From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, linux-kernel@vger.kernel.org,
Scott J Norton <scott.norton@hp.com>,
Douglas Hatch <doug.hatch@hp.com>,
Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt
Date: Tue, 15 Sep 2015 10:24:30 +0200 [thread overview]
Message-ID: <20150915082430.GX16853@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <55F71CC8.3050306@hpe.com>
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.
next prev parent reply other threads:[~2015-09-15 8:24 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-11 18:37 [PATCH v6 0/6] locking/qspinlock: Enhance pvqspinlock performance Waiman Long
2015-09-11 18:37 ` [PATCH v6 1/6] locking/qspinlock: relaxes cmpxchg & xchg ops in native code Waiman Long
2015-09-11 22:27 ` Davidlohr Bueso
2015-09-14 12:06 ` Peter Zijlstra
2015-09-14 18:40 ` Waiman Long
2015-09-14 15:16 ` Waiman Long
2015-09-11 18:37 ` [PATCH v6 2/6] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL Waiman Long
2015-09-18 8:50 ` [tip:locking/core] locking/pvqspinlock: Kick the PV CPU unconditionally when _Q_SLOW_VAL tip-bot for Waiman Long
2015-09-11 18:37 ` [PATCH v6 3/6] locking/pvqspinlock, x86: Optimize PV unlock code path Waiman Long
2015-09-11 18:37 ` [PATCH v6 4/6] locking/pvqspinlock: Collect slowpath lock statistics Waiman Long
2015-09-11 23:13 ` Davidlohr Bueso
2015-09-14 15:25 ` Waiman Long
2015-09-14 21:41 ` Davidlohr Bueso
2015-09-15 3:47 ` Waiman Long
2015-09-11 18:37 ` [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt Waiman Long
2015-09-14 13:57 ` Peter Zijlstra
2015-09-14 19:02 ` Waiman Long
2015-09-14 14:00 ` Peter Zijlstra
2015-09-14 19:15 ` Waiman Long
2015-09-14 19:38 ` Waiman Long
2015-09-15 8:24 ` Peter Zijlstra [this message]
2015-09-15 15:29 ` Waiman Long
2015-09-16 15:01 ` Peter Zijlstra
2015-09-17 15:08 ` Waiman Long
2015-09-14 14:04 ` Peter Zijlstra
2015-09-14 19:19 ` Waiman Long
2015-09-11 18:37 ` [PATCH v6 6/6] locking/pvqspinlock: Queue node adaptive spinning Waiman Long
2015-09-14 14:10 ` Peter Zijlstra
2015-09-14 19:37 ` Waiman Long
2015-09-15 8:38 ` Peter Zijlstra
2015-09-15 15:32 ` Waiman Long
2015-09-16 15:03 ` Peter Zijlstra
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