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SubjectRe: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt
On 09/14/2015 10:00 AM, Peter Zijlstra wrote:
> On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
>> This patch allows one attempt for the lock waiter to steal the lock
>> when entering the PV slowpath. This helps to reduce the performance
>> penalty caused by lock waiter preemption while not having much of
>> the downsides of a real unfair lock.
>> @@ -415,8 +458,12 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
>>
>> for (;; waitcnt++) {
>> for (loop = SPIN_THRESHOLD; loop; loop--) {
>> - if (!READ_ONCE(l->locked))
>> - return;
>> + /*
>> + * Try to acquire the lock when it is free.
>> + */
>> + if (!READ_ONCE(l->locked)&&
>> + (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0))
>> + goto gotlock;
>> cpu_relax();
>> }
>>
> This isn't _once_, this is once per 'wakeup'. And note that interrupts
> unrelated to the kick can equally wake the vCPU up.
>

Oh! There is a minor bug that I shouldn't need to have a second
READ_ONCE() call here.

As this is the queue head, finding the lock free entitles the vCPU to
own the lock. However, because of lock stealing, I can't just write a 1
to the lock and assume thing is all set. That is why I need to use
cmpxchg() to make sure that the queue head vCPU can actually get the
lock without the lock stolen underneath. I don't count that as lock
stealing as it is the rightful owner of the lock.

I am sorry that I should have added a comment to clarify that. Will do
so in the next update.

> void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> {
> :
> /*
> * We touched a (possibly) cold cacheline in the per-cpu queue
node;
> * attempt the trylock once more in the hope someone let go
while we
> * weren't watching.
> */
> if (queued_spin_trylock(lock))
> goto release;

This is the only place where I consider lock stealing happens. Again, I
should have a comment in pv_queued_spin_trylock_unfair() to say where it
will be called.

Cheers,
Longman


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