Messages in this thread | | | Date | Sat, 16 Nov 2019 23:11:13 -0500 | From | Dennis Zhou <> | Subject | Re: [PATCH v2] percpu-refcount: Use normal instead of RCU-sched" |
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On Fri, Nov 08, 2019 at 06:35:53PM +0100, Sebastian Andrzej Siewior 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 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> > --- > On 2019-11-07 12:36:53 [-0500], Dennis Zhou wrote: > > > some RCU section here invoke callbacks which acquire spinlock_t locks. > > > This does not work on RT with disabled preemption. > > > > > > > Yeah, so adding a bit in the commit message about why it's an issue for > > RT kernels with disabled preemption as I don't believe this is an issue > > for non-RT kernels. > > I realized that I had partly in the commit message so I rewrote the > second chapter hopefully covering it all now more explicit. > > v1…v2: Slightly rewriting the second paragraph regarding RT > implications. > > include/linux/percpu-refcount.h | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h > index 7aef0abc194a2..390031e816dcd 100644 > --- a/include/linux/percpu-refcount.h > +++ b/include/linux/percpu-refcount.h > @@ -186,14 +186,14 @@ static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr) > { > unsigned long __percpu *percpu_count; > > - rcu_read_lock_sched(); > + rcu_read_lock(); > > if (__ref_is_percpu(ref, &percpu_count)) > this_cpu_add(*percpu_count, nr); > else > atomic_long_add(nr, &ref->count); > > - rcu_read_unlock_sched(); > + rcu_read_unlock(); > } > > /** > @@ -223,7 +223,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref) > unsigned long __percpu *percpu_count; > bool ret; > > - rcu_read_lock_sched(); > + rcu_read_lock(); > > if (__ref_is_percpu(ref, &percpu_count)) { > this_cpu_inc(*percpu_count); > @@ -232,7 +232,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref) > ret = atomic_long_inc_not_zero(&ref->count); > } > > - rcu_read_unlock_sched(); > + rcu_read_unlock(); > > return ret; > } > @@ -257,7 +257,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) > unsigned long __percpu *percpu_count; > bool ret = false; > > - rcu_read_lock_sched(); > + rcu_read_lock(); > > if (__ref_is_percpu(ref, &percpu_count)) { > this_cpu_inc(*percpu_count); > @@ -266,7 +266,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) > ret = atomic_long_inc_not_zero(&ref->count); > } > > - rcu_read_unlock_sched(); > + rcu_read_unlock(); > > return ret; > } > @@ -285,14 +285,14 @@ static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr) > { > unsigned long __percpu *percpu_count; > > - rcu_read_lock_sched(); > + rcu_read_lock(); > > if (__ref_is_percpu(ref, &percpu_count)) > this_cpu_sub(*percpu_count, nr); > else if (unlikely(atomic_long_sub_and_test(nr, &ref->count))) > ref->release(ref); > > - rcu_read_unlock_sched(); > + rcu_read_unlock(); > } > > /** > -- > 2.24.0 > >
Sorry for sitting on this for so long. I've applied it to for-5.5.
Thanks, Dennis
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