Re: [PATCH RFC ticketlock] v3 Auto-queued ticketlock

From: Lai Jiangshan
Date: Thu Jun 13 2013 - 21:28:43 EST

On 06/14/2013 07:57 AM, Paul E. McKenney wrote:
> On Fri, Jun 14, 2013 at 07:25:57AM +0800, Lai Jiangshan wrote:
>> On Thu, Jun 13, 2013 at 11:22 PM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>>> On Thu, Jun 13, 2013 at 10:55:41AM +0800, Lai Jiangshan wrote:
>>>> On 06/12/2013 11:40 PM, Paul E. McKenney wrote:
>>>>> Breaking up locks is better than implementing high-contention locks, but
>>>>> if we must have high-contention locks, why not make them automatically
>>>>> switch between light-weight ticket locks at low contention and queued
>>>>> locks at high contention? After all, this would remove the need for
>>>>> the developer to predict which locks will be highly contended.
>>>>> This commit allows ticket locks to automatically switch between pure
>>>>> ticketlock and queued-lock operation as needed. If too many CPUs are
>>>>> spinning on a given ticket lock, a queue structure will be allocated
>>>>> and the lock will switch to queued-lock operation. When the lock becomes
>>>>> free, it will switch back into ticketlock operation. The low-order bit
>>>>> of the head counter is used to indicate that the lock is in queued mode,
>>>>> which forces an unconditional mismatch between the head and tail counters.
>>>>> This approach means that the common-case code path under conditions of
>>>>> low contention is very nearly that of a plain ticket lock.
>>>>> A fixed number of queueing structures is statically allocated in an
>>>>> array. The ticket-lock address is used to hash into an initial element,
>>>>> but if that element is already in use, it moves to the next element. If
>>>>> the entire array is already in use, continue to spin in ticket mode.
>>>>> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>>>>> [ paulmck: Eliminate duplicate code and update comments (Steven Rostedt). ]
>>>>> [ paulmck: Address Eric Dumazet review feedback. ]
>>>>> [ paulmck: Use Lai Jiangshan idea to eliminate smp_mb(). ]
>>>>> [ paulmck: Expand ->head_tkt from s32 to s64 (Waiman Long). ]
>>>>> [ paulmck: Move cpu_relax() to main spin loop (Steven Rostedt). ]
>>>>> [ paulmck: Reduce queue-switch contention (Waiman Long). ]
>>>>> [ paulmck: __TKT_SPIN_INC for __ticket_spin_trylock() (Steffen Persvold). ]
>>>>> [ paulmck: Type safety fixes (Steven Rostedt). ]
>>>>> [ paulmck: Pre-check cmpxchg() value (Waiman Long). ]
>>>>> [ paulmck: smp_mb() downgrade to smp_wmb() (Lai Jiangshan). ]
>>>> Hi, Paul,
>>>> I simplify the code and remove the search by encoding the index of struct tkt_q_head
>>>> into lock->tickets.head.
>>>> A) lock->tickets.head(when queued-lock):
>>>> ---------------------------------
>>>> index of struct tkt_q_head |0|1|
>>>> ---------------------------------
>>> Interesting approach! It might reduce queued-mode overhead a bit in
>>> some cases, though I bet that in the common case the first queue element
>>> examined is the right one. More on this below.
>>>> The bit0 = 1 for queued, and the bit1 = 0 is reserved for __ticket_spin_unlock(),
>>>> thus __ticket_spin_unlock() will not change the higher bits of lock->tickets.head.
>>>> B) tqhp->head is for the real value of lock->tickets.head.
>>>> if the last bit of tqhp->head is 1, it means it is the head ticket when started queuing.
>>> But don't you also need the xadd() in __ticket_spin_unlock() to become
>>> a cmpxchg() for this to work? Or is your patch missing your changes to
>>> arch/x86/include/asm/spinlock.h? Either way, this is likely to increase
>>> the no-contention overhead, which might be counterproductive. Wouldn't
>>> hurt to get measurements, though.
>> No, don't need to change __ticket_spin_unlock() in my idea.
>> bit1 in the tickets.head is reserved for __ticket_spin_unlock(),
>> __ticket_spin_unlock() only changes the bit1, it will not change
>> the higher bits. tkt_q_do_wake() will restore the tickets.head.
>> This approach avoids cmpxchg in __ticket_spin_unlock().
> Ah, I did miss that. But doesn't the adjustment in __ticket_spin_lock()
> need to be atomic in order to handle concurrent invocations of
> __ticket_spin_lock()?

I don't understand, do we just discuss about __ticket_spin_unlock() only?
Or does my suggestion hurt __ticket_spin_lock()?

> Either way, it would be good to see the performance effects of this.
> Thanx, Paul
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