From: Waiman Long <waiman.long@hpe.com>
To: Peter Zijlstra <peterz@infradead.org>
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: Thu, 17 Sep 2015 11:08:43 -0400 [thread overview]
Message-ID: <55FAD77B.3040100@hpe.com> (raw)
In-Reply-To: <20150916150131.GC3604@twins.programming.kicks-ass.net>
On 09/16/2015 11:01 AM, Peter Zijlstra wrote:
> On Tue, Sep 15, 2015 at 11:29:14AM -0400, Waiman Long wrote:
>
>> Only the queue head vCPU will be in pv_wait_head() spinning to acquire the
>> lock.
> But what will guarantee fwd progress for the lock that is the head?
>
> Suppose CPU0 becomes head and enters the /* claim the lock */ loop.
>
> Then CPU1 comes in, steals it in pv_wait_head(). CPU1 releases, CPU1
> re-acquires and _again_ steals in pv_wait_head(), etc..
>
> All the while CPU0 doesn't go anywhere.
>
That can't happen. For a given lock, there can only be 1 queue head
spinning on the lock at any instance in time. If CPU0 was the head,
another CPU could not become head until CPU0 got the lock and pass the
MCS lock bit to the next one in the queue. As I said in earlier mail,
the only place where lock stealing can happen is in the
pv_queued_spin_trylock_unfair() function where I purposely inserted a
cpu_relax() to allow an actively spinning queue head CPU a better chance
of getting the lock. Once a CPU enters the queue. It won't try to
acquire the lock until it becomes the head and there is one and only one
head.
Cheers,
Longman
next prev parent reply other threads:[~2015-09-17 15:08 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
2015-09-15 15:29 ` Waiman Long
2015-09-16 15:01 ` Peter Zijlstra
2015-09-17 15:08 ` Waiman Long [this message]
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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