linux-kernel - Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

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Date:   Tue, 4 Feb 2020 12:53:46 -0500
From:   Alex Kogan <alex.kogan@...cle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, linux@...linux.org.uk,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, hpa@...or.com, x86@...nel.org,
        Hanjun Guo <guohanjun@...wei.com>,
        Jan Glauber <jglauber@...vell.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        dave.dice@...cle.com
Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance
 into CNA



> On Feb 4, 2020, at 12:39 PM, Waiman Long <longman@...hat.com> wrote:
> 
> On 2/4/20 12:27 PM, Peter Zijlstra wrote:
>> On Tue, Feb 04, 2020 at 11:54:02AM -0500, Alex Kogan wrote:
>>>> On Feb 3, 2020, at 10:47 AM, Waiman Long <longman@...hat.com> wrote:
>>>> 
>>>> On 2/3/20 10:28 AM, Peter Zijlstra wrote:
>>>>> On Mon, Feb 03, 2020 at 09:59:12AM -0500, Waiman Long wrote:
>>>>>> On 2/3/20 8:45 AM, Peter Zijlstra wrote:
>>>>>>> Presumably you have a workload where CNA is actually a win? That is,
>>>>>>> what inspired you to go down this road? Which actual kernel lock is so
>>>>>>> contended on NUMA machines that we need to do this?
>>> There are quite a few actually. files_struct.file_lock, file_lock_context.flc_lock
>>> and lockref.lock are some concrete examples that get very hot in will-it-scale
>>> benchmarks. 
>> Right, that's all a variant of banging on the same resources across
>> nodes. I'm not sure there's anything fundamental we can fix there.
Not much, except gain that 2x from a better lock.

>> 
>>> And then there are spinlocks in __futex_data.queues, 
>>> which get hot when applications have contended (pthread) locks — 
>>> LevelDB is an example.
>> A numa aware rework of futexes has been on the todo list for years :/
> Now, we are going to get that for free with this patchset:-)
Exactly!!

>> 
>>> Our initial motivation was based on an observation that kernel qspinlock is not 
>>> NUMA-aware. So what, you may ask. Much like people realized in the past that
>>> global spinning is bad for performance, and they switched from ticket lock to
>>> locks with local spinning (e.g., MCS), I think everyone would agree these days that
>>> bouncing a lock (and cache lines in general) across numa nodes is similarly bad.
>>> And as CNA demonstrates, we are easily leaving 2-3x speedups on the table by
>>> doing just that with the current qspinlock.
>> Actual benchmarks with performance numbers are required. It helps
>> motivate the patches as well as gives reviewers clues on how to
>> reproduce / inspect the claims made.
>> 
> I think the cover-letter does have some benchmark results listed.
To clarify on that, I _used to include benchmark results in the cover letter 
for previous revisions. I stopped doing that as the changes between revisions
were rather minor — maybe that is the missing part? If so, my apologies, I can
certainly publish them again.

The point is that we have numbers for actual benchmarks, plus the kernel build
robot has sent quite a few reports on positive improvements in the performance
of AIM7 and other benchmarks due to CNA (plus ARM folks noticed improvement
in their benchmarks, although I think those were mostly microbenchmarks. 
Yet, it is evident that the improvements are cross-platform.)

Regards,
— Alex