[PATCH v2 0/3] kernel/smp.c: add more CSD lock debugging

[PATCH v2 0/3] kernel/smp.c: add more CSD lock debugging

[Juergen Gross]

This patch series was created to help catching a rather long standing
problem with smp_call_function_any() and friends.

Very rarely a remote cpu seems not to execute a queued function and
the cpu queueing that function request will wait forever for the
CSD lock to be released by the remote cpu.

This problem has been observed primarily when running as a guest on
top of KVM or Xen, but there are reports of the same pattern for the
bare metal case, too. It seems to exist since about 2 years now, and
there is not much data available.

What is known up to now is that resending an IPI to the remote cpu is
helping.

The patches are adding more debug data being printed in a hang
situation using a kernel with CONFIG_CSD_LOCK_WAIT_DEBUG configured.
Additionally the debug coding can be controlled via a new parameter
in order to make it easier to use such a kernel in a production
environment without too much negative performance impact. Per default
the debugging additions will be switched off and they can be activated
via the new boot parameter:

csdlock_debug=1 will switch on the basic debugging and IPI resend
csdlock_debug=ext will add additional data printed out in a hang
  situation, but this option will have a larger impact on performance.

I hope that the "ext" setting will help to find the root cause of the
problem.

Juergen Gross (3):
  kernel/smp: add boot parameter for controlling CSD lock debugging
  kernel/smp: prepare more CSD lock debugging
  kernel/smp: add more data to CSD lock debugging

 .../admin-guide/kernel-parameters.txt         |  10 +
 kernel/smp.c                                  | 284 +++++++++++++++++-
 2 files changed, 282 insertions(+), 12 deletions(-)

-- 
2.26.2

[PATCH v2 1/3] kernel/smp: add boot parameter for controlling CSD lock debugging

[Juergen Gross]

Currently CSD lock debugging can be switched on and off via a kernel
config option only. Unfortunately there is at least one problem with
CSD lock handling pending for about 2 years now, which has been seen
in different environments (mostly when running virtualized under KVM
or Xen, at least once on bare metal). Multiple attempts to catch this
issue have finally led to introduction of CSD lock debug code, but
this code is not in use in most distros as it has some impact on
performance.

In order to be able to ship kernels with CONFIG_CSD_LOCK_WAIT_DEBUG
enabled even for production use, add a boot parameter for switching
the debug functionality on. This will reduce any performance impact
of the debug coding to a bare minimum when not being used.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 .../admin-guide/kernel-parameters.txt         |  6 +++
 kernel/smp.c                                  | 38 +++++++++++++++++--
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bab6a8b01202..af9749b866c2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -784,6 +784,12 @@
 	cs89x0_media=	[HW,NET]
 			Format: { rj45 | aui | bnc }
 
+	csdlock_debug=	[KNL] Enable debug add-ons of cross-cpu function call
+			handling. When switched on additional debug data is
+			printed to the console in case a hanging cpu is
+			detected and that cpu is pinged again in order to try
+			to resolve the hang situation.
+
 	dasd=		[HW,NET]
 			See header of drivers/s390/block/dasd_devmap.c.
 
diff --git a/kernel/smp.c b/kernel/smp.c
index aeb0adfa0606..d5f0b21ab55e 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -24,6 +24,7 @@
 #include <linux/sched/clock.h>
 #include <linux/nmi.h>
 #include <linux/sched/debug.h>
+#include <linux/jump_label.h>
 
 #include "smpboot.h"
 #include "sched/smp.h"
@@ -102,6 +103,20 @@ void __init call_function_init(void)
 
 #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
 
+static DEFINE_STATIC_KEY_FALSE(csdlock_debug_enabled);
+
+static int __init csdlock_debug(char *str)
+{
+	unsigned int val = 0;
+
+	get_option(&str, &val);
+	if (val)
+		static_branch_enable(&csdlock_debug_enabled);
+
+	return 0;
+}
+early_param("csdlock_debug", csdlock_debug);
+
 static DEFINE_PER_CPU(call_single_data_t *, cur_csd);
 static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
 static DEFINE_PER_CPU(void *, cur_csd_info);
@@ -110,7 +125,7 @@ static DEFINE_PER_CPU(void *, cur_csd_info);
 static atomic_t csd_bug_count = ATOMIC_INIT(0);
 
 /* Record current CSD work for current CPU, NULL to erase. */
-static void csd_lock_record(call_single_data_t *csd)
+static void __csd_lock_record(call_single_data_t *csd)
 {
 	if (!csd) {
 		smp_mb(); /* NULL cur_csd after unlock. */
@@ -125,7 +140,13 @@ static void csd_lock_record(call_single_data_t *csd)
 		  /* Or before unlock, as the case may be. */
 }
 
-static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd)
+static __always_inline void csd_lock_record(call_single_data_t *csd)
+{
+	if (static_branch_unlikely(&csdlock_debug_enabled))
+		__csd_lock_record(csd);
+}
+
+static int csd_lock_wait_getcpu(call_single_data_t *csd)
 {
 	unsigned int csd_type;
 
@@ -140,7 +161,7 @@ static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd)
  * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
  * so waiting on other types gets much less information.
  */
-static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, int *bug_id)
+static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, int *bug_id)
 {
 	int cpu = -1;
 	int cpux;
@@ -204,7 +225,7 @@ static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd, u64 t
  * previous function call. For multi-cpu calls its even more interesting
  * as we'll have to ensure no other cpu is observing our csd.
  */
-static __always_inline void csd_lock_wait(call_single_data_t *csd)
+static void __csd_lock_wait(call_single_data_t *csd)
 {
 	int bug_id = 0;
 	u64 ts0, ts1;
@@ -218,6 +239,15 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
 	smp_acquire__after_ctrl_dep();
 }
 
+static __always_inline void csd_lock_wait(call_single_data_t *csd)
+{
+	if (static_branch_unlikely(&csdlock_debug_enabled)) {
+		__csd_lock_wait(csd);
+		return;
+	}
+
+	smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
+}
 #else
 static void csd_lock_record(call_single_data_t *csd)
 {
-- 
2.26.2

[PATCH v2 2/3] kernel/smp: prepare more CSD lock debugging

[Juergen Gross]

In order to be able to easily add more CSD lock debugging data to
struct call_function_data->csd move the call_single_data_t element
into a sub-structure.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 kernel/smp.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index d5f0b21ab55e..6d7e6dbe33dc 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -31,8 +31,12 @@
 
 #define CSD_TYPE(_csd)	((_csd)->node.u_flags & CSD_FLAG_TYPE_MASK)
 
+struct cfd_percpu {
+	call_single_data_t	csd;
+};
+
 struct call_function_data {
-	call_single_data_t	__percpu *csd;
+	struct cfd_percpu	__percpu *pcpu;
 	cpumask_var_t		cpumask;
 	cpumask_var_t		cpumask_ipi;
 };
@@ -55,8 +59,8 @@ int smpcfd_prepare_cpu(unsigned int cpu)
 		free_cpumask_var(cfd->cpumask);
 		return -ENOMEM;
 	}
-	cfd->csd = alloc_percpu(call_single_data_t);
-	if (!cfd->csd) {
+	cfd->pcpu = alloc_percpu(struct cfd_percpu);
+	if (!cfd->pcpu) {
 		free_cpumask_var(cfd->cpumask);
 		free_cpumask_var(cfd->cpumask_ipi);
 		return -ENOMEM;
@@ -71,7 +75,7 @@ int smpcfd_dead_cpu(unsigned int cpu)
 
 	free_cpumask_var(cfd->cpumask);
 	free_cpumask_var(cfd->cpumask_ipi);
-	free_percpu(cfd->csd);
+	free_percpu(cfd->pcpu);
 	return 0;
 }
 
@@ -694,7 +698,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 
 	cpumask_clear(cfd->cpumask_ipi);
 	for_each_cpu(cpu, cfd->cpumask) {
-		call_single_data_t *csd = per_cpu_ptr(cfd->csd, cpu);
+		call_single_data_t *csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
 
 		if (cond_func && !cond_func(cpu, info))
 			continue;
@@ -719,7 +723,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		for_each_cpu(cpu, cfd->cpumask) {
 			call_single_data_t *csd;
 
-			csd = per_cpu_ptr(cfd->csd, cpu);
+			csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
 			csd_lock_wait(csd);
 		}
 	}
-- 
2.26.2

[PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging

[Juergen Gross]

In order to help identifying problems with IPI handling and remote
function execution add some more data to IPI debugging code.

There have been multiple reports of cpus looping long times (many
seconds) in smp_call_function_many() waiting for another cpu executing
a function like tlb flushing. Most of these reports have been for
cases where the kernel was running as a guest on top of KVM or Xen
(there are rumours of that happening under VMWare, too, and even on
bare metal).

Finding the root cause hasn't been successful yet, even after more than
2 years of chasing this bug by different developers.

Commit 35feb60474bf4f7 ("kernel/smp: Provide CSD lock timeout
diagnostics") tried to address this by adding some debug code and by
issuing another IPI when a hang was detected. This helped mitigating
the problem (the repeated IPI unlocks the hang), but the root cause is
still unknown.

Current available data suggests that either an IPI wasn't sent when it
should have been, or that the IPI didn't result in the target cpu
executing the queued function (due to the IPI not reaching the cpu,
the IPI handler not being called, or the handler not seeing the queued
request).

Try to add more diagnostic data by introducing a global atomic counter
which is being incremented when doing critical operations (before and
after queueing a new request, when sending an IPI, and when dequeueing
a request). The counter value is stored in percpu variables which can
be printed out when a hang is detected.

The data of the last event (consisting of sequence counter, source
cpu, target cpu, and event type) is stored in a global variable. When
a new event is to be traced, the data of the last event is stored in
the event related percpu location and the global data is updated with
the new event's data. This allows to track two events in one data
location: one by the value of the event data (the event before the
current one), and one by the location itself (the current event).

A typical printout with a detected hang will look like this:

csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
	csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
        csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
        csd: cnt(00008cd): ffff->0006 idle
        csd: cnt(0003668): 0001->0006 queue
        csd: cnt(0003669): 0001->0006 ipi
        csd: cnt(0003e0f): 0007->000a queue
        csd: cnt(0003e10): 0001->ffff ping
        csd: cnt(0003e71): 0003->0000 ping
        csd: cnt(0003e72): ffff->0006 gotipi
        csd: cnt(0003e73): ffff->0006 handle
        csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
        csd: cnt(0003e7f): 0004->0006 ping
        csd: cnt(0003e80): 0001->ffff pinged
        csd: cnt(0003eb2): 0005->0001 noipi
        csd: cnt(0003eb3): 0001->0006 queue
        csd: cnt(0003eb4): 0001->0006 noipi
        csd: cnt now: 0003f00

The idea is to print only relevant entries. Those are all events which
are associated with the hang (so sender side events for the source cpu
of the hanging request, and receiver side events for the target cpu),
and the related events just before those (for adding data needed to
identify a possible race). Printing all available data would be
possible, but this would add large amounts of data printed on larger
configurations.

Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
---
V2:
- add automatic data deciphering and sorting of entries
- add new trace point for leaving flush_smp_call_function_queue()
- add information when finding an empty call_single_queue
---
 .../admin-guide/kernel-parameters.txt         |   4 +
 kernel/smp.c                                  | 234 +++++++++++++++++-
 2 files changed, 234 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index af9749b866c2..89da345ec8f1 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -789,6 +789,10 @@
 			printed to the console in case a hanging cpu is
 			detected and that cpu is pinged again in order to try
 			to resolve the hang situation.
+			0: disable csdlock debugging (default)
+			1: enable basic csdlock debugging (minor impact)
+			ext: enable extended csdlock debugging (more impact,
+			     but more data)
 
 	dasd=		[HW,NET]
 			See header of drivers/s390/block/dasd_devmap.c.
diff --git a/kernel/smp.c b/kernel/smp.c
index 6d7e6dbe33dc..94993cf19ba3 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -31,8 +31,59 @@
 
 #define CSD_TYPE(_csd)	((_csd)->node.u_flags & CSD_FLAG_TYPE_MASK)
 
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+union cfd_seq_cnt {
+	u64		val;
+	struct {
+		u64	src:16;
+		u64	dst:16;
+#define CFD_SEQ_NOCPU	0xffff
+		u64	type:4;
+#define CFD_SEQ_QUEUE	0
+#define CFD_SEQ_IPI	1
+#define CFD_SEQ_NOIPI	2
+#define CFD_SEQ_PING	3
+#define CFD_SEQ_PINGED	4
+#define CFD_SEQ_HANDLE	5
+#define CFD_SEQ_DEQUEUE	6
+#define CFD_SEQ_IDLE	7
+#define CFD_SEQ_GOTIPI	8
+#define CFD_SEQ_HDLEND	9
+		u64	cnt:28;
+	}		u;
+};
+
+static char *seq_type[] = {
+	[CFD_SEQ_QUEUE]		= "queue",
+	[CFD_SEQ_IPI]		= "ipi",
+	[CFD_SEQ_NOIPI]		= "noipi",
+	[CFD_SEQ_PING]		= "ping",
+	[CFD_SEQ_PINGED]	= "pinged",
+	[CFD_SEQ_HANDLE]	= "handle",
+	[CFD_SEQ_DEQUEUE]	= "dequeue (src cpu 0 == empty)",
+	[CFD_SEQ_IDLE]		= "idle",
+	[CFD_SEQ_GOTIPI]	= "gotipi",
+	[CFD_SEQ_HDLEND]	= "hdlend (src cpu 0 == early)",
+};
+
+struct cfd_seq_local {
+	u64	ping;
+	u64	pinged;
+	u64	handle;
+	u64	dequeue;
+	u64	idle;
+	u64	gotipi;
+	u64	hdlend;
+};
+#endif
+
 struct cfd_percpu {
 	call_single_data_t	csd;
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+	u64	seq_queue;
+	u64	seq_ipi;
+	u64	seq_noipi;
+#endif
 };
 
 struct call_function_data {
@@ -108,12 +159,18 @@ void __init call_function_init(void)
 #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
 
 static DEFINE_STATIC_KEY_FALSE(csdlock_debug_enabled);
+static DEFINE_STATIC_KEY_FALSE(csdlock_debug_extended);
 
 static int __init csdlock_debug(char *str)
 {
 	unsigned int val = 0;
 
-	get_option(&str, &val);
+	if (str && !strcmp(str, "ext")) {
+		val = 1;
+		static_branch_enable(&csdlock_debug_extended);
+	} else
+		get_option(&str, &val);
+
 	if (val)
 		static_branch_enable(&csdlock_debug_enabled);
 
@@ -124,9 +181,34 @@ early_param("csdlock_debug", csdlock_debug);
 static DEFINE_PER_CPU(call_single_data_t *, cur_csd);
 static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
 static DEFINE_PER_CPU(void *, cur_csd_info);
+static DEFINE_PER_CPU(struct cfd_seq_local, cfd_seq_local);
 
 #define CSD_LOCK_TIMEOUT (5ULL * NSEC_PER_SEC)
 static atomic_t csd_bug_count = ATOMIC_INIT(0);
+static u64 cfd_seq;
+
+#define CFD_SEQ(s, d, t, c)	\
+	(union cfd_seq_cnt){ .u.src = s, .u.dst = d, .u.type = t, .u.cnt = c }
+
+static u64 cfd_seq_inc(unsigned int src, unsigned int dst, unsigned int type)
+{
+	union cfd_seq_cnt new, old;
+
+	new = CFD_SEQ(src, dst, type, 0);
+
+	do {
+		old.val = READ_ONCE(cfd_seq);
+		new.u.cnt = old.u.cnt + 1;
+	} while (cmpxchg(&cfd_seq, old.val, new.val) != old.val);
+
+	return old.val;
+}
+
+#define cfd_seq_store(var, src, dst, type)				\
+	do {								\
+		if (static_branch_unlikely(&csdlock_debug_extended))	\
+			var = cfd_seq_inc(src, dst, type);		\
+	} while (0)
 
 /* Record current CSD work for current CPU, NULL to erase. */
 static void __csd_lock_record(call_single_data_t *csd)
@@ -160,6 +242,88 @@ static int csd_lock_wait_getcpu(call_single_data_t *csd)
 	return -1;
 }
 
+static void cfd_seq_data_add(u64 val, unsigned int src, unsigned int dst,
+			     unsigned int type, union cfd_seq_cnt *data,
+			     unsigned int *n_data, unsigned int now)
+{
+	union cfd_seq_cnt new[2];
+	unsigned int i, j, k;
+
+	new[0].val = val;
+	new[1] = CFD_SEQ(src, dst, type, new[0].u.cnt + 1);
+
+	for (i = 0; i < 2; i++) {
+		if (new[i].u.cnt <= now)
+			new[i].u.cnt |= 0x80000000U;
+		for (j = 0; j < *n_data; j++) {
+			if (new[i].u.cnt == data[j].u.cnt) {
+				/* Direct read value trumps generated one. */
+				if (i == 0)
+					data[j].val = new[i].val;
+				break;
+			}
+			if (new[i].u.cnt < data[j].u.cnt) {
+				for (k = *n_data; k > j; k--)
+					data[k].val = data[k - 1].val;
+				data[j].val = new[i].val;
+				(*n_data)++;
+				break;
+			}
+		}
+		if (j == *n_data) {
+			data[j].val = new[i].val;
+			(*n_data)++;
+		}
+	}
+}
+
+static const char *csd_lock_get_type(unsigned int type)
+{
+	return (type >= ARRAY_SIZE(seq_type)) ? "?" : seq_type[type];
+}
+
+static void csd_lock_print_extended(call_single_data_t *csd, int cpu)
+{
+	struct cfd_seq_local *seq = &per_cpu(cfd_seq_local, cpu);
+	unsigned int srccpu = csd->node.src;
+	struct call_function_data *cfd = per_cpu_ptr(&cfd_data, srccpu);
+	struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+	unsigned int now;
+	union cfd_seq_cnt data[2 * ARRAY_SIZE(seq_type)];
+	unsigned int n_data = 0, i;
+
+	data[0].val = READ_ONCE(cfd_seq);
+	now = data[0].u.cnt;
+
+	cfd_seq_data_add(pcpu->seq_queue, srccpu, cpu,
+			 CFD_SEQ_QUEUE, data, &n_data, now);
+	cfd_seq_data_add(pcpu->seq_ipi, srccpu, cpu,
+			 CFD_SEQ_IPI, data, &n_data, now);
+	cfd_seq_data_add(pcpu->seq_noipi, srccpu, cpu,
+			 CFD_SEQ_NOIPI, data, &n_data, now);
+	cfd_seq_data_add(per_cpu(cfd_seq_local.ping, srccpu), srccpu,
+			 CFD_SEQ_NOCPU, CFD_SEQ_PING, data, &n_data, now);
+	cfd_seq_data_add(per_cpu(cfd_seq_local.pinged, srccpu), srccpu,
+			 CFD_SEQ_NOCPU, CFD_SEQ_PINGED, data, &n_data, now);
+	cfd_seq_data_add(seq->idle, CFD_SEQ_NOCPU, cpu,
+			 CFD_SEQ_IDLE, data, &n_data, now);
+	cfd_seq_data_add(seq->gotipi, CFD_SEQ_NOCPU, cpu,
+			 CFD_SEQ_GOTIPI, data, &n_data, now);
+	cfd_seq_data_add(seq->handle, CFD_SEQ_NOCPU, cpu,
+			 CFD_SEQ_HANDLE, data, &n_data, now);
+	cfd_seq_data_add(seq->dequeue, CFD_SEQ_NOCPU, cpu,
+			 CFD_SEQ_DEQUEUE, data, &n_data, now);
+	cfd_seq_data_add(seq->hdlend, CFD_SEQ_NOCPU, cpu,
+			 CFD_SEQ_HDLEND, data, &n_data, now);
+
+	for (i = 0; i < n_data; i++) {
+		pr_alert("\tcsd: cnt(%07x): %04x->%04x %s\n",
+			 data[i].u.cnt & ~0x80000000U, data[i].u.src,
+			 data[i].u.dst, csd_lock_get_type(data[i].u.type));
+	}
+	pr_alert("\tcsd: cnt now: %07x\n", now);
+}
+
 /*
  * Complain if too much time spent waiting.  Note that only
  * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
@@ -209,6 +373,8 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
 			 *bug_id, !cpu_cur_csd ? "unresponsive" : "handling this request");
 	}
 	if (cpu >= 0) {
+		if (static_branch_unlikely(&csdlock_debug_extended))
+			csd_lock_print_extended(csd, cpu);
 		if (!trigger_single_cpu_backtrace(cpu))
 			dump_cpu_task(cpu);
 		if (!cpu_cur_csd) {
@@ -252,7 +418,27 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
 
 	smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
 }
+
+static void __smp_call_single_queue_debug(int cpu, struct llist_node *node)
+{
+	unsigned int this_cpu = smp_processor_id();
+	struct cfd_seq_local *seq = this_cpu_ptr(&cfd_seq_local);
+	struct call_function_data *cfd = this_cpu_ptr(&cfd_data);
+	struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+
+	cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
+	if (llist_add(node, &per_cpu(call_single_queue, cpu))) {
+		cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
+		cfd_seq_store(seq->ping, this_cpu, cpu, CFD_SEQ_PING);
+		send_call_function_single_ipi(cpu);
+		cfd_seq_store(seq->pinged, this_cpu, cpu, CFD_SEQ_PINGED);
+	} else {
+		cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
+	}
+}
 #else
+#define cfd_seq_store(var, src, dst, type)
+
 static void csd_lock_record(call_single_data_t *csd)
 {
 }
@@ -290,6 +476,19 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
 
 void __smp_call_single_queue(int cpu, struct llist_node *node)
 {
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+	if (static_branch_unlikely(&csdlock_debug_extended)) {
+		unsigned int type;
+
+		type = CSD_TYPE(container_of(node, call_single_data_t,
+					     node.llist));
+		if (type == CSD_TYPE_SYNC || type == CSD_TYPE_ASYNC) {
+			__smp_call_single_queue_debug(cpu, node);
+			return;
+		}
+	}
+#endif
+
 	/*
 	 * The list addition should be visible before sending the IPI
 	 * handler locks the list to pull the entry off it because of
@@ -348,6 +547,8 @@ static int generic_exec_single(int cpu, call_single_data_t *csd)
  */
 void generic_smp_call_function_single_interrupt(void)
 {
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->gotipi, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_GOTIPI);
 	flush_smp_call_function_queue(true);
 }
 
