The handling of sysrq key can be activated by echoing the key to
/proc/sysrq-trigger or via the magic key sequence typed into a terminal
that is connected to the system in some way (serial, USB or other mean).
In the former case, the handling is done in a user context. In the
latter case, it is likely to be in an interrupt context.
There should be no more than one instance of sysrq key processing via
a terminal, but multiple instances of /proc/sysrq-trigger is possible.
Currently in print_cpu() of kernel/sched/debug.c, sched_debug_lock is
taken with interrupt disabled for the whole duration of the calls to
print_*_stats() and print_rq() which could last for the quite some time
if the information dump happens on the serial console.
If the system has many cpus and the sched_debug_lock is somehow busy
(e.g. parallel sysrq-t), the system may hit a hard lockup panic
depending on the actually serial console implementation of the
system. For instance,
[ 7809.796262] Kernel panic - not syncing: Hard LOCKUP
[ 7809.796264] CPU: 13 PID: 79867 Comm: reproducer.sh Kdump: loaded Tainted: G I --------- - - 4.18.0-301.el8.x86_64 #1
[ 7809.796264] Hardware name: Dell Inc. PowerEdge R640/0W23H8, BIOS 1.4.9 06/29/2018
[ 7809.796265] Call Trace:
[ 7809.796265] <NMI>
[ 7809.796266] dump_stack+0x5c/0x80
[ 7809.796266] panic+0xe7/0x2a9
[ 7809.796267] nmi_panic.cold.9+0xc/0xc
[ 7809.796267] watchdog_overflow_callback.cold.7+0x5c/0x70
[ 7809.796268] __perf_event_overflow+0x52/0xf0
[ 7809.796268] handle_pmi_common+0x204/0x2a0
[ 7809.796269] ? __set_pte_vaddr+0x32/0x50
[ 7809.796269] ? __native_set_fixmap+0x24/0x30
[ 7809.796270] ? ghes_copy_tofrom_phys+0xd3/0x1c0
[ 7809.796271] intel_pmu_handle_irq+0xbf/0x160
[ 7809.796271] perf_event_nmi_handler+0x2d/0x50
[ 7809.796272] nmi_handle+0x63/0x110
[ 7809.796272] default_do_nmi+0x49/0x100
[ 7809.796273] do_nmi+0x17e/0x1e0
[ 7809.796273] end_repeat_nmi+0x16/0x6f
[ 7809.796274] RIP: 0010:native_queued_spin_lock_slowpath+0x5b/0x1d0
[ 7809.796275] Code: 6d f0 0f ba 2f 08 0f 92 c0 0f b6 c0 c1 e0 08 89 c2 8b 07 30 e4 09 d0 a9 00 01 ff ff 75 47 85 c0 74 0e 8b 07 84 c0 74 08 f3 90 <8b> 07 84 c0 75 f8 b8 01 00 00 00 66 89 07 c3 8b 37 81 fe 00 01 00
[ 7809.796276] RSP: 0018:ffffaa54cd887df8 EFLAGS: 00000002
[ 7809.796277] RAX: 0000000000000101 RBX: 0000000000000246 RCX: 0000000000000000
[ 7809.796278] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff936b66d0
[ 7809.796278] RBP: ffffffff9301fb40 R08: 0000000000000004 R09: 000000000000004f
[ 7809.796279] R10: 0000000000000000 R11: ffffaa54cd887cc0 R12: ffff907fd0a29ec0
[ 7809.796280] R13: 0000000000000000 R14: ffffffff926ab7c0 R15: 0000000000000000
[ 7809.796280] ? native_queued_spin_lock_slowpath+0x5b/0x1d0
[ 7809.796281] ? native_queued_spin_lock_slowpath+0x5b/0x1d0
[ 7809.796281] </NMI>
[ 7809.796282] _raw_spin_lock_irqsave+0x32/0x40
[ 7809.796283] print_cpu+0x261/0x7c0
[ 7809.796283] sysrq_sched_debug_show+0x34/0x50
[ 7809.796284] sysrq_handle_showstate+0xc/0x20
[ 7809.796284] __handle_sysrq.cold.11+0x48/0xfb
[ 7809.796285] write_sysrq_trigger+0x2b/0x30
[ 7809.796285] proc_reg_write+0x39/0x60
[ 7809.796286] vfs_write+0xa5/0x1a0
[ 7809.796286] ksys_write+0x4f/0xb0
[ 7809.796287] do_syscall_64+0x5b/0x1a0
[ 7809.796287] entry_SYSCALL_64_after_hwframe+0x65/0xca
[ 7809.796288] RIP: 0033:0x7fabe4ceb648
The purpose of sched_debug_lock is to serialize the use of the global
cgroup_path[] buffer in print_cpu(). The rests of the printk calls don't
need serialization from sched_debug_lock.
