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SubjectRe: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count
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Hi,

On 2020/9/16 0:03, peterz@infradead.org wrote:
> On Tue, Sep 15, 2020 at 05:51:50PM +0200, peterz@infradead.org wrote:
>
>> Anyway, I'll rewrite the Changelog and stuff it in locking/urgent.
>
> How's this?
>
Thanks for that.

> ---
> Subject: locking/percpu-rwsem: Use this_cpu_{inc,dec}() for read_count
> From: Hou Tao <houtao1@huawei.com>
> Date: Tue, 15 Sep 2020 22:07:50 +0800
>
> From: Hou Tao <houtao1@huawei.com>
>
> The __this_cpu*() accessors are (in general) IRQ-unsafe which, given
> that percpu-rwsem is a blocking primitive, should be just fine.
>
> However, file_end_write() is used from IRQ context and will cause
> load-store issues.
>
> Fixing it by using the IRQ-safe this_cpu_*() for operations on
> read_count. This will generate more expensive code on a number of
> platforms, which might cause a performance regression for some of the
> other percpu-rwsem users.
>
> If any such is reported, we can consider alternative solutions.
>
I have simply test the performance impact on both x86 and aarch64.

There is no degradation under x86 (2 sockets, 18 core per sockets, 2 threads per core)

v5.8.9
no writer, reader cn | 18 | 36 | 72
the rate of down_read/up_read per second | 231423957 | 230737381 | 109943028
the rate of down_read/up_read per second (patched) | 232864799 | 233555210 | 109768011

However the performance degradation is huge under aarch64 (4 sockets, 24 core per sockets): nearly 60% lost.

v4.19.111
no writer, reader cn | 24 | 48 | 72 | 96
the rate of down_read/up_read per second | 166129572 | 166064100 | 165963448 | 165203565
the rate of down_read/up_read per second (patched) | 63863506 | 63842132 | 63757267 | 63514920

I will test the aarch64 host by using v5.8 tomorrow.

Regards,
Tao


> Fixes: 70fe2f48152e ("aio: fix freeze protection of aio writes")
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20200915140750.137881-1-houtao1@huawei.com
> ---
> include/linux/percpu-rwsem.h | 8 ++++----
> kernel/locking/percpu-rwsem.c | 4 ++--
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -60,7 +60,7 @@ static inline void percpu_down_read(stru
> * anything we did within this RCU-sched read-size critical section.
> */
> if (likely(rcu_sync_is_idle(&sem->rss)))
> - __this_cpu_inc(*sem->read_count);
> + this_cpu_inc(*sem->read_count);
> else
> __percpu_down_read(sem, false); /* Unconditional memory barrier */
> /*
> @@ -79,7 +79,7 @@ static inline bool percpu_down_read_tryl
> * Same as in percpu_down_read().
> */
> if (likely(rcu_sync_is_idle(&sem->rss)))
> - __this_cpu_inc(*sem->read_count);
> + this_cpu_inc(*sem->read_count);
> else
> ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
> preempt_enable();
> @@ -103,7 +103,7 @@ static inline void percpu_up_read(struct
> * Same as in percpu_down_read().
> */
> if (likely(rcu_sync_is_idle(&sem->rss))) {
> - __this_cpu_dec(*sem->read_count);
> + this_cpu_dec(*sem->read_count);
> } else {
> /*
> * slowpath; reader will only ever wake a single blocked
> @@ -115,7 +115,7 @@ static inline void percpu_up_read(struct
> * aggregate zero, as that is the only time it matters) they
> * will also see our critical section.
> */
> - __this_cpu_dec(*sem->read_count);
> + this_cpu_dec(*sem->read_count);
> rcuwait_wake_up(&sem->writer);
> }
> preempt_enable();
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -45,7 +45,7 @@ EXPORT_SYMBOL_GPL(percpu_free_rwsem);
>
> static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
> {
> - __this_cpu_inc(*sem->read_count);
> + this_cpu_inc(*sem->read_count);
>
> /*
> * Due to having preemption disabled the decrement happens on
> @@ -71,7 +71,7 @@ static bool __percpu_down_read_trylock(s
> if (likely(!atomic_read_acquire(&sem->block)))
> return true;
>
> - __this_cpu_dec(*sem->read_count);
> + this_cpu_dec(*sem->read_count);
>
> /* Prod writer to re-evaluate readers_active_check() */
> rcuwait_wake_up(&sem->writer);
> .
>

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