@@ -375,7 +576,13 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 	lockdep_assert_irqs_disabled();
 
 	head = this_cpu_ptr(&call_single_queue);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->handle, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_HANDLE);
 	entry = llist_del_all(head);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->dequeue,
+		      /* Special meaning of source cpu: 0 == queue empty */
+		      entry ? CFD_SEQ_NOCPU : 0,
+		      smp_processor_id(), CFD_SEQ_DEQUEUE);
 	entry = llist_reverse_order(entry);
 
 	/* There shouldn't be any pending callbacks on an offline CPU. */
@@ -434,8 +641,12 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 		}
 	}
 
-	if (!entry)
+	if (!entry) {
+		cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->hdlend,
+			      0, smp_processor_id(),
+			      CFD_SEQ_HDLEND);
 		return;
+	}
 
 	/*
 	 * Second; run all !SYNC callbacks.
@@ -473,6 +684,9 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 	 */
 	if (entry)
 		sched_ttwu_pending(entry);
+
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->hdlend, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_HDLEND);
 }
 
 void flush_smp_call_function_from_idle(void)
@@ -482,6 +696,8 @@ void flush_smp_call_function_from_idle(void)
 	if (llist_empty(this_cpu_ptr(&call_single_queue)))
 		return;
 
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->idle, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_IDLE);
 	local_irq_save(flags);
 	flush_smp_call_function_queue(true);
 	if (local_softirq_pending())
@@ -698,7 +914,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 
 	cpumask_clear(cfd->cpumask_ipi);
 	for_each_cpu(cpu, cfd->cpumask) {
-		call_single_data_t *csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
+		struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+		call_single_data_t *csd = &pcpu->csd;
 
 		if (cond_func && !cond_func(cpu, info))
 			continue;
@@ -712,12 +929,21 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		csd->node.src = smp_processor_id();
 		csd->node.dst = cpu;
 #endif
-		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu)))
+		cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
+		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
 			__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
+			cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
+		} else {
+			cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
+		}
 	}
 
 	/* Send a message to all CPUs in the map */
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->ping, this_cpu,
+		      CFD_SEQ_NOCPU, CFD_SEQ_PING);
 	arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->pinged, this_cpu,
+		      CFD_SEQ_NOCPU, CFD_SEQ_PINGED);
 
 	if (wait) {
 		for_each_cpu(cpu, cfd->cpumask) {
-- 
2.26.2

[tip: locking/core] locking/csd_lock: Add more data to CSD lock debugging

[tip-bot2 for Juergen Gross]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     6bf3195fdbab92b57f3167101a0b651b93dbeae7
Gitweb:        https://git.kernel.org/tip/6bf3195fdbab92b57f3167101a0b651b93dbeae7
Author:        Juergen Gross <jgross@suse.com>
AuthorDate:    Mon, 01 Mar 2021 11:13:36 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 01 Mar 2021 14:27:59 +01:00

locking/csd_lock: Add more data to CSD lock debugging

In order to help identifying problems with IPI handling and remote
function execution add some more data to IPI debugging code.

There have been multiple reports of CPUs looping long times (many
seconds) in smp_call_function_many() waiting for another CPU executing
a function like tlb flushing. Most of these reports have been for
cases where the kernel was running as a guest on top of KVM or Xen
(there are rumours of that happening under VMWare, too, and even on
bare metal).

Finding the root cause hasn't been successful yet, even after more than
2 years of chasing this bug by different developers.

Commit:

  35feb60474bf4f7 ("kernel/smp: Provide CSD lock timeout diagnostics")

tried to address this by adding some debug code and by issuing another
IPI when a hang was detected. This helped mitigating the problem
(the repeated IPI unlocks the hang), but the root cause is still unknown.

Current available data suggests that either an IPI wasn't sent when it
should have been, or that the IPI didn't result in the target CPU
executing the queued function (due to the IPI not reaching the CPU,
the IPI handler not being called, or the handler not seeing the queued
request).

Try to add more diagnostic data by introducing a global atomic counter
which is being incremented when doing critical operations (before and
after queueing a new request, when sending an IPI, and when dequeueing
a request). The counter value is stored in percpu variables which can
be printed out when a hang is detected.

The data of the last event (consisting of sequence counter, source
CPU, target CPU, and event type) is stored in a global variable. When
a new event is to be traced, the data of the last event is stored in
the event related percpu location and the global data is updated with
the new event's data. This allows to track two events in one data
location: one by the value of the event data (the event before the
current one), and one by the location itself (the current event).

A typical printout with a detected hang will look like this:

csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
	csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
        csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
        csd: cnt(00008cd): ffff->0006 idle
        csd: cnt(0003668): 0001->0006 queue
        csd: cnt(0003669): 0001->0006 ipi
        csd: cnt(0003e0f): 0007->000a queue
        csd: cnt(0003e10): 0001->ffff ping
        csd: cnt(0003e71): 0003->0000 ping
        csd: cnt(0003e72): ffff->0006 gotipi
        csd: cnt(0003e73): ffff->0006 handle
        csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
        csd: cnt(0003e7f): 0004->0006 ping
        csd: cnt(0003e80): 0001->ffff pinged
        csd: cnt(0003eb2): 0005->0001 noipi
        csd: cnt(0003eb3): 0001->0006 queue
        csd: cnt(0003eb4): 0001->0006 noipi
        csd: cnt now: 0003f00

The idea is to print only relevant entries. Those are all events which
are associated with the hang (so sender side events for the source CPU
of the hanging request, and receiver side events for the target CPU),
and the related events just before those (for adding data needed to
identify a possible race). Printing all available data would be
possible, but this would add large amounts of data printed on larger
configurations.

Signed-off-by: Juergen Gross <jgross@suse.com>
[ Minor readability edits. Breaks col80 but is far more readable. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-4-jgross@suse.com
---
 Documentation/admin-guide/kernel-parameters.txt |   4 +-
 kernel/smp.c                                    | 226 ++++++++++++++-
 2 files changed, 226 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 98dbffa..1fe9d38 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -789,6 +789,10 @@
 			printed to the console in case a hanging CPU is
 			detected, and that CPU is pinged again in order to try
 			to resolve the hang situation.
+			0: disable csdlock debugging (default)
+			1: enable basic csdlock debugging (minor impact)
+			ext: enable extended csdlock debugging (more impact,
+			     but more data)
 
 	dasd=		[HW,NET]
 			See header of drivers/s390/block/dasd_devmap.c.
diff --git a/kernel/smp.c b/kernel/smp.c
index 6d7e6db..f472ef6 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -31,8 +31,59 @@
 
 #define CSD_TYPE(_csd)	((_csd)->node.u_flags & CSD_FLAG_TYPE_MASK)
 
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+union cfd_seq_cnt {
+	u64		val;
+	struct {
+		u64	src:16;
+		u64	dst:16;
+#define CFD_SEQ_NOCPU	0xffff
+		u64	type:4;
+#define CFD_SEQ_QUEUE	0
+#define CFD_SEQ_IPI	1
+#define CFD_SEQ_NOIPI	2
+#define CFD_SEQ_PING	3
+#define CFD_SEQ_PINGED	4
+#define CFD_SEQ_HANDLE	5
+#define CFD_SEQ_DEQUEUE	6
+#define CFD_SEQ_IDLE	7
+#define CFD_SEQ_GOTIPI	8
+#define CFD_SEQ_HDLEND	9
+		u64	cnt:28;
+	}		u;
+};
+
+static char *seq_type[] = {
+	[CFD_SEQ_QUEUE]		= "queue",
+	[CFD_SEQ_IPI]		= "ipi",
+	[CFD_SEQ_NOIPI]		= "noipi",
+	[CFD_SEQ_PING]		= "ping",
+	[CFD_SEQ_PINGED]	= "pinged",
+	[CFD_SEQ_HANDLE]	= "handle",
+	[CFD_SEQ_DEQUEUE]	= "dequeue (src CPU 0 == empty)",
+	[CFD_SEQ_IDLE]		= "idle",
+	[CFD_SEQ_GOTIPI]	= "gotipi",
+	[CFD_SEQ_HDLEND]	= "hdlend (src CPU 0 == early)",
+};
+
+struct cfd_seq_local {
+	u64	ping;
+	u64	pinged;
+	u64	handle;
+	u64	dequeue;
+	u64	idle;
+	u64	gotipi;
+	u64	hdlend;
+};
+#endif
+
 struct cfd_percpu {
 	call_single_data_t	csd;
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+	u64	seq_queue;
+	u64	seq_ipi;
+	u64	seq_noipi;
+#endif
 };
 
 struct call_function_data {
@@ -108,12 +159,18 @@ void __init call_function_init(void)
 #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
 
 static DEFINE_STATIC_KEY_FALSE(csdlock_debug_enabled);
+static DEFINE_STATIC_KEY_FALSE(csdlock_debug_extended);
 
 static int __init csdlock_debug(char *str)
 {
 	unsigned int val = 0;
 
-	get_option(&str, &val);
+	if (str && !strcmp(str, "ext")) {
+		val = 1;
+		static_branch_enable(&csdlock_debug_extended);
+	} else
+		get_option(&str, &val);
+
 	if (val)
 		static_branch_enable(&csdlock_debug_enabled);
 
@@ -124,9 +181,34 @@ early_param("csdlock_debug", csdlock_debug);
 static DEFINE_PER_CPU(call_single_data_t *, cur_csd);
 static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
 static DEFINE_PER_CPU(void *, cur_csd_info);
+static DEFINE_PER_CPU(struct cfd_seq_local, cfd_seq_local);
 
 #define CSD_LOCK_TIMEOUT (5ULL * NSEC_PER_SEC)
 static atomic_t csd_bug_count = ATOMIC_INIT(0);
+static u64 cfd_seq;
+
+#define CFD_SEQ(s, d, t, c)	\
+	(union cfd_seq_cnt){ .u.src = s, .u.dst = d, .u.type = t, .u.cnt = c }
+
+static u64 cfd_seq_inc(unsigned int src, unsigned int dst, unsigned int type)
+{
+	union cfd_seq_cnt new, old;
+
+	new = CFD_SEQ(src, dst, type, 0);
+
+	do {
+		old.val = READ_ONCE(cfd_seq);
+		new.u.cnt = old.u.cnt + 1;
+	} while (cmpxchg(&cfd_seq, old.val, new.val) != old.val);
+
+	return old.val;
+}
+
+#define cfd_seq_store(var, src, dst, type)				\
+	do {								\
+		if (static_branch_unlikely(&csdlock_debug_extended))	\
+			var = cfd_seq_inc(src, dst, type);		\
+	} while (0)
 
