Messages in this thread | | | Date | Mon, 14 Sep 2015 15:15:20 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt |
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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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