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Message-ID: <a2baa104-6526-a129-0409-e66ebc098e2a@redhat.com>
Date: Tue, 23 Apr 2019 10:31:42 -0400
From: Waiman Long <llong@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
Will Deacon <will.deacon@....com>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org, x86@...nel.org,
Davidlohr Bueso <dave@...olabs.net>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Tim Chen <tim.c.chen@...ux.intel.com>,
huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count
negative
On 4/23/19 10:17 AM, Peter Zijlstra wrote:
> On Sun, Apr 21, 2019 at 05:07:56PM -0400, Waiman Long wrote:
>
>> How about the following chunks to disable preemption temporarily for the
>> increment-check-decrement sequence?
>>
>> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
>> index dd92b1a93919..4cc03ac66e13 100644
>> --- a/include/linux/preempt.h
>> +++ b/include/linux/preempt.h
>> @@ -250,6 +250,8 @@ do { \
>> #define preempt_enable_notrace() barrier()
>> #define preemptible() 0
>>
>> +#define __preempt_disable_nop /* preempt_disable() is nop */
>> +
>> #endif /* CONFIG_PREEMPT_COUNT */
>>
>> #ifdef MODULE
>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
>> index 043fd29b7534..54029e6af17b 100644
>> --- a/kernel/locking/rwsem.c
>> +++ b/kernel/locking/rwsem.c
>> @@ -256,11 +256,64 @@ static inline struct task_struct
>> *rwsem_get_owner(struct r
>> return (struct task_struct *) (cowner
>> ? cowner | (sowner & RWSEM_NONSPINNABLE) : sowner);
>> }
>> +
>> +/*
>> + * If __preempt_disable_nop is defined, calling preempt_disable() and
>> + * preempt_enable() directly is the most efficient way. Otherwise, it may
>> + * be more efficient to disable and enable interrupt instead for disabling
>> + * preemption tempoarily.
>> + */
>> +#ifdef __preempt_disable_nop
>> +#define disable_preemption() preempt_disable()
>> +#define enable_preemption() preempt_enable()
>> +#else
>> +#define disable_preemption() local_irq_disable()
>> +#define enable_preemption() local_irq_enable()
>> +#endif
> I'm not aware of an architecture where disabling interrupts is faster
> than disabling preemption.
I have actually done some performance test measuring the effects of
disabling interrupt and preemption on readers (on x86-64 system).
Threads Before patch Disable irq Disable preemption
------- ------------ ----------- ------------------
1 9,088 8,766 9,172
2 9,296 9,169 8,707
4 11,192 11,205 10,712
8 11,329 11,332 11,213
For uncontended case, disable interrupt is slower. The slowdown is gone
once the rwsem becomes contended. So it may not be a good idea to
disable interrupt as a proxy of disabling preemption.
BTW, preemption count is not enabled in typical distro production
kernels like RHEL. So preempt_disable() is just a barrier. It is turned
on in the debug kernel, though.
>> +/*
>> + * When the owner task structure pointer is merged into couunt, less bits
>> + * will be available for readers. Therefore, there is a very slight chance
>> + * that the reader count may overflow. We try to prevent that from
>> happening
>> + * by checking for the MS bit of the count and failing the trylock attempt
>> + * if this bit is set.
>> + *
>> + * With preemption enabled, there is a remote possibility that preemption
>> + * can happen in the narrow timing window between incrementing and
>> + * decrementing the reader count and the task is put to sleep for a
>> + * considerable amount of time. If sufficient number of such unfortunate
>> + * sequence of events happen, we may still overflow the reader count.
>> + * To avoid such possibility, we have to disable preemption for the
>> + * whole increment-check-decrement sequence.
>> + *
>> + * The function returns true if there are too many readers and the count
>> + * has already been properly decremented so the reader must go directly
>> + * into the wait list.
>> + */
>> +static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cnt)
>> +{
>> + bool wait = false; /* Wait now flag */
>> +
>> + disable_preemption();
>> + *cnt = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
>> &sem->count);
>> + if (unlikely(*cnt < 0)) {
>> + atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
>> + wait = true;
>> + }
>> + enable_preemption();
>> + return wait;
>> +}
>> #else /* !CONFIG_RWSEM_OWNER_COUNT */
> This also means you have to ensure CONFIG_NR_CPUS < 32K for
> RWSEM_OWNER_COUNT.
Yes, that can be done.
>
>> static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
>> {
>> return READ_ONCE(sem->owner);
>> }
>> +
>> +static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cnt)
>> +{
>> + *cnt = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
>> &sem->count);
>> + return false;
>> +}
>> #endif /* CONFIG_RWSEM_OWNER_COUNT */
>>
>> /*
>> @@ -981,32 +1034,18 @@ static inline void clear_wr_nonspinnable(struct
>> rw_semaph
>> * Wait for the read lock to be granted
>> */
>> static struct rw_semaphore __sched *
>> -rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count)
>> +rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, const
>> bool wait)
>> {
>> - long adjustment = -RWSEM_READER_BIAS;
>> + long count, adjustment = -RWSEM_READER_BIAS;
>> bool wake = false;
>> struct rwsem_waiter waiter;
>> DEFINE_WAKE_Q(wake_q);
>>
>> - if (unlikely(count < 0)) {
>> + if (unlikely(wait)) {
>> /*
>> - * The sign bit has been set meaning that too many active
>> - * readers are present. We need to decrement reader count &
>> - * enter wait queue immediately to avoid overflowing the
>> - * reader count.
>> - *
>> - * As preemption is not disabled, there is a remote
>> - * possibility that preemption can happen in the narrow
>> - * timing window between incrementing and decrementing
>> - * the reader count and the task is put to sleep for a
>> - * considerable amount of time. If sufficient number
>> - * of such unfortunate sequence of events happen, we
>> - * may still overflow the reader count. It is extremely
>> - * unlikey, though. If this is a concern, we should consider
>> - * disable preemption during this timing window to make
>> - * sure that such unfortunate event will not happen.
>> + * The reader count has already been decremented and the
>> + * reader should go directly into the wait list now.
>> */
>> - atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
>> adjustment = 0;
>> goto queue;
>> }
>> @@ -1358,11 +1397,12 @@ static struct rw_semaphore
>> *rwsem_downgrade_wake(struct
>> */
>> inline void __down_read(struct rw_semaphore *sem)
>> {
>> - long tmp = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
>> - &sem->count);
>> + long tmp;
>> + bool wait;
>>
>> + wait = rwsem_read_trylock(sem, &tmp);
>> if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) {
>> - rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, tmp);
>> + rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, wait);
>> DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
>> } else {
>> rwsem_set_reader_owned(sem);
> I think I prefer that function returning/taking the bias/adjustment
> value instead of a bool, if it is all the same.
Sure, I can do that.
Cheers,
Longman
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