 /* Record current CSD work for current CPU, NULL to erase. */
 static void __csd_lock_record(call_single_data_t *csd)
@@ -160,6 +242,80 @@ static int csd_lock_wait_getcpu(call_single_data_t *csd)
 	return -1;
 }
 
+static void cfd_seq_data_add(u64 val, unsigned int src, unsigned int dst,
+			     unsigned int type, union cfd_seq_cnt *data,
+			     unsigned int *n_data, unsigned int now)
+{
+	union cfd_seq_cnt new[2];
+	unsigned int i, j, k;
+
+	new[0].val = val;
+	new[1] = CFD_SEQ(src, dst, type, new[0].u.cnt + 1);
+
+	for (i = 0; i < 2; i++) {
+		if (new[i].u.cnt <= now)
+			new[i].u.cnt |= 0x80000000U;
+		for (j = 0; j < *n_data; j++) {
+			if (new[i].u.cnt == data[j].u.cnt) {
+				/* Direct read value trumps generated one. */
+				if (i == 0)
+					data[j].val = new[i].val;
+				break;
+			}
+			if (new[i].u.cnt < data[j].u.cnt) {
+				for (k = *n_data; k > j; k--)
+					data[k].val = data[k - 1].val;
+				data[j].val = new[i].val;
+				(*n_data)++;
+				break;
+			}
+		}
+		if (j == *n_data) {
+			data[j].val = new[i].val;
+			(*n_data)++;
+		}
+	}
+}
+
+static const char *csd_lock_get_type(unsigned int type)
+{
+	return (type >= ARRAY_SIZE(seq_type)) ? "?" : seq_type[type];
+}
+
+static void csd_lock_print_extended(call_single_data_t *csd, int cpu)
+{
+	struct cfd_seq_local *seq = &per_cpu(cfd_seq_local, cpu);
+	unsigned int srccpu = csd->node.src;
+	struct call_function_data *cfd = per_cpu_ptr(&cfd_data, srccpu);
+	struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+	unsigned int now;
+	union cfd_seq_cnt data[2 * ARRAY_SIZE(seq_type)];
+	unsigned int n_data = 0, i;
+
+	data[0].val = READ_ONCE(cfd_seq);
+	now = data[0].u.cnt;
+
+	cfd_seq_data_add(pcpu->seq_queue,			srccpu, cpu,	       CFD_SEQ_QUEUE,  data, &n_data, now);
+	cfd_seq_data_add(pcpu->seq_ipi,				srccpu, cpu,	       CFD_SEQ_IPI,    data, &n_data, now);
+	cfd_seq_data_add(pcpu->seq_noipi,			srccpu, cpu,	       CFD_SEQ_NOIPI,  data, &n_data, now);
+
+	cfd_seq_data_add(per_cpu(cfd_seq_local.ping, srccpu),	srccpu, CFD_SEQ_NOCPU, CFD_SEQ_PING,   data, &n_data, now);
+	cfd_seq_data_add(per_cpu(cfd_seq_local.pinged, srccpu), srccpu, CFD_SEQ_NOCPU, CFD_SEQ_PINGED, data, &n_data, now);
+
+	cfd_seq_data_add(seq->idle,    CFD_SEQ_NOCPU, cpu, CFD_SEQ_IDLE,    data, &n_data, now);
+	cfd_seq_data_add(seq->gotipi,  CFD_SEQ_NOCPU, cpu, CFD_SEQ_GOTIPI,  data, &n_data, now);
+	cfd_seq_data_add(seq->handle,  CFD_SEQ_NOCPU, cpu, CFD_SEQ_HANDLE,  data, &n_data, now);
+	cfd_seq_data_add(seq->dequeue, CFD_SEQ_NOCPU, cpu, CFD_SEQ_DEQUEUE, data, &n_data, now);
+	cfd_seq_data_add(seq->hdlend,  CFD_SEQ_NOCPU, cpu, CFD_SEQ_HDLEND,  data, &n_data, now);
+
+	for (i = 0; i < n_data; i++) {
+		pr_alert("\tcsd: cnt(%07x): %04x->%04x %s\n",
+			 data[i].u.cnt & ~0x80000000U, data[i].u.src,
+			 data[i].u.dst, csd_lock_get_type(data[i].u.type));
+	}
+	pr_alert("\tcsd: cnt now: %07x\n", now);
+}
+
 /*
  * Complain if too much time spent waiting.  Note that only
  * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
@@ -209,6 +365,8 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
 			 *bug_id, !cpu_cur_csd ? "unresponsive" : "handling this request");
 	}
 	if (cpu >= 0) {
+		if (static_branch_unlikely(&csdlock_debug_extended))
+			csd_lock_print_extended(csd, cpu);
 		if (!trigger_single_cpu_backtrace(cpu))
 			dump_cpu_task(cpu);
 		if (!cpu_cur_csd) {
@@ -252,7 +410,27 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
 
 	smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
 }
+
+static void __smp_call_single_queue_debug(int cpu, struct llist_node *node)
+{
+	unsigned int this_cpu = smp_processor_id();
+	struct cfd_seq_local *seq = this_cpu_ptr(&cfd_seq_local);
+	struct call_function_data *cfd = this_cpu_ptr(&cfd_data);
+	struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+
+	cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
+	if (llist_add(node, &per_cpu(call_single_queue, cpu))) {
+		cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
+		cfd_seq_store(seq->ping, this_cpu, cpu, CFD_SEQ_PING);
+		send_call_function_single_ipi(cpu);
+		cfd_seq_store(seq->pinged, this_cpu, cpu, CFD_SEQ_PINGED);
+	} else {
+		cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
+	}
+}
 #else
+#define cfd_seq_store(var, src, dst, type)
+
 static void csd_lock_record(call_single_data_t *csd)
 {
 }
@@ -290,6 +468,19 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
 
 void __smp_call_single_queue(int cpu, struct llist_node *node)
 {
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+	if (static_branch_unlikely(&csdlock_debug_extended)) {
+		unsigned int type;
+
+		type = CSD_TYPE(container_of(node, call_single_data_t,
+					     node.llist));
+		if (type == CSD_TYPE_SYNC || type == CSD_TYPE_ASYNC) {
+			__smp_call_single_queue_debug(cpu, node);
+			return;
+		}
+	}
+#endif
+
 	/*
 	 * The list addition should be visible before sending the IPI
 	 * handler locks the list to pull the entry off it because of
@@ -348,6 +539,8 @@ static int generic_exec_single(int cpu, call_single_data_t *csd)
  */
 void generic_smp_call_function_single_interrupt(void)
 {
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->gotipi, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_GOTIPI);
 	flush_smp_call_function_queue(true);
 }
 
@@ -375,7 +568,13 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 	lockdep_assert_irqs_disabled();
 
 	head = this_cpu_ptr(&call_single_queue);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->handle, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_HANDLE);
 	entry = llist_del_all(head);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->dequeue,
+		      /* Special meaning of source cpu: 0 == queue empty */
+		      entry ? CFD_SEQ_NOCPU : 0,
+		      smp_processor_id(), CFD_SEQ_DEQUEUE);
 	entry = llist_reverse_order(entry);
 
 	/* There shouldn't be any pending callbacks on an offline CPU. */
@@ -434,8 +633,12 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 		}
 	}
 
-	if (!entry)
+	if (!entry) {
+		cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->hdlend,
+			      0, smp_processor_id(),
+			      CFD_SEQ_HDLEND);
 		return;
+	}
 
 	/*
 	 * Second; run all !SYNC callbacks.
@@ -473,6 +676,9 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 	 */
 	if (entry)
 		sched_ttwu_pending(entry);
+
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->hdlend, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_HDLEND);
 }
 
 void flush_smp_call_function_from_idle(void)
@@ -482,6 +688,8 @@ void flush_smp_call_function_from_idle(void)
 	if (llist_empty(this_cpu_ptr(&call_single_queue)))
 		return;
 
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->idle, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_IDLE);
 	local_irq_save(flags);
 	flush_smp_call_function_queue(true);
 	if (local_softirq_pending())
@@ -698,7 +906,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 
 	cpumask_clear(cfd->cpumask_ipi);
 	for_each_cpu(cpu, cfd->cpumask) {
-		call_single_data_t *csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
+		struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+		call_single_data_t *csd = &pcpu->csd;
 
 		if (cond_func && !cond_func(cpu, info))
 			continue;
@@ -712,12 +921,21 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		csd->node.src = smp_processor_id();
 		csd->node.dst = cpu;
 #endif
-		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu)))
+		cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
+		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
 			__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
+			cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
+		} else {
+			cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
+		}
 	}
 
 	/* Send a message to all CPUs in the map */
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->ping, this_cpu,
+		      CFD_SEQ_NOCPU, CFD_SEQ_PING);
 	arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->pinged, this_cpu,
+		      CFD_SEQ_NOCPU, CFD_SEQ_PINGED);
 
 	if (wait) {
 		for_each_cpu(cpu, cfd->cpumask) {

[tip: locking/core] locking/csd_lock: Prepare more CSD lock debugging

[tip-bot2 for Juergen Gross]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     b3e3bc34b1e938c6447fa8b646010c4016be7fad
Gitweb:        https://git.kernel.org/tip/b3e3bc34b1e938c6447fa8b646010c4016be7fad
Author:        Juergen Gross <jgross@suse.com>
AuthorDate:    Mon, 01 Mar 2021 11:13:35 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 01 Mar 2021 14:27:58 +01:00

locking/csd_lock: Prepare more CSD lock debugging

In order to be able to easily add more CSD lock debugging data to
struct call_function_data->csd move the call_single_data_t element
into a sub-structure.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-3-jgross@suse.com
---
 kernel/smp.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index d5f0b21..6d7e6db 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -31,8 +31,12 @@
 
 #define CSD_TYPE(_csd)	((_csd)->node.u_flags & CSD_FLAG_TYPE_MASK)
 
+struct cfd_percpu {
+	call_single_data_t	csd;
+};
+
 struct call_function_data {
-	call_single_data_t	__percpu *csd;
+	struct cfd_percpu	__percpu *pcpu;
 	cpumask_var_t		cpumask;
 	cpumask_var_t		cpumask_ipi;
 };
@@ -55,8 +59,8 @@ int smpcfd_prepare_cpu(unsigned int cpu)
 		free_cpumask_var(cfd->cpumask);
 		return -ENOMEM;
 	}
-	cfd->csd = alloc_percpu(call_single_data_t);
-	if (!cfd->csd) {
+	cfd->pcpu = alloc_percpu(struct cfd_percpu);
+	if (!cfd->pcpu) {
 		free_cpumask_var(cfd->cpumask);
 		free_cpumask_var(cfd->cpumask_ipi);
 		return -ENOMEM;
@@ -71,7 +75,7 @@ int smpcfd_dead_cpu(unsigned int cpu)
 
 	free_cpumask_var(cfd->cpumask);
 	free_cpumask_var(cfd->cpumask_ipi);
-	free_percpu(cfd->csd);
+	free_percpu(cfd->pcpu);
 	return 0;
 }
 
@@ -694,7 +698,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 
 	cpumask_clear(cfd->cpumask_ipi);
 	for_each_cpu(cpu, cfd->cpumask) {
-		call_single_data_t *csd = per_cpu_ptr(cfd->csd, cpu);
+		call_single_data_t *csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
 
 		if (cond_func && !cond_func(cpu, info))
 			continue;
@@ -719,7 +723,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		for_each_cpu(cpu, cfd->cpumask) {
 			call_single_data_t *csd;
 
-			csd = per_cpu_ptr(cfd->csd, cpu);
+			csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
 			csd_lock_wait(csd);
 		}
 	}

[tip: locking/core] locking/csd_lock: Add boot parameter for controlling CSD lock debugging

[tip-bot2 for Juergen Gross]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     4b816578c16b92b68fb9842dcec0bc2fdc2b36d8
Gitweb:        https://git.kernel.org/tip/4b816578c16b92b68fb9842dcec0bc2fdc2b36d8
Author:        Juergen Gross <jgross@suse.com>
AuthorDate:    Mon, 01 Mar 2021 11:13:34 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 01 Mar 2021 14:27:58 +01:00

locking/csd_lock: Add boot parameter for controlling CSD lock debugging

Currently CSD lock debugging can be switched on and off via a kernel
config option only. Unfortunately there is at least one problem with
CSD lock handling pending for about 2 years now, which has been seen
in different environments (mostly when running virtualized under KVM
or Xen, at least once on bare metal). Multiple attempts to catch this
issue have finally led to introduction of CSD lock debug code, but
this code is not in use in most distros as it has some impact on
performance.

