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Date:   Fri, 20 Nov 2020 12:26:58 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH 3/5] locking/rwsem: Enable reader optimistic lock stealing

On 11/20/20 9:36 AM, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 10:04:27PM -0500, Waiman Long wrote:
>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
>> index ee374ae061c3..930dd4af3639 100644
>> --- a/kernel/locking/rwsem.c
>> +++ b/kernel/locking/rwsem.c
>> @@ -957,6 +957,12 @@ static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
>>   	}
>>   	return false;
>>   }
>> +
>> +static inline bool osq_is_empty(struct rw_semaphore *sem)
>> +{
>> +	return !osq_is_locked(&sem->osq);
>> +}
>> +
>>   #else
>>   static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
>>   					   unsigned long nonspinnable)
>> @@ -977,6 +983,10 @@ static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
>>   	return false;
>>   }
>>   
>> +static inline bool osq_is_empty(sem)
>> +{
>> +	return false;
>> +}
> Hurph, the naming seems to suggest this ought to be in osq_lock.h, but
> it really is part of rwsem, it captures the lack of osq member for this
> configuration.
>
> How about: rwsem_no_spinners() instead ?
Yes, sure. Will make the name change.
>
>>   static inline int
>>   rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
>>   {
>> @@ -1007,6 +1017,22 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count)
>>   	   !(count & RWSEM_WRITER_LOCKED))
>>   		goto queue;
>>   
>> +	/*
>> +	 * Reader optimistic lock stealing
>> +	 *
>> +	 * We can take the read lock directly without doing
>> +	 * rwsem_optimistic_spin() if the conditions are right.
>> +	 * Also wake up other readers if it is the first reader.
>> +	 */
>> +	if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF)) &&
>> +	    osq_is_empty(sem)) {
>> +		rwsem_set_reader_owned(sem);
>> +		lockevent_inc(rwsem_rlock_steal);
>> +		if (rcnt == 1)
>> +			goto wake_readers;
>> +		return sem;
>> +	}
> AFAICT this saves at least 3 atomic ops; how common is this case
> (you did add a counter but forgot to mention this).
>
Right, I should have mentioned the counter results.

Below is the relevant counter stats for a test system that have been up 
for more than 21 hours:

rwsem_opt_rlock=11792583 (optmistically acquired read lock)
rwsem_rlock=193357272 (slowpath acquired read lock)
rwsem_rlock_steal=44795149 (lock stealing)

So lock stealing represents about 17.9% of the total read lock acquired 
in non-fast path. I ran some microbenchmark test on the system before, 
so it may skew a bit to the high side. Anyway, this is not an 
insignificant amount.

Cheers,
Longman


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