linux-kernel - Re: [PATCH 2/2] locking/pvqspinlock: Optionally store lock holder cpu into lock

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Date:   Mon, 13 Jul 2020 14:17:58 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>, x86@...nel.org
Subject: Re: [PATCH 2/2] locking/pvqspinlock: Optionally store lock holder cpu
 into lock

Excerpts from Waiman Long's message of July 13, 2020 9:05 am:
> On 7/12/20 1:34 PM, Peter Zijlstra wrote:
>> On Sat, Jul 11, 2020 at 02:21:28PM -0400, Waiman Long wrote:
>>> The previous patch enables native qspinlock to store lock holder cpu
>>> number into the lock word when the lock is acquired via the slowpath.
>>> Since PV qspinlock uses atomic unlock, allowing the fastpath and
>>> slowpath to put different values into the lock word will further slow
>>> down the performance. This is certainly undesirable.
>>>
>>> The only way we can do that without too much performance impact is to
>>> make fastpath and slowpath put in the same value. Still there is a slight
>>> performance overhead in the additional access to a percpu variable in the
>>> fastpath as well as the less optimized x86-64 PV qspinlock unlock path.
>>>
>>> A new config option QUEUED_SPINLOCKS_CPUINFO is now added to enable
>>> distros to decide if they want to enable lock holder cpu information in
>>> the lock itself for both native and PV qspinlocks across both fastpath
>>> and slowpath. If this option is not configureed, only native qspinlocks
>>> in the slowpath will put the lock holder cpu information in the lock
>>> word.
>> And this kills it,.. if it doesn't make unconditional sense, we're not
>> going to do this. It's just too ugly.
>>
> You mean it has to be unconditional, no option config if we want to do 
> it. Right?
> 
> It can certainly be made unconditional after I figure out how to make 
> the optimized PV unlock code work.

Sorry I've not had a lot of time to get back to this thread and test
things -- don't spend loads of effort or complexity on it until we get
some more numbers. I did see some worse throughput results (with no
attention to fairness) with the PV spin lock, but it was a really quick
limited few tests, I need to get something a bit more substantial.

I do very much appreciate your help with the powerpc patches, and
interest in the PV issues though. I'll try to find more time to help
out.

Thanks,
Nick