In order to be able to ship kernels with CONFIG_CSD_LOCK_WAIT_DEBUG
enabled even for production use, add a boot parameter for switching
the debug functionality on. This will reduce any performance impact
of the debug coding to a bare minimum when not being used.

Signed-off-by: Juergen Gross <jgross@suse.com>
[ Minor edits. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-2-jgross@suse.com
---
 Documentation/admin-guide/kernel-parameters.txt |  6 +++-
 kernel/smp.c                                    | 38 ++++++++++++++--
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0454572..98dbffa 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -784,6 +784,12 @@
 	cs89x0_media=	[HW,NET]
 			Format: { rj45 | aui | bnc }
 
+	csdlock_debug=	[KNL] Enable debug add-ons of cross-CPU function call
+			handling. When switched on, additional debug data is
+			printed to the console in case a hanging CPU is
+			detected, and that CPU is pinged again in order to try
+			to resolve the hang situation.
+
 	dasd=		[HW,NET]
 			See header of drivers/s390/block/dasd_devmap.c.
 
diff --git a/kernel/smp.c b/kernel/smp.c
index aeb0adf..d5f0b21 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -24,6 +24,7 @@
 #include <linux/sched/clock.h>
 #include <linux/nmi.h>
 #include <linux/sched/debug.h>
+#include <linux/jump_label.h>
 
 #include "smpboot.h"
 #include "sched/smp.h"
@@ -102,6 +103,20 @@ void __init call_function_init(void)
 
 #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
 
+static DEFINE_STATIC_KEY_FALSE(csdlock_debug_enabled);
+
+static int __init csdlock_debug(char *str)
+{
+	unsigned int val = 0;
+
+	get_option(&str, &val);
+	if (val)
+		static_branch_enable(&csdlock_debug_enabled);
+
+	return 0;
+}
+early_param("csdlock_debug", csdlock_debug);
+
 static DEFINE_PER_CPU(call_single_data_t *, cur_csd);
 static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
 static DEFINE_PER_CPU(void *, cur_csd_info);
@@ -110,7 +125,7 @@ static DEFINE_PER_CPU(void *, cur_csd_info);
 static atomic_t csd_bug_count = ATOMIC_INIT(0);
 
 /* Record current CSD work for current CPU, NULL to erase. */
-static void csd_lock_record(call_single_data_t *csd)
+static void __csd_lock_record(call_single_data_t *csd)
 {
 	if (!csd) {
 		smp_mb(); /* NULL cur_csd after unlock. */
@@ -125,7 +140,13 @@ static void csd_lock_record(call_single_data_t *csd)
 		  /* Or before unlock, as the case may be. */
 }
 
-static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd)
+static __always_inline void csd_lock_record(call_single_data_t *csd)
+{
+	if (static_branch_unlikely(&csdlock_debug_enabled))
+		__csd_lock_record(csd);
+}
+
+static int csd_lock_wait_getcpu(call_single_data_t *csd)
 {
 	unsigned int csd_type;
 
@@ -140,7 +161,7 @@ static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd)
  * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
  * so waiting on other types gets much less information.
  */
-static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, int *bug_id)
+static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, int *bug_id)
 {
 	int cpu = -1;
 	int cpux;
@@ -204,7 +225,7 @@ static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd, u64 t
  * previous function call. For multi-cpu calls its even more interesting
  * as we'll have to ensure no other cpu is observing our csd.
  */
-static __always_inline void csd_lock_wait(call_single_data_t *csd)
+static void __csd_lock_wait(call_single_data_t *csd)
 {
 	int bug_id = 0;
 	u64 ts0, ts1;
@@ -218,6 +239,15 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
 	smp_acquire__after_ctrl_dep();
 }
 
+static __always_inline void csd_lock_wait(call_single_data_t *csd)
+{
+	if (static_branch_unlikely(&csdlock_debug_enabled)) {
+		__csd_lock_wait(csd);
+		return;
+	}
+
+	smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
+}
 #else
 static void csd_lock_record(call_single_data_t *csd)
 {

Re: [PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging

[Peter Zijlstra]

On Mon, Mar 01, 2021 at 11:13:36AM +0100, Juergen Gross wrote:
> In order to help identifying problems with IPI handling and remote
> function execution add some more data to IPI debugging code.
> 
> There have been multiple reports of cpus looping long times (many
> seconds) in smp_call_function_many() waiting for another cpu executing
> a function like tlb flushing. Most of these reports have been for
> cases where the kernel was running as a guest on top of KVM or Xen
> (there are rumours of that happening under VMWare, too, and even on
> bare metal).
> 
> Finding the root cause hasn't been successful yet, even after more than
> 2 years of chasing this bug by different developers.
> 
> Commit 35feb60474bf4f7 ("kernel/smp: Provide CSD lock timeout
> diagnostics") tried to address this by adding some debug code and by
> issuing another IPI when a hang was detected. This helped mitigating
> the problem (the repeated IPI unlocks the hang), but the root cause is
> still unknown.
> 
> Current available data suggests that either an IPI wasn't sent when it
> should have been, or that the IPI didn't result in the target cpu
> executing the queued function (due to the IPI not reaching the cpu,
> the IPI handler not being called, or the handler not seeing the queued
> request).
> 
> Try to add more diagnostic data by introducing a global atomic counter
> which is being incremented when doing critical operations (before and
> after queueing a new request, when sending an IPI, and when dequeueing
> a request). The counter value is stored in percpu variables which can
> be printed out when a hang is detected.
> 
> The data of the last event (consisting of sequence counter, source
> cpu, target cpu, and event type) is stored in a global variable. When
> a new event is to be traced, the data of the last event is stored in
> the event related percpu location and the global data is updated with
> the new event's data. This allows to track two events in one data
> location: one by the value of the event data (the event before the
> current one), and one by the location itself (the current event).
> 
> A typical printout with a detected hang will look like this:
> 
> csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
> 	csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
>         csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
>         csd: cnt(00008cd): ffff->0006 idle
>         csd: cnt(0003668): 0001->0006 queue
>         csd: cnt(0003669): 0001->0006 ipi
>         csd: cnt(0003e0f): 0007->000a queue
>         csd: cnt(0003e10): 0001->ffff ping
>         csd: cnt(0003e71): 0003->0000 ping
>         csd: cnt(0003e72): ffff->0006 gotipi
>         csd: cnt(0003e73): ffff->0006 handle
>         csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
>         csd: cnt(0003e7f): 0004->0006 ping
>         csd: cnt(0003e80): 0001->ffff pinged
>         csd: cnt(0003eb2): 0005->0001 noipi
>         csd: cnt(0003eb3): 0001->0006 queue
>         csd: cnt(0003eb4): 0001->0006 noipi
>         csd: cnt now: 0003f00
> 
> The idea is to print only relevant entries. Those are all events which
> are associated with the hang (so sender side events for the source cpu
> of the hanging request, and receiver side events for the target cpu),
> and the related events just before those (for adding data needed to
> identify a possible race). Printing all available data would be
> possible, but this would add large amounts of data printed on larger
> configurations.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> Tested-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> V2:
> - add automatic data deciphering and sorting of entries
> - add new trace point for leaving flush_smp_call_function_queue()
> - add information when finding an empty call_single_queue

They do not apply on top of these:

  https://lkml.kernel.org/r/20210220231712.2475218-2-namit@vmware.com

:-/

> @@ -290,6 +476,19 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
>  
>  void __smp_call_single_queue(int cpu, struct llist_node *node)
>  {
> +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
> +	if (static_branch_unlikely(&csdlock_debug_extended)) {
> +		unsigned int type;
> +
> +		type = CSD_TYPE(container_of(node, call_single_data_t,
> +					     node.llist));
> +		if (type == CSD_TYPE_SYNC || type == CSD_TYPE_ASYNC) {
> +			__smp_call_single_queue_debug(cpu, node);
> +			return;
> +		}
> +	}
> +#endif

This really ought to be in generic_exec_single(), because there we know
the type matches.