Calling printk() with interrupt disabled can still be problematic if
multiple instances are running. Allocating a stack buffer of PATH_MAX
bytes is not feasible because of the limited size of the kernel stack.
The print_cpu() function has two callers - sched_debug_show() and
sysrq_sched_debug_show(). The solution implemented in this patch is to
allow all sched_debug_show() callers to contend for sched_debug_lock and
use the full size group_path[] as their SEQ_printf() calls will be much
faster. However only one sysrq_sched_debug_show() caller that output to
the slow console will be allowed to use group_path[]. Another parallel
console writer will have to use a shorter stack buffer instead. Since
the console output will be garbled anyway, truncation of some cgroup
paths shouldn't be a big issue.
Fixes: efe25c2c7b3a ("sched: Reinstate group names in /proc/sched_debug")
Signed-off-by: Waiman Long <[email protected]>
---
kernel/sched/debug.c | 54 +++++++++++++++++++++++++++++++++-----------
1 file changed, 41 insertions(+), 13 deletions(-)
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 486f403a778b..5d021b247998 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -8,8 +8,6 @@
*/
#include "sched.h"
-static DEFINE_SPINLOCK(sched_debug_lock);
-
/*
* This allows printing both to /proc/sched_debug and
* to the console
@@ -470,16 +468,49 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
#endif
#ifdef CONFIG_CGROUP_SCHED
+static DEFINE_SPINLOCK(sched_debug_lock);
static char group_path[PATH_MAX];
+static enum {
+ TOKEN_NONE,
+ TOKEN_ACQUIRED,
+ TOKEN_NA /* Not applicable */
+} console_token = TOKEN_ACQUIRED;
-static char *task_group_path(struct task_group *tg)
+static void task_group_path(struct task_group *tg, char *path, int plen)
{
- if (autogroup_path(tg, group_path, PATH_MAX))
- return group_path;
+ if (autogroup_path(tg, path, plen))
+ return;
- cgroup_path(tg->css.cgroup, group_path, PATH_MAX);
+ cgroup_path(tg->css.cgroup, path, plen);
+}
- return group_path;
+/*
+ * All the print_cpu() callers from sched_debug_show() will be allowed
+ * to contend for sched_debug_lock and use group_path[] as their SEQ_printf()
+ * calls will be much faster. However only one print_cpu() caller from
+ * sysrq_sched_debug_show() which outputs to the console will be allowed
+ * to use group_path[]. Another parallel console writer will have to use
+ * a shorter stack buffer instead. Since the console output will be garbled
+ * anyway, truncation of some cgroup paths shouldn't be a big issue.
+ */
+#define SEQ_printf_task_group_path(m, tg, fmt...) \
+{ \
+ unsigned long flags; \
+ int token = m ? TOKEN_NA \
+ : xchg_acquire(&console_token, TOKEN_NONE); \
+ \
+ if (token == TOKEN_NONE) { \
+ char buf[128]; \
+ task_group_path(tg, buf, sizeof(buf)); \
+ SEQ_printf(m, fmt, buf); \
+ } else { \
+ spin_lock_irqsave(&sched_debug_lock, flags); \
+ task_group_path(tg, group_path, sizeof(group_path)); \
+ SEQ_printf(m, fmt, group_path); \
+ spin_unlock_irqrestore(&sched_debug_lock, flags); \
+ if (token == TOKEN_ACQUIRED) \
+ smp_store_release(&console_token, token); \
+ } \
}
#endif
@@ -506,7 +537,7 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
SEQ_printf(m, " %d %d", task_node(p), task_numa_group_id(p));
#endif
#ifdef CONFIG_CGROUP_SCHED
- SEQ_printf(m, " %s", task_group_path(task_group(p)));
+ SEQ_printf_task_group_path(m, task_group(p), " %s")
#endif
SEQ_printf(m, "\n");
@@ -543,7 +574,7 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
#ifdef CONFIG_FAIR_GROUP_SCHED
SEQ_printf(m, "\n");
- SEQ_printf(m, "cfs_rq[%d]:%s\n", cpu, task_group_path(cfs_rq->tg));
