From: "Steven Rostedt (VMware)" Date: Mon, 9 Jul 2018 17:48:54 -0400 Subject: [PATCH] cgroup/tracing: Move taking of spin lock out of trace event handlers Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.18/older/patches-4.18.16-rt9.tar.xz [ Upstream commit e4f8d81c738db6d3ffdabfb8329aa2feaa310699 ] It is unwise to take spin locks from the handlers of trace events. Mainly, because they can introduce lockups, because it introduces locks in places that are normally not tested. Worse yet, because trace events are tucked away in the include/trace/events/ directory, locks that are taken there are forgotten about. As a general rule, I tell people never to take any locks in a trace event handler. Several cgroup trace event handlers call cgroup_path() which eventually takes the kernfs_rename_lock spinlock. This injects the spinlock in the code without people realizing it. It also can cause issues for the PREEMPT_RT patch, as the spinlock becomes a mutex, and the trace event handlers are called with preemption disabled. By moving the calculation of the cgroup_path() out of the trace event handlers and into a macro (surrounded by a trace_cgroup_##type##_enabled()), then we could place the cgroup_path into a string, and pass that to the trace event. Not only does this remove the taking of the spinlock out of the trace event handler, but it also means that the cgroup_path() only needs to be called once (it is currently called twice, once to get the length to reserver the buffer for, and once again to get the path itself. Now it only needs to be done once. Reported-by: Sebastian Andrzej Siewior Signed-off-by: Steven Rostedt (VMware) Signed-off-by: Tejun Heo Signed-off-by: Sebastian Andrzej Siewior --- include/trace/events/cgroup.h | 47 +++++++++++++++++++--------------------- kernel/cgroup/cgroup-internal.h | 26 ++++++++++++++++++++++ kernel/cgroup/cgroup-v1.c | 4 +-- kernel/cgroup/cgroup.c | 12 +++++----- 4 files changed, 58 insertions(+), 31 deletions(-) --- 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", @@ -79,45 +77,45 @@ 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, - TP_PROTO(struct cgroup *dst_cgrp, struct task_struct *task, bool threadgroup), + TP_PROTO(struct cgroup *dst_cgrp, const char *path, + struct task_struct *task, bool threadgroup), - TP_ARGS(dst_cgrp, task, threadgroup), + TP_ARGS(dst_cgrp, path, task, threadgroup), TP_STRUCT__entry( __field( int, dst_root ) __field( int, dst_id ) __field( int, dst_level ) - __dynamic_array(char, dst_path, - cgroup_path(dst_cgrp, NULL, 0) + 1) __field( int, pid ) + __string( dst_path, path ) __string( comm, task->comm ) ), @@ -125,8 +123,7 @@ DECLARE_EVENT_CLASS(cgroup_migrate, __entry->dst_root = dst_cgrp->root->hierarchy_id; __entry->dst_id = dst_cgrp->id; __entry->dst_level = dst_cgrp->level; - cgroup_path(dst_cgrp, __get_dynamic_array(dst_path), - __get_dynamic_array_len(dst_path)); + __assign_str(dst_path, path); __entry->pid = task->pid; __assign_str(comm, task->comm); ), @@ -138,16 +135,18 @@ DECLARE_EVENT_CLASS(cgroup_migrate, DEFINE_EVENT(cgroup_migrate, cgroup_attach_task, - TP_PROTO(struct cgroup *dst_cgrp, struct task_struct *task, bool threadgroup), + TP_PROTO(struct cgroup *dst_cgrp, const char *path, + struct task_struct *task, bool threadgroup), - TP_ARGS(dst_cgrp, task, threadgroup) + TP_ARGS(dst_cgrp, path, task, threadgroup) ); DEFINE_EVENT(cgroup_migrate, cgroup_transfer_tasks, - TP_PROTO(struct cgroup *dst_cgrp, struct task_struct *task, bool threadgroup), + TP_PROTO(struct cgroup *dst_cgrp, const char *path, + struct task_struct *task, bool threadgroup), - TP_ARGS(dst_cgrp, task, threadgroup) + TP_ARGS(dst_cgrp, path, task, threadgroup) ); #endif /* _TRACE_CGROUP_H */ --- a/kernel/cgroup/cgroup-internal.h +++ b/kernel/cgroup/cgroup-internal.h @@ -8,6 +8,32 @@ #include #include +#define TRACE_CGROUP_PATH_LEN 1024 +extern spinlock_t trace_cgroup_path_lock; +extern char trace_cgroup_path[TRACE_CGROUP_PATH_LEN]; + +/* + * cgroup_path() takes a spin lock. It is good practice not to take + * spin locks within trace point handlers, as they are mostly hidden + * from normal view. As cgroup_path() can take the kernfs_rename_lock + * spin lock, it is best to not call that function from the trace event + * handler. + * + * Note: trace_cgroup_##type##_enabled() is a static branch that will only + * be set when the trace event is enabled. + */ +#define TRACE_CGROUP_PATH(type, cgrp, ...) \ + 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, \ + ##__VA_ARGS__); \ + 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 --- a/kernel/cgroup/cgroup-v1.c +++ b/kernel/cgroup/cgroup-v1.c @@ -135,7 +135,7 @@ int cgroup_transfer_tasks(struct cgroup if (task) { ret = cgroup_migrate(task, false, &mgctx); if (!ret) - trace_cgroup_transfer_tasks(to, task, false); + TRACE_CGROUP_PATH(transfer_tasks, to, task, false); put_task_struct(task); } } while (task && !ret); @@ -865,7 +865,7 @@ static int cgroup1_rename(struct kernfs_ ret = kernfs_rename(kn, new_parent, new_name_str); if (!ret) - trace_cgroup_rename(cgrp); + TRACE_CGROUP_PATH(rename, cgrp); mutex_unlock(&cgroup_mutex); --- 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. @@ -2638,7 +2641,7 @@ int cgroup_attach_task(struct cgroup *ds cgroup_migrate_finish(&mgctx); if (!ret) - trace_cgroup_attach_task(dst_cgrp, leader, threadgroup); + TRACE_CGROUP_PATH(attach_task, dst_cgrp, leader, threadgroup); return ret; } @@ -4641,7 +4644,7 @@ static void css_release_work_fn(struct w struct cgroup *tcgrp; /* cgroup release path */ - trace_cgroup_release(cgrp); + TRACE_CGROUP_PATH(release, cgrp); if (cgroup_on_dfl(cgrp)) cgroup_rstat_flush(cgrp); @@ -4984,7 +4987,7 @@ int cgroup_mkdir(struct kernfs_node *par if (ret) goto out_destroy; - trace_cgroup_mkdir(cgrp); + TRACE_CGROUP_PATH(mkdir, cgrp); /* let's create and online css's */ kernfs_activate(kn); @@ -5172,9 +5175,8 @@ int cgroup_rmdir(struct kernfs_node *kn) return 0; ret = cgroup_destroy_locked(cgrp); - if (!ret) - trace_cgroup_rmdir(cgrp); + TRACE_CGROUP_PATH(rmdir, cgrp); cgroup_kn_unlock(kn); return ret;