linux-kernel - Re: [PATCH] locking/qspinlock: Optimize pending state waiting for unlock

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Date:   Sat, 24 Dec 2022 22:29:29 -0500
From:   Waiman Long <longman@...hat.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        Guo Ren <guoren@...ux.alibaba.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] locking/qspinlock: Optimize pending state waiting for
 unlock

On 12/24/22 21:57, Guo Ren wrote:
> On Sun, Dec 25, 2022 at 9:55 AM Waiman Long <longman@...hat.com> wrote:
>> On 12/24/22 07:05, guoren@...nel.org wrote:
>>> From: Guo Ren <guoren@...ux.alibaba.com>
>>>
>>> When we're pending, we only care about lock value. The xchg_tail
>>> wouldn't affect the pending state. That means the hardware thread
>>> could stay in a sleep state and leaves the rest execution units'
>>> resources of pipeline to other hardware threads. This optimization
>>> may work only for SMT scenarios because the granularity between
>>> cores is cache-block.
> Please have a look at the comment I've written.
>
>>> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
>>> Signed-off-by: Guo Ren <guoren@...nel.org>
>>> Cc: Waiman Long <longman@...hat.com>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Cc: Boqun Feng <boqun.feng@...il.com>
>>> Cc: Will Deacon <will@...nel.org>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> ---
>>>    kernel/locking/qspinlock.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>>> index 2b23378775fe..ebe6b8ec7cb3 100644
>>> --- a/kernel/locking/qspinlock.c
>>> +++ b/kernel/locking/qspinlock.c
>>> @@ -371,7 +371,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>>>        /*
>>>         * We're pending, wait for the owner to go away.
>>>         *
>>> -      * 0,1,1 -> 0,1,0
>>> +      * 0,1,1 -> *,1,0
>>>         *
>>>         * this wait loop must be a load-acquire such that we match the
>>>         * store-release that clears the locked bit and create lock
>> Yes, we don't care about the tail.
>>> @@ -380,7 +380,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>>>         * barriers.
>>>         */
>>>        if (val & _Q_LOCKED_MASK)
>>> -             atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK));
>>> +             smp_cond_load_acquire(&lock->locked, !VAL);
>>>
>>>        /*
>>>         * take ownership and clear the pending bit.
>> We may save an AND operation here which may be a cycle or two.  I
>> remember that it may be more costly to load a byte instead of an integer
>> in some arches. So it doesn't seem like that much of an optimization
>> from my point of view.
> The reason is, of course, not here. See my commit comment.
>
>> I know that arm64 will enter a low power state in
>> this *cond_load_acquire() loop, but I believe any change in the state of
>> the the lock cacheline will wake it up. So it doesn't really matter if
>> you are checking a byte or an int.
> The situation is the SMT scenarios in the same core. Not an entering
> low-power state situation. Of course, the granularity between cores is
> "cacheline", but the granularity between SMT hw threads of the same
> core could be "byte" which internal LSU handles. For example, when a
> hw-thread yields the resources of the core to other hw-threads, this
> patch could help the hw-thread stay in the sleep state and prevent it
> from being woken up by other hw-threads xchg_tail.
>
> Finally, from the software semantic view, does the patch make it more
> accurate? (We don't care about the tail here.)

Thanks for the clarification.

I am not arguing for the simplification part. I just want to clarify my 
limited understanding of how the CPU hardware are actually dealing with 
these conditions.

With that, I am fine with this patch. It would be nice if you can 
elaborate a bit more in your commit log.

Acked-by: Waiman Long <longman@...hat.com>