+ SEQ_printf_task_group_path(m, cfs_rq->tg, "cfs_rq[%d]:%s\n", cpu);
#else
SEQ_printf(m, "\n");
SEQ_printf(m, "cfs_rq[%d]:\n", cpu);
@@ -614,7 +645,7 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
{
#ifdef CONFIG_RT_GROUP_SCHED
SEQ_printf(m, "\n");
- SEQ_printf(m, "rt_rq[%d]:%s\n", cpu, task_group_path(rt_rq->tg));
+ SEQ_printf_task_group_path(m, rt_rq->tg, "rt_rq[%d]:%s\n", cpu);
#else
SEQ_printf(m, "\n");
SEQ_printf(m, "rt_rq[%d]:\n", cpu);
@@ -666,7 +697,6 @@ void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq)
static void print_cpu(struct seq_file *m, int cpu)
{
struct rq *rq = cpu_rq(cpu);
- unsigned long flags;
#ifdef CONFIG_X86
{
@@ -717,13 +747,11 @@ do { \
}
#undef P
- spin_lock_irqsave(&sched_debug_lock, flags);
print_cfs_stats(m, cpu);
print_rt_stats(m, cpu);
print_dl_stats(m, cpu);
print_rq(m, rq, cpu);
- spin_unlock_irqrestore(&sched_debug_lock, flags);
SEQ_printf(m, "\n");
}
--
2.18.1
On Mon, 5 Apr 2021 19:42:03 -0400
Waiman Long <[email protected]> wrote:
> +/*
> + * All the print_cpu() callers from sched_debug_show() will be allowed
> + * to contend for sched_debug_lock and use group_path[] as their SEQ_printf()
> + * calls will be much faster. However only one print_cpu() caller from
> + * sysrq_sched_debug_show() which outputs to the console will be allowed
> + * to use group_path[]. Another parallel console writer will have to use
> + * a shorter stack buffer instead. Since the console output will be garbled
> + * anyway, truncation of some cgroup paths shouldn't be a big issue.
> + */
> +#define SEQ_printf_task_group_path(m, tg, fmt...) \
> +{ \
> + unsigned long flags; \
> + int token = m ? TOKEN_NA \
> + : xchg_acquire(&console_token, TOKEN_NONE); \
> + \
> + if (token == TOKEN_NONE) { \
> + char buf[128]; \
> + task_group_path(tg, buf, sizeof(buf)); \
> + SEQ_printf(m, fmt, buf); \
> + } else { \
> + spin_lock_irqsave(&sched_debug_lock, flags); \
> + task_group_path(tg, group_path, sizeof(group_path)); \
> + SEQ_printf(m, fmt, group_path); \
> + spin_unlock_irqrestore(&sched_debug_lock, flags); \
> + if (token == TOKEN_ACQUIRED) \
> + smp_store_release(&console_token, token); \
> + } \
> }
> #endif
And you said my suggestion was complex!
I'll let others review this.
-- Steve
On 4/5/21 8:18 PM, Steven Rostedt wrote:
> On Mon, 5 Apr 2021 19:42:03 -0400
> Waiman Long <[email protected]> wrote:
>
>> +/*
>> + * All the print_cpu() callers from sched_debug_show() will be allowed
>> + * to contend for sched_debug_lock and use group_path[] as their SEQ_printf()
>> + * calls will be much faster. However only one print_cpu() caller from
>> + * sysrq_sched_debug_show() which outputs to the console will be allowed
>> + * to use group_path[]. Another parallel console writer will have to use
>> + * a shorter stack buffer instead. Since the console output will be garbled
>> + * anyway, truncation of some cgroup paths shouldn't be a big issue.
>> + */
>> +#define SEQ_printf_task_group_path(m, tg, fmt...) \
>> +{ \
>> + unsigned long flags; \
>> + int token = m ? TOKEN_NA \
>> + : xchg_acquire(&console_token, TOKEN_NONE); \
>> + \
>> + if (token == TOKEN_NONE) { \
>> + char buf[128]; \
>> + task_group_path(tg, buf, sizeof(buf)); \
>> + SEQ_printf(m, fmt, buf); \
>> + } else { \
>> + spin_lock_irqsave(&sched_debug_lock, flags); \
>> + task_group_path(tg, group_path, sizeof(group_path)); \
>> + SEQ_printf(m, fmt, group_path); \
>> + spin_unlock_irqrestore(&sched_debug_lock, flags); \
>> + if (token == TOKEN_ACQUIRED) \
>> + smp_store_release(&console_token, token); \
>> + } \
>> }
>> #endif
> And you said my suggestion was complex!
