Messages in this thread | | | Date | Thu, 7 Nov 2019 10:13:19 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH] percpu-refcount: Use normal instead of RCU-sched" |
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On 2019-10-02 13:22:53 [+0200], To linux-kernel@vger.kernel.org wrote: > This is a revert of commit > a4244454df129 ("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 callback must not acquire any sleeping locks like spinlock_t on > PREEMPT_RT which is the case with some of the users. > > 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.
a gentle ping.
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > > Benchmark https://breakpoint.cc/percpu_test.patch
Sebastian
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