = get_nmi_reason();
+ char buf[64];
+
+ sprintf(buf, "NMI received for unknown reason %02x\n", reason);
+ die_nmi(buf, regs, 1); /* Always panic here */
+ return 0;
+}
+
+static int do_nmi_callback(struct pt_regs *regs, int cpu)
+{
+ if (unknown_nmi_panic)
+ return unknown_nmi_panic_callback(regs, cpu);
+ return 0;
+}
+#endif
+
static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
{
unsigned char reason = 0;
@@ -405,8 +432,8 @@ static notrace __kprobes void default_do
*/
if (nmi_watchdog_tick(regs, reason))
return;
- if (!do_nmi_callback(regs, cpu))
#endif /* !CONFIG_LOCKUP_DETECTOR */
+ if (!do_nmi_callback(regs, cpu))
unknown_nmi_error(reason, regs);
#else
unknown_nmi_error(reason, regs);
Index: linux-2.6/kernel/sysctl.c
===================================================================
--- linux-2.6.orig/kernel/sysctl.c
+++ linux-2.6/kernel/sysctl.c
@@ -741,7 +741,7 @@ static struct ctl_table kern_table[] = {
.extra2 = &one,
},
#endif
-#if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86) && !defined(CONFIG_LOCKUP_DETECTOR)
+#if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86)
{
.procname = "unknown_nmi_panic",
.data = &unknown_nmi_panic,
@@ -749,6 +749,8 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec,
},
+#endif
+#if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86) && !defined(CONFIG_LOCKUP_DETECTOR)
{
.procname = "nmi_watchdog",
.data = &nmi_watchdog_enabled,
Index: linux-2.6/arch/x86/include/asm/nmi.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/nmi.h
+++ linux-2.6/arch/x86/include/asm/nmi.h
@@ -7,14 +7,6 @@

#ifdef ARCH_HAS_NMI_WATCHDOG

-/**
- * do_nmi_callback
- *
- * Check to see if a callback exists and execute it. Return 1
- * if the handler exists and was handled successfully.
- */
-int do_nmi_callback(struct pt_regs *regs, int cpu);
-
extern void die_nmi(char *str, struct pt_regs *regs, int do_panic);
extern int check_nmi_watchdog(void);
#if !defined(CONFIG_LOCKUP_DETECTOR)


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