The pid_revalidate() function requires dropping from RCU into REF lookup
mode. When many threads are resolving paths within /proc in parallel,
this can result in heavy spinlock contention as each thread tries to
grab a reference to the /proc dentry lock (and drop it shortly
thereafter).
Allow the pid_revalidate() function to execute under LOOKUP_RCU. When
updates must be made to the inode due to the owning task performing
setuid(), drop out of RCU and into REF mode. Remove the call to
security_task_to_inode(), since we can rely on the call from
proc_pid_make_inode().
Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
I'd like to use this patch as an RFC on this approach for reducing spinlock
contention during many parallel path lookups in the /proc filesystem. The
contention can be triggered by, for example, running ~100 parallel instances of
"TZ=/etc/localtime ps -fe >/dev/null" on a 100CPU machine. The %sys utilization
in such a case reaches around 90%, and profiles show two code paths with high
utilization:
walk_component
lookup_fast
unlazy_child
legitimize_root
__legitimize_path
lockref_get_not_dead
terminate_walk
dput
dput
By applying this patch, %sys utilization falls to around 60% under the same
workload. Although this particular workload is a bit contrived, we have seen
some monitoring scripts which produced high %sys time due to this contention.
Changes from v2:
- Remove get_pid_task_rcu_user() and get_proc_task_rcu(), since they were
unnecessary.
- Remove the call to security_task_to_inode().
fs/proc/base.c | 47 +++++++++++++++++++++++++++++++++--------------
1 file changed, 33 insertions(+), 14 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index ebea9501afb8..833d55a59e20 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1813,12 +1813,28 @@ int pid_getattr(const struct path *path, struct kstat *stat,
/*
* Set <pid>/... inode ownership (can change due to setuid(), etc.)
*/
-void pid_update_inode(struct task_struct *task, struct inode *inode)
+static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
+ unsigned int flags)
{
- task_dump_owner(task, inode->i_mode, &inode->i_uid, &inode->i_gid);
+ kuid_t uid;
+ kgid_t gid;
+
+ task_dump_owner(task, inode->i_mode, &uid, &gid);
+ if (uid_eq(uid, inode->i_uid) && gid_eq(gid, inode->i_gid) &&
+ !(inode->i_mode & (S_ISUID | S_ISGID)))
+ return 1;
+ if (flags & LOOKUP_RCU)
+ return -ECHILD;
+ inode->i_uid = uid;
+ inode->i_gid = gid;
inode->i_mode &= ~(S_ISUID | S_ISGID);
- security_task_to_inode(task, inode);
+ return 1;
+}
+
+void pid_update_inode(struct task_struct *task, struct inode *inode)
+{
+ do_pid_update_inode(task, inode, 0);
}
/*
@@ -1830,19 +1846,22 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
{
struct inode *inode;
struct task_struct *task;
+ int rv = 0;
- if (flags & LOOKUP_RCU)
- return -ECHILD;
-
- inode = d_inode(dentry);
- task = get_proc_task(inode);
-
- if (task) {
- pid_update_inode(task, inode);
- put_task_struct(task);
- return 1;
+ if (flags & LOOKUP_RCU) {
+ inode = d_inode_rcu(dentry);
+ task = pid_task(proc_pid(inode), PIDTYPE_PID);
+ if (task)
+ rv = do_pid_update_inode(task, inode, flags);
+ } else {
+ inode = d_inode(dentry);
+ task = get_proc_task(inode);
+ if (task) {
+ rv = do_pid_update_inode(task, inode, flags);
+ put_task_struct(task);
+ }
}
- return 0;
+ return rv;
}
static inline bool proc_inode_is_dead(struct inode *inode)
--
2.25.1