Re: [PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging

[Jürgen Groß]

[-- Attachment #1.1.1: Type: text/plain, Size: 4317 bytes --]

On 01.03.21 18:07, Peter Zijlstra wrote:
> On Mon, Mar 01, 2021 at 11:13:36AM +0100, Juergen Gross wrote:
>> In order to help identifying problems with IPI handling and remote
>> function execution add some more data to IPI debugging code.
>>
>> There have been multiple reports of cpus looping long times (many
>> seconds) in smp_call_function_many() waiting for another cpu executing
>> a function like tlb flushing. Most of these reports have been for
>> cases where the kernel was running as a guest on top of KVM or Xen
>> (there are rumours of that happening under VMWare, too, and even on
>> bare metal).
>>
>> Finding the root cause hasn't been successful yet, even after more than
>> 2 years of chasing this bug by different developers.
>>
>> Commit 35feb60474bf4f7 ("kernel/smp: Provide CSD lock timeout
>> diagnostics") tried to address this by adding some debug code and by
>> issuing another IPI when a hang was detected. This helped mitigating
>> the problem (the repeated IPI unlocks the hang), but the root cause is
>> still unknown.
>>
>> Current available data suggests that either an IPI wasn't sent when it
>> should have been, or that the IPI didn't result in the target cpu
>> executing the queued function (due to the IPI not reaching the cpu,
>> the IPI handler not being called, or the handler not seeing the queued
>> request).
>>
>> Try to add more diagnostic data by introducing a global atomic counter
>> which is being incremented when doing critical operations (before and
>> after queueing a new request, when sending an IPI, and when dequeueing
>> a request). The counter value is stored in percpu variables which can
>> be printed out when a hang is detected.
>>
>> The data of the last event (consisting of sequence counter, source
>> cpu, target cpu, and event type) is stored in a global variable. When
>> a new event is to be traced, the data of the last event is stored in
>> the event related percpu location and the global data is updated with
>> the new event's data. This allows to track two events in one data
>> location: one by the value of the event data (the event before the
>> current one), and one by the location itself (the current event).
>>
>> A typical printout with a detected hang will look like this:
>>
>> csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
>> 	csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
>>          csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
>>          csd: cnt(00008cd): ffff->0006 idle
>>          csd: cnt(0003668): 0001->0006 queue
>>          csd: cnt(0003669): 0001->0006 ipi
>>          csd: cnt(0003e0f): 0007->000a queue
>>          csd: cnt(0003e10): 0001->ffff ping
>>          csd: cnt(0003e71): 0003->0000 ping
>>          csd: cnt(0003e72): ffff->0006 gotipi
>>          csd: cnt(0003e73): ffff->0006 handle
>>          csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
>>          csd: cnt(0003e7f): 0004->0006 ping
>>          csd: cnt(0003e80): 0001->ffff pinged
>>          csd: cnt(0003eb2): 0005->0001 noipi
>>          csd: cnt(0003eb3): 0001->0006 queue
>>          csd: cnt(0003eb4): 0001->0006 noipi
>>          csd: cnt now: 0003f00
>>
>> The idea is to print only relevant entries. Those are all events which
>> are associated with the hang (so sender side events for the source cpu
>> of the hanging request, and receiver side events for the target cpu),
>> and the related events just before those (for adding data needed to
>> identify a possible race). Printing all available data would be
>> possible, but this would add large amounts of data printed on larger
>> configurations.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> Tested-by: Paul E. McKenney <paulmck@kernel.org>
>> ---
>> V2:
>> - add automatic data deciphering and sorting of entries
>> - add new trace point for leaving flush_smp_call_function_queue()
>> - add information when finding an empty call_single_queue
> 
> They do not apply on top of these:
> 
>    https://lkml.kernel.org/r/20210220231712.2475218-2-namit@vmware.com

They are already in tip locking/core (Ingo applied them).


Juergen


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Re: [PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging

[Peter Zijlstra]

On Mon, Mar 01, 2021 at 08:16:12PM +0100, Jürgen Groß wrote:
> >    https://lkml.kernel.org/r/20210220231712.2475218-2-namit@vmware.com
> 
> They are already in tip locking/core (Ingo applied them).

I'm very tempted to undo that :-(

Re: [PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging

[Jürgen Groß]

[-- Attachment #1.1.1: Type: text/plain, Size: 4965 bytes --]

On 01.03.21 18:07, Peter Zijlstra wrote:
> On Mon, Mar 01, 2021 at 11:13:36AM +0100, Juergen Gross wrote:
>> In order to help identifying problems with IPI handling and remote
>> function execution add some more data to IPI debugging code.
>>
>> There have been multiple reports of cpus looping long times (many
>> seconds) in smp_call_function_many() waiting for another cpu executing
>> a function like tlb flushing. Most of these reports have been for
>> cases where the kernel was running as a guest on top of KVM or Xen
>> (there are rumours of that happening under VMWare, too, and even on
>> bare metal).
>>
>> Finding the root cause hasn't been successful yet, even after more than
>> 2 years of chasing this bug by different developers.
>>
>> Commit 35feb60474bf4f7 ("kernel/smp: Provide CSD lock timeout
>> diagnostics") tried to address this by adding some debug code and by
>> issuing another IPI when a hang was detected. This helped mitigating
>> the problem (the repeated IPI unlocks the hang), but the root cause is
>> still unknown.
>>
>> Current available data suggests that either an IPI wasn't sent when it
>> should have been, or that the IPI didn't result in the target cpu
>> executing the queued function (due to the IPI not reaching the cpu,
>> the IPI handler not being called, or the handler not seeing the queued
>> request).
>>
>> Try to add more diagnostic data by introducing a global atomic counter
>> which is being incremented when doing critical operations (before and
>> after queueing a new request, when sending an IPI, and when dequeueing
>> a request). The counter value is stored in percpu variables which can
>> be printed out when a hang is detected.
>>
>> The data of the last event (consisting of sequence counter, source
>> cpu, target cpu, and event type) is stored in a global variable. When
>> a new event is to be traced, the data of the last event is stored in
>> the event related percpu location and the global data is updated with
>> the new event's data. This allows to track two events in one data
>> location: one by the value of the event data (the event before the
>> current one), and one by the location itself (the current event).
>>
>> A typical printout with a detected hang will look like this:
>>
>> csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
>> 	csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
>>          csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
>>          csd: cnt(00008cd): ffff->0006 idle
>>          csd: cnt(0003668): 0001->0006 queue
>>          csd: cnt(0003669): 0001->0006 ipi
>>          csd: cnt(0003e0f): 0007->000a queue
>>          csd: cnt(0003e10): 0001->ffff ping
>>          csd: cnt(0003e71): 0003->0000 ping
>>          csd: cnt(0003e72): ffff->0006 gotipi
>>          csd: cnt(0003e73): ffff->0006 handle
>>          csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
>>          csd: cnt(0003e7f): 0004->0006 ping
>>          csd: cnt(0003e80): 0001->ffff pinged
>>          csd: cnt(0003eb2): 0005->0001 noipi
>>          csd: cnt(0003eb3): 0001->0006 queue
>>          csd: cnt(0003eb4): 0001->0006 noipi
>>          csd: cnt now: 0003f00
>>
>> The idea is to print only relevant entries. Those are all events which
>> are associated with the hang (so sender side events for the source cpu
>> of the hanging request, and receiver side events for the target cpu),
>> and the related events just before those (for adding data needed to
>> identify a possible race). Printing all available data would be
>> possible, but this would add large amounts of data printed on larger
>> configurations.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> Tested-by: Paul E. McKenney <paulmck@kernel.org>
>> ---
>> V2:
>> - add automatic data deciphering and sorting of entries
>> - add new trace point for leaving flush_smp_call_function_queue()
>> - add information when finding an empty call_single_queue
> 
> They do not apply on top of these:
> 
>    https://lkml.kernel.org/r/20210220231712.2475218-2-namit@vmware.com
> 
> :-/
> 
>> @@ -290,6 +476,19 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
>>   
>>   void __smp_call_single_queue(int cpu, struct llist_node *node)
>>   {
>> +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
>> +	if (static_branch_unlikely(&csdlock_debug_extended)) {
>> +		unsigned int type;
>> +
>> +		type = CSD_TYPE(container_of(node, call_single_data_t,
>> +					     node.llist));
>> +		if (type == CSD_TYPE_SYNC || type == CSD_TYPE_ASYNC) {
>> +			__smp_call_single_queue_debug(cpu, node);
>> +			return;
>> +		}
>> +	}
>> +#endif
> 
> This really ought to be in generic_exec_single(), because there we know
> the type matches.

You are right. This is much better.


Juergen

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Re: [PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging

[Ingo Molnar]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Mar 01, 2021 at 08:16:12PM +0100, Jürgen Groß wrote:
> > >    https://lkml.kernel.org/r/20210220231712.2475218-2-namit@vmware.com
> > 
> > They are already in tip locking/core (Ingo applied them).
> 
> I'm very tempted to undo that :-(

Sorry about that - I'm sorting out the conflicts with the concurrent TLB 
flush series.

Thanks,

	Ingo

Re: [PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging

[Ingo Molnar]

* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Mon, Mar 01, 2021 at 08:16:12PM +0100, Jürgen Groß wrote:
> > > >    https://lkml.kernel.org/r/20210220231712.2475218-2-namit@vmware.com
> > > 
> > > They are already in tip locking/core (Ingo applied them).
> > 
> > I'm very tempted to undo that :-(
> 
> Sorry about that - I'm sorting out the conflicts with the concurrent TLB 
> flush series.

I've resolved them in:

  8dbac5da2796: ("Merge branch 'locking/core' into x86/mm, to resolve conflicts")

Will push it out after a bit of testing.

Thanks,

	Ingo

[tip: locking/core] locking/csd_lock: Add boot parameter for controlling CSD lock debugging

[tip-bot2 for Juergen Gross]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     8d0968cc6b8ffd8496c2ebffdfdc801f949a85e5
Gitweb:        https://git.kernel.org/tip/8d0968cc6b8ffd8496c2ebffdfdc801f949a85e5
Author:        Juergen Gross <jgross@suse.com>
AuthorDate:    Mon, 01 Mar 2021 11:13:34 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 06 Mar 2021 12:49:48 +01:00

locking/csd_lock: Add boot parameter for controlling CSD lock debugging

Currently CSD lock debugging can be switched on and off via a kernel
config option only. Unfortunately there is at least one problem with
CSD lock handling pending for about 2 years now, which has been seen
in different environments (mostly when running virtualized under KVM
or Xen, at least once on bare metal). Multiple attempts to catch this
issue have finally led to introduction of CSD lock debug code, but
this code is not in use in most distros as it has some impact on
performance.

In order to be able to ship kernels with CONFIG_CSD_LOCK_WAIT_DEBUG
enabled even for production use, add a boot parameter for switching
the debug functionality on. This will reduce any performance impact
of the debug coding to a bare minimum when not being used.

