Re: [PATCH v16 13/14] pvqspinlock: Improve slowpath performance by avoiding cmpxchg

From: Waiman Long <waiman.long@hp.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org, Rik van Riel <riel@redhat.com>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	Oleg Nesterov <oleg@redhat.com>,
	kvm@vger.kernel.org, Daniel J Blueman <daniel@numascale.com>,
	x86@kernel.org, Paolo Bonzini <paolo.bonzini@gmail.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	Scott J Norton <scott.norton@hp.com>,
	Ingo Molnar <mingo@redhat.com>,
	David Vrabel <david.vrabel@citrix.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	xen-devel@lists.xenproject.org,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [PATCH v16 13/14] pvqspinlock: Improve slowpath performance by avoiding cmpxchg
Date: Mon, 04 May 2015 13:18:17 -0400	[thread overview]
Message-ID: <5547A9D9.3020205__18802.7335251265$1430760004$gmane$org@hp.com> (raw)
In-Reply-To: <20150504140551.GJ23123@twins.programming.kicks-ass.net>

On 05/04/2015 10:05 AM, Peter Zijlstra wrote:
> On Thu, Apr 30, 2015 at 02:49:26PM -0400, Waiman Long wrote:
>> On 04/29/2015 02:11 PM, Peter Zijlstra wrote:
>>> On Fri, Apr 24, 2015 at 02:56:42PM -0400, Waiman Long wrote:
>>>> In the pv_scan_next() function, the slow cmpxchg atomic operation is
>>>> performed even if the other CPU is not even close to being halted. This
>>>> extra cmpxchg can harm slowpath performance.
>>>>
>>>> This patch introduces the new mayhalt flag to indicate if the other
>>>> spinning CPU is close to being halted or not. The current threshold
>>>> for x86 is 2k cpu_relax() calls. If this flag is not set, the other
>>>> spinning CPU will have at least 2k more cpu_relax() calls before
>>>> it can enter the halt state. This should give enough time for the
>>>> setting of the locked flag in struct mcs_spinlock to propagate to
>>>> that CPU without using atomic op.
>>> Yuck! I'm not at all sure you can make assumptions like that. And the
>>> worst part is, if it goes wrong the borkage is subtle and painful.
>> I do think the code is OK. However, you are right that if my reasoning is
>> incorrect, the resulting bug will be really subtle.
> So I do not think its correct, imagine the fabrics used for the 4096 cpu
> SGI machine, now add some serious traffic to them. There is no saying
> your random 2k relax loop will be enough to propagate the change.
>
> Equally, another arch (this is generic code) might have starvation
> issues on its inter-cpu fabric and delay the store just long enough.
>
> The thing is, one should _never_ rely on timing for correctness, _ever_.
>

Yes, you are right. Having a dependency on timing can be dangerous.

>> So I am going to
>> withdraw this particular patch as it has no functional impact to the overall
>> patch series. Please let me know if you have any other comments on other
>> parts of the series and I will send send out a new series without this
>> particular patch.
> Please wait a little while, I've queued the 'basic' patches, once that
> settles in tip we can look at the others.
>
> Also, I have some local changes (sorry, I could not help mysef) I should
> post, I've been somewhat delayed by illness.

Sure. I will wait until you finish your tip test.

I am sorry to hear that you are bothered with illness. I hope you get 
well by now.

Cheers,
Longman