linux-kernel - Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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Date:	Tue, 15 Sep 2015 10:24:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

On Mon, Sep 14, 2015 at 03:15:20PM -0400, Waiman Long wrote:
> 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.

> > 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.

But you're not adding that..

What you did add is a steal in pv_wait_head(), and its not even once per
pv_wait_head, its inside the spin loop (I read it wrong yesterday).

So that makes the entire Changelog complete crap. There isn't _one_
attempt, and there is absolutely no fairness left.
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