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Date: Mon, 9 Jul 2018 15:01:54 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: tglx@...utronix.de, Clark Williams <williams@...hat.com>,
linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: cgroup trace events acquire sleeping locks
On Mon, 9 Jul 2018 18:38:05 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> Clark showed me this:
>
> | BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:974
> | in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: systemd
> | 5 locks held by systemd/1:
> | #0: (sb_writers#7){.+.+}, at: [< (ptrval)>] mnt_want_write+0x1f/0x50
> | #1: (&type->i_mutex_dir_key#3/1){+.+.}, at: [< (ptrval)>] do_rmdir+0x14d/0x1f0
> | #2: (&type->i_mutex_dir_key#5){++++}, at: [< (ptrval)>] vfs_rmdir+0x50/0x150
> | #3: (cgroup_mutex){+.+.}, at: [< (ptrval)>] cgroup_kn_lock_live+0xfb/0x200
> | #4: (kernfs_rename_lock){+.+.}, at: [< (ptrval)>] kernfs_path_from_node+0x23/0x50
> | Preemption disabled at:
> | [<ffffffff8dc994c5>] migrate_enable+0x95/0x340
> | CPU: 1 PID: 1 Comm: systemd Not tainted 4.16.18-rt9+ #173
> | Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.11.1-1 04/01/2014
> | Call Trace:
> | dump_stack+0x70/0xa7
> | ___might_sleep+0x159/0x240
> | rt_spin_lock+0x4e/0x60
> | ? kernfs_path_from_node+0x23/0x50
> | kernfs_path_from_node+0x23/0x50
> | trace_event_raw_event_cgroup+0x54/0x110
> | cgroup_rmdir+0xdb/0x1a0
> | kernfs_iop_rmdir+0x53/0x80
> | vfs_rmdir+0x74/0x150
> | do_rmdir+0x191/0x1f0
> | SyS_rmdir+0x11/0x20
> | do_syscall_64+0x73/0x220
> | entry_SYSCALL_64_after_hwframe+0x42/0xb7
>
> which is the trace_cgroup_rmdir() trace event in cgroup_rmdir(). The
> trace event invokes cgroup_path() which acquires a spin_lock_t and this
> is invoked within a preempt_disable()ed section.
Correct. And I wish no trace event took spin locks.
>
> It says "Preemption disabled at" migrate_enable() but this is not true.
> A printk() just before the lock reports preempt_count() of two and
> sometimes one. I *think*
> - one is from rcu_read_lock_sched_notrace() in __DO_TRACE()
> - the second is from preempt_disable_notrace() in ring_buffer_lock_reserve()
>
> I would prefer not to turn kernfs_rename_lock into raw_spin_lock_t. We
> had a similar problem with a i915 trace event which eventually vanished
> (and before I just disabled it).
>
> So how likely are chances that we can use rcu_read_lock() in __DO_TRACE()?
Not very.
> And how likely are chances that the preempt_disable() in
> ring_buffer_lock_reserve() could be avoided while the trace event is
> invoked?
Even less likely. The design of the ring buffer is based on not being
able to be preempted.
>
> I guess nothing of this is easy peasy. Any suggestions?
>
One solution, albeit not so pretty, is to move the grabbing of the
path, outside the trace event. But this should work.
