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SubjectRe: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field
On 05/09/2016 04:27 AM, Peter Zijlstra wrote:
> On Fri, May 06, 2016 at 08:20:24PM -0400, Waiman Long wrote:
>> @@ -391,9 +386,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> * When there's no owner, we might have preempted between the
>> * owner acquiring the lock and setting the owner field. If
>> * we're an RT task that will live-lock because we won't let
>> + * the owner complete. We also quit if the lock is owned by
>> + * readers.
> Maybe also note why we quit on readers.

Sure. Will do so.

>> */
>> + if (rwsem_is_reader_owned(owner) ||
>> + (!owner&& (need_resched() || rt_task(current))))
>> break;
>>
>> /*
>
>> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
>> index 870ed9a..d7fea18 100644
>> --- a/kernel/locking/rwsem.h
>> +++ b/kernel/locking/rwsem.h
>> @@ -1,3 +1,20 @@
>> +/*
>> + * The owner field of the rw_semaphore structure will be set to
>> + * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
>> + * the owner field when it unlocks. A reader, on the other hand, will
>> + * not touch the owner field when it unlocks.
>> + *
>> + * In essence, the owner field now has the following 3 states:
>> + * 1) 0
>> + * - lock is free or the owner hasn't set the field yet
>> + * 2) RWSEM_READER_OWNED
>> + * - lock is currently or previously owned by readers (lock is free
>> + * or not set by owner yet)
>> + * 3) Other non-zero value
>> + * - a writer owns the lock
>> + */
>> +#define RWSEM_READER_OWNED 1UL
> #define RWSEM_READER_OWNED ((struct task_struct *)1UL)

Will make the change.

>> +
>> #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>> static inline void rwsem_set_owner(struct rw_semaphore *sem)
>> {
>> @@ -9,6 +26,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
>> sem->owner = NULL;
>> }
>>
>> +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
>> +{
>> + /*
>> + * We check the owner value first to make sure that we will only
>> + * do a write to the rwsem cacheline when it is really necessary
>> + * to minimize cacheline contention.
>> + */
>> + if (sem->owner != (struct task_struct *)RWSEM_READER_OWNED)
>> + sem->owner = (struct task_struct *)RWSEM_READER_OWNED;
> How much if anything did this optimization matter?

I hadn't run any performance test to verify the effective of this
change. For a reader-heavy rwsem, this change should be able to save
quite a lot of needless write to the rwsem cacheline.

>> +}
>> +
>> +static inline bool rwsem_is_writer_owned(struct task_struct *owner)
>> +{
>> + return (unsigned long)owner> RWSEM_READER_OWNED;
>> +}
> Tad too clever that; what does GCC generate if you write the obvious:
>
> return owner&& owner != RWSEM_READER_OWNER;

You are right. GCC is intelligent enough to make the necessary
optimization. I will revert it to this form which is more obvious.

>> +
>> +static inline bool rwsem_is_reader_owned(struct task_struct *owner)
>> +{
>> + return owner == (struct task_struct *)RWSEM_READER_OWNED;
>> +}
> So I don't particularly like these names; they read like they take a
> rwsem as argument, but they don't.
>
> Would something like: rwsem_owner_is_{reader,writer}() make more sense?

Yes, these names look good to me.

Cheers,
Longman

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