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

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Date:   Tue, 4 Feb 2020 12:39:30 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Alex Kogan <alex.kogan@...cle.com>
Cc:     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 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.
>
>> 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:-)
>
>> 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.  Are
you saying that some benchmark results should be put into individual
patches themselves?

Cheers,
Longman