linux-kernel - Re: [PATCH] locking/rwsem: Allow slowpath writer to ignore handoff bit if not set by first waiter

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Date:   Wed, 22 Jun 2022 16:07:42 -0400
From:   Waiman Long <longman@...hat.com>
To:     john.p.donnelly@...cle.com, Hillf Danton <hdanton@...a.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/rwsem: Allow slowpath writer to ignore handoff
 bit if not set by first waiter

On 6/22/22 13:48, john.p.donnelly@...cle.com wrote:
> On 4/27/22 8:23 PM, Hillf Danton wrote:
>> On  Wed, 27 Apr 2022 13:31:24 -0400 Waiman Long wrote:
>>> With commit d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling 
>>> more
>>> consistent"), the writer that sets the handoff bit can be interrupted
>>> out without clearing the bit if the wait queue isn't empty. This 
>>> disables
>>> reader and writer optimistic lock spinning and stealing.
>>>
>>> Now if a non-first writer in the queue is somehow woken up or first
>>> entering the waiting loop, it can't acquire the lock.  This is not
>>> the case before that commit as the writer that set the handoff bit
>>> will clear it when exiting out via the out_nolock path. This is less
>>> efficient as the busy rwsem stays in an unlock state for a longer time.
>>>
>>> This patch allows a non-first writer to ignore the handoff bit if it
>>> is not originally set or initiated by the first waiter.
>>>
>>> Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more 
>>> consistent")
>>> Signed-off-by: Waiman Long <longman@...hat.com>
>>> ---
>>>   kernel/locking/rwsem.c | 30 ++++++++++++++++++++----------
>>>   1 file changed, 20 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
>>> index 9d1db4a54d34..65f0262f635e 100644
>>> --- a/kernel/locking/rwsem.c
>>> +++ b/kernel/locking/rwsem.c
>>> @@ -335,8 +335,6 @@ struct rwsem_waiter {
>>>       struct task_struct *task;
>>>       enum rwsem_waiter_type type;
>>>       unsigned long timeout;
>>> -
>>> -    /* Writer only, not initialized in reader */
>>>       bool handoff_set;
>>>   };
>>>   #define rwsem_first_waiter(sem) \
>>> @@ -459,10 +457,12 @@ static void rwsem_mark_wake(struct 
>>> rw_semaphore *sem,
>>>                * to give up the lock), request a HANDOFF to
>>>                * force the issue.
>>>                */
>>> -            if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
>>> -                time_after(jiffies, waiter->timeout)) {
>>> -                adjustment -= RWSEM_FLAG_HANDOFF;
>>> -                lockevent_inc(rwsem_rlock_handoff);
>>> +            if (time_after(jiffies, waiter->timeout)) {
>>> +                if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
>>> +                    adjustment -= RWSEM_FLAG_HANDOFF;
>>> +                    lockevent_inc(rwsem_rlock_handoff);
>>> +                }
>>> +                waiter->handoff_set = true;
>>>               }
>>
>> Handoff is tracked in both sem->count and waiter->handoff_set,
>>
>>> atomic_long_add(-adjustment, &sem->count);
>>> @@ -599,7 +599,7 @@ rwsem_del_wake_waiter(struct rw_semaphore *sem, 
>>> struct rwsem_waiter *waiter,
>>>   static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
>>>                       struct rwsem_waiter *waiter)
>>>   {
>>> -    bool first = rwsem_first_waiter(sem) == waiter;
>>> +    struct rwsem_waiter *first = rwsem_first_waiter(sem);
>>>       long count, new;
>>>         lockdep_assert_held(&sem->wait_lock);
>>> @@ -609,11 +609,20 @@ static inline bool rwsem_try_write_lock(struct 
>>> rw_semaphore *sem,
>>>           bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>>>             if (has_handoff) {
>>> -            if (!first)
>>> +            /*
>>> +             * Honor handoff bit and yield only when the first
>>> +             * waiter is the one that set it. Otherwisee, we
>>> +             * still try to acquire the rwsem.
>>> +             */
>>> +            if (first->handoff_set && (waiter != first))
>>>                   return false;
>>
>> and checked against both parties, thus in a simpler manner 
>> RWSEM_FLAG_HANDOFF
>> in sem->count means the first waiter has been waiting for lock long 
>> enough.
>>
>> Feel free to ignore the comment given the Fixes tag above.
>>
>> Hillf
>>>   -            /* First waiter inherits a previously set handoff bit */
>>> -            waiter->handoff_set = true;
>>> +            /*
>>> +             * First waiter can inherit a previously set handoff
>>> +             * bit and spin on rwsem if lock acquisition fails.
>>> +             */
>>> +            if (waiter == first)
>>> +                waiter->handoff_set = true;
>>>           }
>>>             new = count;
>>> @@ -1027,6 +1036,7 @@ rwsem_down_read_slowpath(struct rw_semaphore 
>>> *sem, long count, unsigned int stat
>>>       waiter.task = current;
>>>       waiter.type = RWSEM_WAITING_FOR_READ;
>>>       waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
>>> +    waiter.handoff_set = false;
>>>         raw_spin_lock_irq(&sem->wait_lock);
>>>       if (list_empty(&sem->wait_list)) {
>>> -- 
>>> 2.27.0
>
>
> Was this ever added ?
>
> I don't see it in
>
>
> a111daf0c53ae 2022-06-19 | Linux 5.19-rc3

This patch hasn't been taken up by upstream yet. I have reposted a v2 
with update to the patch description.

Cheers,
Longman