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 on d_locrkef as each thread
tries to grab a reference to the /proc dentry (and drop it shortly
thereafter).
Allow the pid_revalidate() function to execute under LOOKUP_RCU. When
updates must be made to the inode, drop out of RCU and into REF mode.
Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
When running running ~100 parallel instances of "TZ=/etc/localtime ps -fe
>/dev/null" on a 100CPU machine, the %sys utilization reaches 90%, and perf
shows the following code path as being responsible for heavy contention on
the d_lockref spinlock:
walk_component()
lookup_fast()
unlazy_child()
lockref_get_not_dead(&nd->path.dentry->d_lockref)
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 similarly high %sys time due to this
contention.
Changes from v3:
- Rather than call pid_update_inode() with flags, create
proc_inode_needs_update() to determine whether the call can be skipped.
- Restore the call to the security hook (see next patch).
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 | 35 +++++++++++++++++++++++++----------
1 file changed, 25 insertions(+), 10 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index b3422cda2a91..4b246e9bd5df 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1968,6 +1968,20 @@ void pid_update_inode(struct task_struct *task, struct inode *inode)
security_task_to_inode(task, inode);
}
+/* See if we can avoid the above call. Assumes RCU lock held */
+static bool pid_inode_needs_update(struct task_struct *task, struct inode *inode)
+{
+ kuid_t uid;
+ kgid_t gid;
+
+ if (inode->i_mode & (S_ISUID | S_ISGID))
+ return true;
+ task_dump_owner(task, inode->i_mode, &uid, &gid);
+ if (!uid_eq(uid, inode->i_uid) || !gid_eq(gid, inode->i_gid))
+ return true;
+ return false;
+}
+
/*
* Rewrite the inode's ownerships here because the owning task may have
* performed a setuid(), etc.
@@ -1977,19 +1991,20 @@ 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);
-
+ rcu_read_lock();
+ inode = d_inode_rcu(dentry);
+ task = pid_task(proc_pid(inode), PIDTYPE_PID);
if (task) {
- pid_update_inode(task, inode);
- put_task_struct(task);
- return 1;
+ rv = 1;
+ if ((flags & LOOKUP_RCU) && pid_inode_needs_update(task, inode))
+ rv = -ECHILD;
+ else if (!(flags & LOOKUP_RCU))
+ pid_update_inode(task, inode);
}
- return 0;
+ rcu_read_unlock();
+ return rv;
}
static inline bool proc_inode_is_dead(struct inode *inode)
--
2.25.1
