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Sebastian Andrzej Siewior authored
This is a revert of commit a4244454 ("percpu-refcount: use RCU-sched insted of normal RCU") which claims the only reason for using RCU-sched is "rcu_read_[un]lock() … are slightly more expensive than preempt_disable/enable()" and "As the RCU critical sections are extremely short, using sched-RCU shouldn't have any latency implications." The problem with using RCU-sched here is that it disables preemption and the release callback (called from percpu_ref_put_many()) must not acquire any sleeping locks like spinlock_t. This breaks PREEMPT_RT because some of the users acquire spinlock_t locks in their callbacks. Using rcu_read_lock() on PREEMPTION=n kernels is not any different compared to rcu_read_lock_sched(). On PREEMPTION=y kernels there are already performance issues due to additional preemption points. Looking at the code, the rcu_read_lock() is just an increment and unlock is almost just a decrement unless there is something special to do. Both are functions while disabling preemption is inlined. Doing a small benchmark, the minimal amount of time required was mostly the same. The average time required was higher due to the higher MAX value (which could be preemption). With DEBUG_PREEMPT=y it is rcu_read_lock_sched() that takes a little longer due to the additional debug code. Convert back to normal RCU. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Dennis Zhou <dennis@kernel.org>
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