>
> I'll let others review this.
>
This patch is actually inspired by your suggestion, though it is
structured differently from your approach. I really want to thank you
for your valuable feedback.
I realized that printing to a sequence file wasn't really a problem,
only printing to console can be problematic. That is why I decided to
allow unlimited use of group_path[] for those users and only one console
writer is allowed to use it. As for calling touch_nmi_watchdog(), I am
still thinking where will be the right place to do it, but it can be
done with a separate patch, if needed.
Cheers,
Longman
On Mon, Apr 05, 2021 at 07:42:03PM -0400, Waiman Long wrote:
> The handling of sysrq key can be activated by echoing the key to
> /proc/sysrq-trigger or via the magic key sequence typed into a terminal
> that is connected to the system in some way (serial, USB or other mean).
> In the former case, the handling is done in a user context. In the
> latter case, it is likely to be in an interrupt context.
> [ 7809.796281] </NMI>
> [ 7809.796282] _raw_spin_lock_irqsave+0x32/0x40
> [ 7809.796283] print_cpu+0x261/0x7c0
> [ 7809.796283] sysrq_sched_debug_show+0x34/0x50
> [ 7809.796284] sysrq_handle_showstate+0xc/0x20
> [ 7809.796284] __handle_sysrq.cold.11+0x48/0xfb
> [ 7809.796285] write_sysrq_trigger+0x2b/0x30
> [ 7809.796285] proc_reg_write+0x39/0x60
> [ 7809.796286] vfs_write+0xa5/0x1a0
> [ 7809.796286] ksys_write+0x4f/0xb0
> [ 7809.796287] do_syscall_64+0x5b/0x1a0
> [ 7809.796287] entry_SYSCALL_64_after_hwframe+0x65/0xca
> [ 7809.796288] RIP: 0033:0x7fabe4ceb648
>
> The purpose of sched_debug_lock is to serialize the use of the global
> cgroup_path[] buffer in print_cpu(). The rests of the printk calls don't
> need serialization from sched_debug_lock.
> The print_cpu() function has two callers - sched_debug_show() and
> sysrq_sched_debug_show().
So what idiot is doing sysrq and that proc file at the same time? Why is
it a problem now?
> @@ -470,16 +468,49 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
> #endif
>
> #ifdef CONFIG_CGROUP_SCHED
> +static DEFINE_SPINLOCK(sched_debug_lock);
> static char group_path[PATH_MAX];
> +static enum {
> + TOKEN_NONE,
> + TOKEN_ACQUIRED,
> + TOKEN_NA /* Not applicable */
> +} console_token = TOKEN_ACQUIRED;
> +/*
> + * All the print_cpu() callers from sched_debug_show() will be allowed
> + * to contend for sched_debug_lock and use group_path[] as their SEQ_printf()
> + * calls will be much faster. However only one print_cpu() caller from
> + * sysrq_sched_debug_show() which outputs to the console will be allowed
> + * to use group_path[]. Another parallel console writer will have to use
> + * a shorter stack buffer instead. Since the console output will be garbled
> + * anyway, truncation of some cgroup paths shouldn't be a big issue.
> + */
> +#define SEQ_printf_task_group_path(m, tg, fmt...) \
> +{ \
> + unsigned long flags; \
> + int token = m ? TOKEN_NA \
> + : xchg_acquire(&console_token, TOKEN_NONE); \
> + \
> + if (token == TOKEN_NONE) { \
> + char buf[128]; \
> + task_group_path(tg, buf, sizeof(buf)); \
> + SEQ_printf(m, fmt, buf); \
> + } else { \
> + spin_lock_irqsave(&sched_debug_lock, flags); \
> + task_group_path(tg, group_path, sizeof(group_path)); \
> + SEQ_printf(m, fmt, group_path); \
> + spin_unlock_irqrestore(&sched_debug_lock, flags); \
> + if (token == TOKEN_ACQUIRED) \
> + smp_store_release(&console_token, token); \
> + } \
> }
This is disgusting... you have an open-coded test-and-set lock like
thing *AND* a spinlock, what gives?
What's wrong with something simple like this?