-- Steve
diff --git a/include/trace/events/cgroup.h b/include/trace/events/cgroup.h
index d74722c2ac8b..d149f3a42122 100644
--- a/include/trace/events/cgroup.h
+++ b/include/trace/events/cgroup.h
@@ -53,24 +53,22 @@ DEFINE_EVENT(cgroup_root, cgroup_remount,
DECLARE_EVENT_CLASS(cgroup,
- TP_PROTO(struct cgroup *cgrp),
+ TP_PROTO(struct cgroup *cgrp, const char *path),
- TP_ARGS(cgrp),
+ TP_ARGS(cgrp, path),
TP_STRUCT__entry(
__field( int, root )
__field( int, id )
__field( int, level )
- __dynamic_array(char, path,
- cgroup_path(cgrp, NULL, 0) + 1)
+ __string( path, path )
),
TP_fast_assign(
__entry->root = cgrp->root->hierarchy_id;
__entry->id = cgrp->id;
__entry->level = cgrp->level;
- cgroup_path(cgrp, __get_dynamic_array(path),
- __get_dynamic_array_len(path));
+ __assign_str(path, path);
),
TP_printk("root=%d id=%d level=%d path=%s",
@@ -79,30 +77,30 @@ DECLARE_EVENT_CLASS(cgroup,
DEFINE_EVENT(cgroup, cgroup_mkdir,
- TP_PROTO(struct cgroup *cgroup),
+ TP_PROTO(struct cgroup *cgrp, const char *path),
- TP_ARGS(cgroup)
+ TP_ARGS(cgrp, path)
);
DEFINE_EVENT(cgroup, cgroup_rmdir,
- TP_PROTO(struct cgroup *cgroup),
+ TP_PROTO(struct cgroup *cgrp, const char *path),
- TP_ARGS(cgroup)
+ TP_ARGS(cgrp, path)
);
DEFINE_EVENT(cgroup, cgroup_release,
- TP_PROTO(struct cgroup *cgroup),
+ TP_PROTO(struct cgroup *cgrp, const char *path),
- TP_ARGS(cgroup)
+ TP_ARGS(cgrp, path)
);
DEFINE_EVENT(cgroup, cgroup_rename,
- TP_PROTO(struct cgroup *cgroup),
+ TP_PROTO(struct cgroup *cgrp, const char *path),
- TP_ARGS(cgroup)
+ TP_ARGS(cgrp, path)
);
DECLARE_EVENT_CLASS(cgroup_migrate,
diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 77ff1cd6a252..d0ad35796ea1 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -8,6 +8,21 @@
#include <linux/list.h>
#include <linux/refcount.h>
+#define TRACE_CGROUP_PATH_LEN 1024
+extern spinlock_t trace_cgroup_path_lock;
+extern char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
+
+#define TRACE_CGROUP_PATH(cgrp, type) \
+ do { \
+ if (trace_cgroup_##type##_enabled()) { \
+ spin_lock(&trace_cgroup_path_lock); \
+ cgroup_path(cgrp, trace_cgroup_path, \
+ TRACE_CGROUP_PATH_LEN); \
+ trace_cgroup_##type(cgrp, trace_cgroup_path); \
+ spin_unlock(&trace_cgroup_path_lock); \
+ } \
+ } while (0)
+
/*
* A cgroup can be associated with multiple css_sets as different tasks may
* belong to different cgroups on different hierarchies. In the other
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 8b4f0768efd6..4f87670c8e4b 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -865,7 +865,7 @@ static int cgroup1_rename(struct kernfs_node *kn, struct kernfs_node *new_parent
ret = kernfs_rename(kn, new_parent, new_name_str);
if (!ret)
- trace_cgroup_rename(cgrp);
+ TRACE_CGROUP_PATH(cgrp, rename);
mutex_unlock(&cgroup_mutex);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 077370bf8964..a3be1b46693c 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -83,6 +83,9 @@ EXPORT_SYMBOL_GPL(cgroup_mutex);
EXPORT_SYMBOL_GPL(css_set_lock);
#endif
+DEFINE_SPINLOCK(trace_cgroup_path_lock);
+char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
+
/*
* Protects cgroup_idr and css_idr so that IDs can be released without
* grabbing cgroup_mutex.
@@ -4634,7 +4637,7 @@ static void css_release_work_fn(struct work_struct *work)
struct cgroup *tcgrp;
/* cgroup release path */
- trace_cgroup_release(cgrp);
+ TRACE_CGROUP_PATH(cgrp, release);
if (cgroup_on_dfl(cgrp))
cgroup_rstat_flush(cgrp);
@@ -4977,7 +4980,7 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
if (ret)
goto out_destroy;
- trace_cgroup_mkdir(cgrp);
+ TRACE_CGROUP_PATH(cgrp, mkdir);
/* let's create and online css's */
kernfs_activate(kn);
@@ -5165,9 +5168,8 @@ int cgroup_rmdir(struct kernfs_node *kn)
return 0;
ret = cgroup_destroy_locked(cgrp);
-
if (!ret)
- trace_cgroup_rmdir(cgrp);
+ TRACE_CGROUP_PATH(cgrp, rmdir);
cgroup_kn_unlock(kn);
return ret;
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