On Thu, Feb 14, 2019 at 04:07:37PM +0800, Huang, Ying wrote: > Before, we choose to use stop_machine() to reduce the overhead of hot > path (page fault handler) as much as possible. But now, I found > rcu_read_lock_sched() is just a wrapper of preempt_disable(). So maybe > we can switch to RCU version now.
rcu_read_lock looks more efficient than rcu_read_lock_sched. So for this purpose in the fast path rcu_read_lock()/unlock() should be the preferred methods, no need to force preempt_disable() (except for debug purposes if sleep debug is enabled). Server builds are done with voluntary preempt (no preempt shouldn't even exist as config option) and there rcu_read_lock might be just a noop. Against a fast path rcu_read_lock/unlock before the consolidation synchronize_rcu would have been enough, now after the consolidation even more certain that it's enough because it's equivalent to _mult.