Signed-off-by: Juergen Gross <jgross@suse.com>
[ Minor edits. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-2-jgross@suse.com
---
 Documentation/admin-guide/kernel-parameters.txt |  6 +++-
 kernel/smp.c                                    | 38 ++++++++++++++--
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0454572..98dbffa 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -784,6 +784,12 @@
 	cs89x0_media=	[HW,NET]
 			Format: { rj45 | aui | bnc }
 
+	csdlock_debug=	[KNL] Enable debug add-ons of cross-CPU function call
+			handling. When switched on, additional debug data is
+			printed to the console in case a hanging CPU is
+			detected, and that CPU is pinged again in order to try
+			to resolve the hang situation.
+
 	dasd=		[HW,NET]
 			See header of drivers/s390/block/dasd_devmap.c.
 
diff --git a/kernel/smp.c b/kernel/smp.c
index aeb0adf..d5f0b21 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -24,6 +24,7 @@
 #include <linux/sched/clock.h>
 #include <linux/nmi.h>
 #include <linux/sched/debug.h>
+#include <linux/jump_label.h>
 
 #include "smpboot.h"
 #include "sched/smp.h"
@@ -102,6 +103,20 @@ void __init call_function_init(void)
 
 #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
 
+static DEFINE_STATIC_KEY_FALSE(csdlock_debug_enabled);
+
+static int __init csdlock_debug(char *str)
+{
+	unsigned int val = 0;
+
+	get_option(&str, &val);
+	if (val)
+		static_branch_enable(&csdlock_debug_enabled);
+
+	return 0;
+}
+early_param("csdlock_debug", csdlock_debug);
+
 static DEFINE_PER_CPU(call_single_data_t *, cur_csd);
 static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
 static DEFINE_PER_CPU(void *, cur_csd_info);
@@ -110,7 +125,7 @@ static DEFINE_PER_CPU(void *, cur_csd_info);
 static atomic_t csd_bug_count = ATOMIC_INIT(0);
 
 /* Record current CSD work for current CPU, NULL to erase. */
-static void csd_lock_record(call_single_data_t *csd)
+static void __csd_lock_record(call_single_data_t *csd)
 {
 	if (!csd) {
 		smp_mb(); /* NULL cur_csd after unlock. */
@@ -125,7 +140,13 @@ static void csd_lock_record(call_single_data_t *csd)
 		  /* Or before unlock, as the case may be. */
 }
 
-static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd)
+static __always_inline void csd_lock_record(call_single_data_t *csd)
+{
+	if (static_branch_unlikely(&csdlock_debug_enabled))
+		__csd_lock_record(csd);
+}
+
+static int csd_lock_wait_getcpu(call_single_data_t *csd)
 {
 	unsigned int csd_type;
 
@@ -140,7 +161,7 @@ static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd)
  * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
  * so waiting on other types gets much less information.
  */
-static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, int *bug_id)
+static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, int *bug_id)
 {
 	int cpu = -1;
 	int cpux;
@@ -204,7 +225,7 @@ static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd, u64 t
  * previous function call. For multi-cpu calls its even more interesting
  * as we'll have to ensure no other cpu is observing our csd.
  */
-static __always_inline void csd_lock_wait(call_single_data_t *csd)
+static void __csd_lock_wait(call_single_data_t *csd)
 {
 	int bug_id = 0;
 	u64 ts0, ts1;
@@ -218,6 +239,15 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
 	smp_acquire__after_ctrl_dep();
 }
 
+static __always_inline void csd_lock_wait(call_single_data_t *csd)
+{
+	if (static_branch_unlikely(&csdlock_debug_enabled)) {
+		__csd_lock_wait(csd);
+		return;
+	}
+
+	smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
+}
 #else
 static void csd_lock_record(call_single_data_t *csd)
 {

[tip: locking/core] locking/csd_lock: Add more data to CSD lock debugging

[tip-bot2 for Juergen Gross]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     a5aabace5fb8abf2adcfcf0fe54c089b20d71755
Gitweb:        https://git.kernel.org/tip/a5aabace5fb8abf2adcfcf0fe54c089b20d71755
Author:        Juergen Gross <jgross@suse.com>
AuthorDate:    Mon, 01 Mar 2021 11:13:36 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 06 Mar 2021 12:49:48 +01:00

locking/csd_lock: Add more data to CSD lock debugging

In order to help identifying problems with IPI handling and remote
function execution add some more data to IPI debugging code.

There have been multiple reports of CPUs looping long times (many
seconds) in smp_call_function_many() waiting for another CPU executing
a function like tlb flushing. Most of these reports have been for
cases where the kernel was running as a guest on top of KVM or Xen
(there are rumours of that happening under VMWare, too, and even on
bare metal).

Finding the root cause hasn't been successful yet, even after more than
2 years of chasing this bug by different developers.

Commit:

  35feb60474bf4f7 ("kernel/smp: Provide CSD lock timeout diagnostics")

tried to address this by adding some debug code and by issuing another
IPI when a hang was detected. This helped mitigating the problem
(the repeated IPI unlocks the hang), but the root cause is still unknown.

Current available data suggests that either an IPI wasn't sent when it
should have been, or that the IPI didn't result in the target CPU
executing the queued function (due to the IPI not reaching the CPU,
the IPI handler not being called, or the handler not seeing the queued
request).

Try to add more diagnostic data by introducing a global atomic counter
which is being incremented when doing critical operations (before and
after queueing a new request, when sending an IPI, and when dequeueing
a request). The counter value is stored in percpu variables which can
be printed out when a hang is detected.

The data of the last event (consisting of sequence counter, source
CPU, target CPU, and event type) is stored in a global variable. When
a new event is to be traced, the data of the last event is stored in
the event related percpu location and the global data is updated with
the new event's data. This allows to track two events in one data
location: one by the value of the event data (the event before the
current one), and one by the location itself (the current event).

A typical printout with a detected hang will look like this:

csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
	csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
        csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
        csd: cnt(00008cd): ffff->0006 idle
        csd: cnt(0003668): 0001->0006 queue
        csd: cnt(0003669): 0001->0006 ipi
        csd: cnt(0003e0f): 0007->000a queue
        csd: cnt(0003e10): 0001->ffff ping
        csd: cnt(0003e71): 0003->0000 ping
        csd: cnt(0003e72): ffff->0006 gotipi
        csd: cnt(0003e73): ffff->0006 handle
        csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
        csd: cnt(0003e7f): 0004->0006 ping
        csd: cnt(0003e80): 0001->ffff pinged
        csd: cnt(0003eb2): 0005->0001 noipi
        csd: cnt(0003eb3): 0001->0006 queue
        csd: cnt(0003eb4): 0001->0006 noipi
        csd: cnt now: 0003f00

The idea is to print only relevant entries. Those are all events which
are associated with the hang (so sender side events for the source CPU
of the hanging request, and receiver side events for the target CPU),
and the related events just before those (for adding data needed to
identify a possible race). Printing all available data would be
possible, but this would add large amounts of data printed on larger
configurations.

Signed-off-by: Juergen Gross <jgross@suse.com>
[ Minor readability edits. Breaks col80 but is far more readable. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-4-jgross@suse.com
---
 Documentation/admin-guide/kernel-parameters.txt |   4 +-
 kernel/smp.c                                    | 226 ++++++++++++++-
 2 files changed, 226 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 98dbffa..1fe9d38 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -789,6 +789,10 @@
 			printed to the console in case a hanging CPU is
 			detected, and that CPU is pinged again in order to try
 			to resolve the hang situation.
+			0: disable csdlock debugging (default)
+			1: enable basic csdlock debugging (minor impact)
+			ext: enable extended csdlock debugging (more impact,
+			     but more data)
 
 	dasd=		[HW,NET]
 			See header of drivers/s390/block/dasd_devmap.c.
diff --git a/kernel/smp.c b/kernel/smp.c
index 6d7e6db..f472ef6 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -31,8 +31,59 @@
 
 #define CSD_TYPE(_csd)	((_csd)->node.u_flags & CSD_FLAG_TYPE_MASK)
 
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+union cfd_seq_cnt {
+	u64		val;
+	struct {
+		u64	src:16;
+		u64	dst:16;
+#define CFD_SEQ_NOCPU	0xffff
+		u64	type:4;
+#define CFD_SEQ_QUEUE	0
+#define CFD_SEQ_IPI	1
+#define CFD_SEQ_NOIPI	2
+#define CFD_SEQ_PING	3
+#define CFD_SEQ_PINGED	4
+#define CFD_SEQ_HANDLE	5
+#define CFD_SEQ_DEQUEUE	6
+#define CFD_SEQ_IDLE	7
+#define CFD_SEQ_GOTIPI	8
+#define CFD_SEQ_HDLEND	9
+		u64	cnt:28;
+	}		u;
+};
+
+static char *seq_type[] = {
+	[CFD_SEQ_QUEUE]		= "queue",
+	[CFD_SEQ_IPI]		= "ipi",
+	[CFD_SEQ_NOIPI]		= "noipi",
+	[CFD_SEQ_PING]		= "ping",
+	[CFD_SEQ_PINGED]	= "pinged",
+	[CFD_SEQ_HANDLE]	= "handle",
+	[CFD_SEQ_DEQUEUE]	= "dequeue (src CPU 0 == empty)",
+	[CFD_SEQ_IDLE]		= "idle",
+	[CFD_SEQ_GOTIPI]	= "gotipi",
+	[CFD_SEQ_HDLEND]	= "hdlend (src CPU 0 == early)",
+};
+
+struct cfd_seq_local {
+	u64	ping;
+	u64	pinged;
+	u64	handle;
+	u64	dequeue;
+	u64	idle;
+	u64	gotipi;
+	u64	hdlend;
+};
+#endif
+
 struct cfd_percpu {
 	call_single_data_t	csd;
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+	u64	seq_queue;
+	u64	seq_ipi;
+	u64	seq_noipi;
+#endif
 };
 
 struct call_function_data {
@@ -108,12 +159,18 @@ void __init call_function_init(void)
 #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
 
 static DEFINE_STATIC_KEY_FALSE(csdlock_debug_enabled);
+static DEFINE_STATIC_KEY_FALSE(csdlock_debug_extended);
 
 static int __init csdlock_debug(char *str)
 {
 	unsigned int val = 0;
 
-	get_option(&str, &val);
+	if (str && !strcmp(str, "ext")) {
+		val = 1;
+		static_branch_enable(&csdlock_debug_extended);
+	} else
+		get_option(&str, &val);
+
 	if (val)
 		static_branch_enable(&csdlock_debug_enabled);
 
@@ -124,9 +181,34 @@ early_param("csdlock_debug", csdlock_debug);
 static DEFINE_PER_CPU(call_single_data_t *, cur_csd);
 static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
 static DEFINE_PER_CPU(void *, cur_csd_info);
+static DEFINE_PER_CPU(struct cfd_seq_local, cfd_seq_local);
 
 #define CSD_LOCK_TIMEOUT (5ULL * NSEC_PER_SEC)
 static atomic_t csd_bug_count = ATOMIC_INIT(0);
+static u64 cfd_seq;
+
+#define CFD_SEQ(s, d, t, c)	\
+	(union cfd_seq_cnt){ .u.src = s, .u.dst = d, .u.type = t, .u.cnt = c }
+
+static u64 cfd_seq_inc(unsigned int src, unsigned int dst, unsigned int type)
+{
+	union cfd_seq_cnt new, old;
+
+	new = CFD_SEQ(src, dst, type, 0);
+
+	do {
+		old.val = READ_ONCE(cfd_seq);
+		new.u.cnt = old.u.cnt + 1;
+	} while (cmpxchg(&cfd_seq, old.val, new.val) != old.val);
+
+	return old.val;
+}
+
+#define cfd_seq_store(var, src, dst, type)				\
+	do {								\
+		if (static_branch_unlikely(&csdlock_debug_extended))	\
+			var = cfd_seq_inc(src, dst, type);		\
+	} while (0)
 
 /* Record current CSD work for current CPU, NULL to erase. */
 static void __csd_lock_record(call_single_data_t *csd)
@@ -160,6 +242,80 @@ static int csd_lock_wait_getcpu(call_single_data_t *csd)
 	return -1;
 }
 
