ebiederm@xmission.com (Eric W. Biederman) writes:

Stephen Brennan [off-list ref] writes:

quoted

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 (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.

So rather than get_task_rcu_user.  I think what we want is a function
that verifies task->rcu_users > 0.

Which frankly is just "pid_task(proc_pid(inode), PIDTYPE_PID)".

Which is something that we can do unconditionally in pid_revalidate.

Skipping the update of the inode is probably the only thing that needs
to be skipped.

It looks like the code can safely rely on the the security_task_to_inode
in proc_pid_make_inode and remove the security_task_to_inode in
pid_update_inode.

This makes sense, I'll get rid of the get_task_rcu_user() stuff in a v2.

quoted

Signed-off-by: Stephen Brennan <redacted>
---

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:

Do you have a real world work-load that is behaves something like this
micro benchmark?  I am just curious how severe the problem you are
trying to solve is.

We have seen this issue occur internally with monitoring scripts
(perhaps a bit misconfigured, I'll admit). However I don't have an exact
sample workload that I can give you.

Thanks,
Stephen