From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hp.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 v2 6/6] locking/pvqspinlock: Queue node adaptive spinning
Date: Wed, 15 Jul 2015 12:01:41 +0200 [thread overview]
Message-ID: <20150715100141.GI2859@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <1436926417-20256-7-git-send-email-Waiman.Long@hp.com>
On Tue, Jul 14, 2015 at 10:13:37PM -0400, Waiman Long wrote:
> +static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> {
> struct pv_node *pn = (struct pv_node *)node;
> + struct pv_node *pp = (struct pv_node *)prev;
> + bool wait_early, can_wait_early;
> int loop;
>
> for (;;) {
> - for (loop = SPIN_THRESHOLD; loop; loop--) {
> + /*
> + * Spin less if the previous vCPU was in the halted state
> + * and it is not the queue head.
> + */
> + can_wait_early = (pn->waithist > PV_WAITHIST_THRESHOLD);
> + wait_early = can_wait_early && !READ_ONCE(prev->locked) &&
> + (READ_ONCE(pp->state) == vcpu_halted);
> + loop = wait_early ? QNODE_SPIN_THRESHOLD_SHORT
> + : QNODE_SPIN_THRESHOLD;
> + for (; loop; loop--, cpu_relax()) {
> + bool halted;
> +
> if (READ_ONCE(node->locked))
> return;
> - cpu_relax();
> +
> + if (!can_wait_early || (loop & QNODE_SPIN_CHECK_MASK))
> + continue;
> +
> + /*
> + * Look for state transition at previous node.
> + *
> + * running => halted:
> + * call pv_wait() now if kick-ahead is enabled
> + * or reduce spin threshold to
> + * QNODE_SPIN_THRESHOLD_SHORT or less.
> + * halted => running:
> + * reset spin threshold to QNODE_SPIN_THRESHOLD
> + */
> + halted = (READ_ONCE(pp->state) == vcpu_halted) &&
> + !READ_ONCE(prev->locked);
> + if (wait_early == halted)
> + continue;
> + wait_early = halted;
> +
> + if (!wait_early)
> + loop = QNODE_SPIN_THRESHOLD;
> + else if (pv_kick_ahead)
> + break;
> + else if (loop > QNODE_SPIN_THRESHOLD_SHORT)
> + loop = QNODE_SPIN_THRESHOLD_SHORT;
> }
> + if (wait_early)
> + pvstat_inc(pvstat_wait_early);
> +
> + /*
> + * A pv_wait while !wait_early has higher weight than when
> + * wait_early is true.
> + */
> + if (pn->waithist < PV_WAITHIST_MAX)
> + pn->waithist += wait_early ? 1 : PV_WAIT_INC;
So when you looked at this patch, you didn't go like, OMFG!?
So what was wrong with something like:
static inline int pv_spin_threshold(struct pv_node *prev)
{
if (READ_ONCE(prev->locked)) /* it can run, wait for it */
return SPIN_THRESHOLD;
if (READ_ONCE(prev->state) == vcpu_halted) /* its not running, do not wait */
return 1;
return SPIN_THRESHOLD;
}
static void pv_wait_head(...)
{
for (;;) {
for (loop = pv_spin_threshold(pp); loop; loop--) {
if (READ_ONCE(node->locked))
return;
cpu_relax();
}
if (!lp) {
...
}
pv_wait(&l->locked, _Q_SLOW_VAL);
}
}
What part of: "keep it simple" and "gradual complexity" have you still
not grasped?
next prev parent reply other threads:[~2015-07-15 10:02 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-15 2:13 [PATCH 0/6 v2] locking/qspinlock: Enhance pvqspinlock performance Waiman Long
2015-07-15 2:13 ` [PATCH v2 1/6] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL Waiman Long
2015-07-15 9:10 ` Peter Zijlstra
2015-07-16 0:18 ` Waiman Long
2015-07-16 5:42 ` Peter Zijlstra
2015-07-16 14:07 ` Waiman Long
2015-07-16 15:04 ` Waiman Long
2015-07-16 15:10 ` Will Deacon
2015-08-03 16:59 ` [tip:locking/core] locking/Documentation: Clarify failed cmpxchg( ) memory ordering semantics tip-bot for Will Deacon
2015-08-03 17:36 ` Davidlohr Bueso
2015-07-15 2:13 ` [PATCH v2 2/6] locking/pvqspinlock: Add pending bit support Waiman Long
2015-07-15 2:13 ` [PATCH v2 3/6] locking/pvqspinlock: Collect slowpath lock statistics Waiman Long
2015-07-15 2:13 ` [PATCH v2 4/6] locking/pvqspinlock: Allow vCPUs kick-ahead Waiman Long
2015-07-15 9:39 ` Peter Zijlstra
2015-07-16 2:01 ` Waiman Long
2015-07-16 5:46 ` Peter Zijlstra
2015-07-16 14:51 ` Waiman Long
2015-07-15 2:13 ` [PATCH v2 5/6] locking/pvqspinlock: Opportunistically defer kicking to unlock time Waiman Long
2015-07-15 6:14 ` Raghavendra K T
2015-07-15 10:03 ` Peter Zijlstra
2015-07-16 2:18 ` Waiman Long
2015-07-16 5:49 ` Peter Zijlstra
2015-07-15 2:13 ` [PATCH v2 6/6] locking/pvqspinlock: Queue node adaptive spinning Waiman Long
2015-07-15 10:01 ` Peter Zijlstra [this message]
2015-07-16 2:13 ` Waiman Long
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