+static void cfd_seq_data_add(u64 val, unsigned int src, unsigned int dst,
+			     unsigned int type, union cfd_seq_cnt *data,
+			     unsigned int *n_data, unsigned int now)
+{
+	union cfd_seq_cnt new[2];
+	unsigned int i, j, k;
+
+	new[0].val = val;
+	new[1] = CFD_SEQ(src, dst, type, new[0].u.cnt + 1);
+
+	for (i = 0; i < 2; i++) {
+		if (new[i].u.cnt <= now)
+			new[i].u.cnt |= 0x80000000U;
+		for (j = 0; j < *n_data; j++) {
+			if (new[i].u.cnt == data[j].u.cnt) {
+				/* Direct read value trumps generated one. */
+				if (i == 0)
+					data[j].val = new[i].val;
+				break;
+			}
+			if (new[i].u.cnt < data[j].u.cnt) {
+				for (k = *n_data; k > j; k--)
+					data[k].val = data[k - 1].val;
+				data[j].val = new[i].val;
+				(*n_data)++;
+				break;
+			}
+		}
+		if (j == *n_data) {
+			data[j].val = new[i].val;
+			(*n_data)++;
+		}
+	}
+}
+
+static const char *csd_lock_get_type(unsigned int type)
+{
+	return (type >= ARRAY_SIZE(seq_type)) ? "?" : seq_type[type];
+}
+
+static void csd_lock_print_extended(call_single_data_t *csd, int cpu)
+{
+	struct cfd_seq_local *seq = &per_cpu(cfd_seq_local, cpu);
+	unsigned int srccpu = csd->node.src;
+	struct call_function_data *cfd = per_cpu_ptr(&cfd_data, srccpu);
+	struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+	unsigned int now;
+	union cfd_seq_cnt data[2 * ARRAY_SIZE(seq_type)];
+	unsigned int n_data = 0, i;
+
+	data[0].val = READ_ONCE(cfd_seq);
+	now = data[0].u.cnt;
+
+	cfd_seq_data_add(pcpu->seq_queue,			srccpu, cpu,	       CFD_SEQ_QUEUE,  data, &n_data, now);
+	cfd_seq_data_add(pcpu->seq_ipi,				srccpu, cpu,	       CFD_SEQ_IPI,    data, &n_data, now);
+	cfd_seq_data_add(pcpu->seq_noipi,			srccpu, cpu,	       CFD_SEQ_NOIPI,  data, &n_data, now);
+
+	cfd_seq_data_add(per_cpu(cfd_seq_local.ping, srccpu),	srccpu, CFD_SEQ_NOCPU, CFD_SEQ_PING,   data, &n_data, now);
+	cfd_seq_data_add(per_cpu(cfd_seq_local.pinged, srccpu), srccpu, CFD_SEQ_NOCPU, CFD_SEQ_PINGED, data, &n_data, now);
+
+	cfd_seq_data_add(seq->idle,    CFD_SEQ_NOCPU, cpu, CFD_SEQ_IDLE,    data, &n_data, now);
+	cfd_seq_data_add(seq->gotipi,  CFD_SEQ_NOCPU, cpu, CFD_SEQ_GOTIPI,  data, &n_data, now);
+	cfd_seq_data_add(seq->handle,  CFD_SEQ_NOCPU, cpu, CFD_SEQ_HANDLE,  data, &n_data, now);
+	cfd_seq_data_add(seq->dequeue, CFD_SEQ_NOCPU, cpu, CFD_SEQ_DEQUEUE, data, &n_data, now);
+	cfd_seq_data_add(seq->hdlend,  CFD_SEQ_NOCPU, cpu, CFD_SEQ_HDLEND,  data, &n_data, now);
+
+	for (i = 0; i < n_data; i++) {
+		pr_alert("\tcsd: cnt(%07x): %04x->%04x %s\n",
+			 data[i].u.cnt & ~0x80000000U, data[i].u.src,
+			 data[i].u.dst, csd_lock_get_type(data[i].u.type));
+	}
+	pr_alert("\tcsd: cnt now: %07x\n", now);
+}
+
 /*
  * Complain if too much time spent waiting.  Note that only
  * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
@@ -209,6 +365,8 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
 			 *bug_id, !cpu_cur_csd ? "unresponsive" : "handling this request");
 	}
 	if (cpu >= 0) {
+		if (static_branch_unlikely(&csdlock_debug_extended))
+			csd_lock_print_extended(csd, cpu);
 		if (!trigger_single_cpu_backtrace(cpu))
 			dump_cpu_task(cpu);
 		if (!cpu_cur_csd) {
@@ -252,7 +410,27 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
 
 	smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
 }
+
+static void __smp_call_single_queue_debug(int cpu, struct llist_node *node)
+{
+	unsigned int this_cpu = smp_processor_id();
+	struct cfd_seq_local *seq = this_cpu_ptr(&cfd_seq_local);
+	struct call_function_data *cfd = this_cpu_ptr(&cfd_data);
+	struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+
+	cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
+	if (llist_add(node, &per_cpu(call_single_queue, cpu))) {
+		cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
+		cfd_seq_store(seq->ping, this_cpu, cpu, CFD_SEQ_PING);
+		send_call_function_single_ipi(cpu);
+		cfd_seq_store(seq->pinged, this_cpu, cpu, CFD_SEQ_PINGED);
+	} else {
+		cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
+	}
+}
 #else
+#define cfd_seq_store(var, src, dst, type)
+
 static void csd_lock_record(call_single_data_t *csd)
 {
 }
@@ -290,6 +468,19 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
 
 void __smp_call_single_queue(int cpu, struct llist_node *node)
 {
+#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
+	if (static_branch_unlikely(&csdlock_debug_extended)) {
+		unsigned int type;
+
+		type = CSD_TYPE(container_of(node, call_single_data_t,
+					     node.llist));
+		if (type == CSD_TYPE_SYNC || type == CSD_TYPE_ASYNC) {
+			__smp_call_single_queue_debug(cpu, node);
+			return;
+		}
+	}
+#endif
+
 	/*
 	 * The list addition should be visible before sending the IPI
 	 * handler locks the list to pull the entry off it because of
@@ -348,6 +539,8 @@ static int generic_exec_single(int cpu, call_single_data_t *csd)
  */
 void generic_smp_call_function_single_interrupt(void)
 {
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->gotipi, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_GOTIPI);
 	flush_smp_call_function_queue(true);
 }
 
@@ -375,7 +568,13 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 	lockdep_assert_irqs_disabled();
 
 	head = this_cpu_ptr(&call_single_queue);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->handle, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_HANDLE);
 	entry = llist_del_all(head);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->dequeue,
+		      /* Special meaning of source cpu: 0 == queue empty */
+		      entry ? CFD_SEQ_NOCPU : 0,
+		      smp_processor_id(), CFD_SEQ_DEQUEUE);
 	entry = llist_reverse_order(entry);
 
 	/* There shouldn't be any pending callbacks on an offline CPU. */
@@ -434,8 +633,12 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 		}
 	}
 
-	if (!entry)
+	if (!entry) {
+		cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->hdlend,
+			      0, smp_processor_id(),
+			      CFD_SEQ_HDLEND);
 		return;
+	}
 
 	/*
 	 * Second; run all !SYNC callbacks.
@@ -473,6 +676,9 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)
 	 */
 	if (entry)
 		sched_ttwu_pending(entry);
+
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->hdlend, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_HDLEND);
 }
 
 void flush_smp_call_function_from_idle(void)
@@ -482,6 +688,8 @@ void flush_smp_call_function_from_idle(void)
 	if (llist_empty(this_cpu_ptr(&call_single_queue)))
 		return;
 
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->idle, CFD_SEQ_NOCPU,
+		      smp_processor_id(), CFD_SEQ_IDLE);
 	local_irq_save(flags);
 	flush_smp_call_function_queue(true);
 	if (local_softirq_pending())
@@ -698,7 +906,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 
 	cpumask_clear(cfd->cpumask_ipi);
 	for_each_cpu(cpu, cfd->cpumask) {
-		call_single_data_t *csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
+		struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
+		call_single_data_t *csd = &pcpu->csd;
 
 		if (cond_func && !cond_func(cpu, info))
 			continue;
@@ -712,12 +921,21 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		csd->node.src = smp_processor_id();
 		csd->node.dst = cpu;
 #endif
-		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu)))
+		cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
+		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
 			__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
+			cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
+		} else {
+			cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
+		}
 	}
 
 	/* Send a message to all CPUs in the map */
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->ping, this_cpu,
+		      CFD_SEQ_NOCPU, CFD_SEQ_PING);
 	arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
+	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->pinged, this_cpu,
+		      CFD_SEQ_NOCPU, CFD_SEQ_PINGED);
 
 	if (wait) {
 		for_each_cpu(cpu, cfd->cpumask) {

[tip: locking/core] locking/csd_lock: Prepare more CSD lock debugging

[tip-bot2 for Juergen Gross]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     de7b09ef658d637eed0584eaba30884e409aef31
Gitweb:        https://git.kernel.org/tip/de7b09ef658d637eed0584eaba30884e409aef31
Author:        Juergen Gross <jgross@suse.com>
AuthorDate:    Mon, 01 Mar 2021 11:13:35 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 06 Mar 2021 12:49:48 +01:00

locking/csd_lock: Prepare more CSD lock debugging

In order to be able to easily add more CSD lock debugging data to
struct call_function_data->csd move the call_single_data_t element
into a sub-structure.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-3-jgross@suse.com
---
 kernel/smp.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index d5f0b21..6d7e6db 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -31,8 +31,12 @@
 
 #define CSD_TYPE(_csd)	((_csd)->node.u_flags & CSD_FLAG_TYPE_MASK)
 
+struct cfd_percpu {
+	call_single_data_t	csd;
+};
+
 struct call_function_data {
-	call_single_data_t	__percpu *csd;
+	struct cfd_percpu	__percpu *pcpu;
 	cpumask_var_t		cpumask;
 	cpumask_var_t		cpumask_ipi;
 };
@@ -55,8 +59,8 @@ int smpcfd_prepare_cpu(unsigned int cpu)
 		free_cpumask_var(cfd->cpumask);
 		return -ENOMEM;
 	}
-	cfd->csd = alloc_percpu(call_single_data_t);
-	if (!cfd->csd) {
+	cfd->pcpu = alloc_percpu(struct cfd_percpu);
+	if (!cfd->pcpu) {
 		free_cpumask_var(cfd->cpumask);
 		free_cpumask_var(cfd->cpumask_ipi);
 		return -ENOMEM;
@@ -71,7 +75,7 @@ int smpcfd_dead_cpu(unsigned int cpu)
 
 	free_cpumask_var(cfd->cpumask);
 	free_cpumask_var(cfd->cpumask_ipi);
-	free_percpu(cfd->csd);
+	free_percpu(cfd->pcpu);
 	return 0;
 }
 
@@ -694,7 +698,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 
 	cpumask_clear(cfd->cpumask_ipi);
 	for_each_cpu(cpu, cfd->cpumask) {
-		call_single_data_t *csd = per_cpu_ptr(cfd->csd, cpu);
+		call_single_data_t *csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
 
 		if (cond_func && !cond_func(cpu, info))
 			continue;
@@ -719,7 +723,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		for_each_cpu(cpu, cfd->cpumask) {
 			call_single_data_t *csd;
 
-			csd = per_cpu_ptr(cfd->csd, cpu);
+			csd = &per_cpu_ptr(cfd->pcpu, cpu)->csd;
 			csd_lock_wait(csd);
 		}
 	}