---
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 4b49cc2af5c4..2ac2977f3b96 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -8,8 +8,6 @@
*/
#include "sched.h"
-static DEFINE_SPINLOCK(sched_debug_lock);
-
/*
* This allows printing both to /proc/sched_debug and
* to the console
@@ -470,6 +468,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
#endif
#ifdef CONFIG_CGROUP_SCHED
+static DEFINE_SPINLOCK(group_path_lock);
static char group_path[PATH_MAX];
static char *task_group_path(struct task_group *tg)
@@ -481,6 +480,22 @@ static char *task_group_path(struct task_group *tg)
return group_path;
}
+
+#define SEQ_printf_task_group_path(m, tg) \
+do { \
+ if (spin_trylock(&group_path_lock)) { \
+ task_group_path(tg, group_path, sizeof(group_path)); \
+ SEQ_printf(m, "%s", group_path); \
+ spin_unlock(&group_path_lock); \
+ } else { \
+ SEQ_printf(m, "looser!"); \
+ }
+} while (0)
+
+#else
+
+#define SEQ_printf_task_group_path(m, tg) do { } while (0)
+
#endif
static void
@@ -505,9 +520,8 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
#ifdef CONFIG_NUMA_BALANCING
SEQ_printf(m, " %d %d", task_node(p), task_numa_group_id(p));
#endif
-#ifdef CONFIG_CGROUP_SCHED
- SEQ_printf(m, " %s", task_group_path(task_group(p)));
-#endif
+ SEQ_printf(m, " ");
+ SEQ_printf_task_group_path(m, task_group(p));
SEQ_printf(m, "\n");
}
@@ -541,13 +555,10 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
struct sched_entity *last;
unsigned long flags;
-#ifdef CONFIG_FAIR_GROUP_SCHED
SEQ_printf(m, "\n");
- SEQ_printf(m, "cfs_rq[%d]:%s\n", cpu, task_group_path(cfs_rq->tg));
-#else
+ SEQ_printf(m, "cfs_rq[%d]:", cpu);
+ SEQ_printf_task_group_path(m, cfs_rq->tg);
SEQ_printf(m, "\n");
- SEQ_printf(m, "cfs_rq[%d]:\n", cpu);
-#endif
SEQ_printf(m, " .%-30s: %Ld.%06ld\n", "exec_clock",
SPLIT_NS(cfs_rq->exec_clock));
@@ -612,13 +623,10 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
{
-#ifdef CONFIG_RT_GROUP_SCHED
SEQ_printf(m, "\n");
- SEQ_printf(m, "rt_rq[%d]:%s\n", cpu, task_group_path(rt_rq->tg));
-#else
+ SEQ_printf(m, "rt_rq[%d]:", cpu);
+ SEQ_printf_task_group_path(m, rt_rq->tg);
SEQ_printf(m, "\n");
- SEQ_printf(m, "rt_rq[%d]:\n", cpu);
-#endif
#define P(x) \
SEQ_printf(m, " .%-30s: %Ld\n", #x, (long long)(rt_rq->x))
@@ -666,7 +674,6 @@ void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq)
static void print_cpu(struct seq_file *m, int cpu)
{
struct rq *rq = cpu_rq(cpu);
- unsigned long flags;
#ifdef CONFIG_X86
{
@@ -717,13 +724,11 @@ do { \
}
#undef P
- spin_lock_irqsave(&sched_debug_lock, flags);
print_cfs_stats(m, cpu);
print_rt_stats(m, cpu);
print_dl_stats(m, cpu);
print_rq(m, rq, cpu);
- spin_unlock_irqrestore(&sched_debug_lock, flags);
SEQ_printf(m, "\n");
}
On 4/6/21 5:15 AM, Peter Zijlstra wrote:
> On Mon, Apr 05, 2021 at 07:42:03PM -0400, Waiman Long wrote:
>> The handling of sysrq key can be activated by echoing the key to
>> /proc/sysrq-trigger or via the magic key sequence typed into a terminal
>> that is connected to the system in some way (serial, USB or other mean).
>> In the former case, the handling is done in a user context. In the
>> latter case, it is likely to be in an interrupt context.
>> [ 7809.796281] </NMI>
>> [ 7809.796282] _raw_spin_lock_irqsave+0x32/0x40
>> [ 7809.796283] print_cpu+0x261/0x7c0
>> [ 7809.796283] sysrq_sched_debug_show+0x34/0x50
>> [ 7809.796284] sysrq_handle_showstate+0xc/0x20
>> [ 7809.796284] __handle_sysrq.cold.11+0x48/0xfb
>> [ 7809.796285] write_sysrq_trigger+0x2b/0x30
>> [ 7809.796285] proc_reg_write+0x39/0x60
>> [ 7809.796286] vfs_write+0xa5/0x1a0
>> [ 7809.796286] ksys_write+0x4f/0xb0
>> [ 7809.796287] do_syscall_64+0x5b/0x1a0
>> [ 7809.796287] entry_SYSCALL_64_after_hwframe+0x65/0xca
>> [ 7809.796288] RIP: 0033:0x7fabe4ceb648
>>
>> The purpose of sched_debug_lock is to serialize the use of the global
>> cgroup_path[] buffer in print_cpu(). The rests of the printk calls don't
>> need serialization from sched_debug_lock.
>> The print_cpu() function has two callers - sched_debug_show() and
>> sysrq_sched_debug_show().
> So what idiot is doing sysrq and that proc file at the same time? Why is
> it a problem now?
We got a customer bug report on watchdog panic because a process somehow
stay within the sched_debug_lock for too long. I don't know what exactly
the customer was doing, but we can reproduce the panic just by having 2
parallel "echo t > /proc/sysrq-trigger" commands. This happens on
certain selected systems. I was using some Dell systems for my testing.
Of course, it is not sensible to have more than one sysrq happening at
the same time. However, a parallel thread accessing /proc/sched_debug is
certainly possible. That invokes similar code path.
>
>> @@ -470,16 +468,49 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
>> #endif
>>
>> #ifdef CONFIG_CGROUP_SCHED
>> +static DEFINE_SPINLOCK(sched_debug_lock);
>> static char group_path[PATH_MAX];
>> +static enum {
>> + TOKEN_NONE,
>> + TOKEN_ACQUIRED,
>> + TOKEN_NA /* Not applicable */
>> +} console_token = TOKEN_ACQUIRED;
>> +/*
>> + * All the print_cpu() callers from sched_debug_show() will be allowed
>> + * to contend for sched_debug_lock and use group_path[] as their SEQ_printf()
>> + * calls will be much faster. However only one print_cpu() caller from
>> + * sysrq_sched_debug_show() which outputs to the console will be allowed
>> + * to use group_path[]. Another parallel console writer will have to use
>> + * a shorter stack buffer instead. Since the console output will be garbled
>> + * anyway, truncation of some cgroup paths shouldn't be a big issue.
>> + */
>> +#define SEQ_printf_task_group_path(m, tg, fmt...) \
>> +{ \
>> + unsigned long flags; \
>> + int token = m ? TOKEN_NA \
>> + : xchg_acquire(&console_token, TOKEN_NONE); \
>> + \
>> + if (token == TOKEN_NONE) { \
>> + char buf[128]; \
>> + task_group_path(tg, buf, sizeof(buf)); \
>> + SEQ_printf(m, fmt, buf); \
>> + } else { \
>> + spin_lock_irqsave(&sched_debug_lock, flags); \
>> + task_group_path(tg, group_path, sizeof(group_path)); \
>> + SEQ_printf(m, fmt, group_path); \
>> + spin_unlock_irqrestore(&sched_debug_lock, flags); \
>> + if (token == TOKEN_ACQUIRED) \
>> + smp_store_release(&console_token, token); \
>> + } \
>> }
> This is disgusting... you have an open-coded test-and-set lock like
> thing *AND* a spinlock, what gives?
>
>
> What's wrong with something simple like this?
>
> ---
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 4b49cc2af5c4..2ac2977f3b96 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -8,8 +8,6 @@
> */
> #include "sched.h"
>
> -static DEFINE_SPINLOCK(sched_debug_lock);
> -
> /*
> * This allows printing both to /proc/sched_debug and
> * to the console
> @@ -470,6 +468,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
> #endif
>
> #ifdef CONFIG_CGROUP_SCHED
> +static DEFINE_SPINLOCK(group_path_lock);
> static char group_path[PATH_MAX];
>
> static char *task_group_path(struct task_group *tg)
> @@ -481,6 +480,22 @@ static char *task_group_path(struct task_group *tg)
>
> return group_path;
> }
> +
> +#define SEQ_printf_task_group_path(m, tg) \
> +do { \
> + if (spin_trylock(&group_path_lock)) { \
> + task_group_path(tg, group_path, sizeof(group_path)); \
> + SEQ_printf(m, "%s", group_path); \
> + spin_unlock(&group_path_lock); \
> + } else { \
> + SEQ_printf(m, "looser!"); \
> + }
> +} while (0)
I have no problem with using this as a possible solution as long as
others agree. Of course, I won't use the term "looser". I will be more
polite:-)
The current patch allows all /proc/sched_debug users and one sysrq user
to use the full group_path[] buffer. I can certainly change it to allow
1 user to use the full size path and the rests running the risk of path
truncation.
Cheers,
Longman