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Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Will Deacon <will@kernel.org>,
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
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Date:   Sat, 25 Jul 2020 13:36:29 -0400
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On 7/25/20 1:26 PM, Peter Zijlstra wrote:
> On Fri, Jul 24, 2020 at 03:10:59PM -0400, Waiman Long wrote:
>> On 7/24/20 4:16 AM, Will Deacon wrote:
>>> On Thu, Jul 23, 2020 at 08:47:59PM +0200, peterz@infradead.org wrote:
>>>> On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long wrote:
>>>>> BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch?
>>>>> I will have to update the patch to fix the reported 0-day test problem, but
>>>>> I want to collect other feedback before sending out v3.
>>>> I want to say I hate it all, it adds instructions to a path we spend an
>>>> aweful lot of time optimizing without really getting anything back for
>>>> it.
>>>>
>>>> Will, how do you feel about it?
>>> I can see it potentially being useful for debugging, but I hate the
>>> limitation to 256 CPUs. Even arm64 is hitting that now.
>> After thinking more about that, I think we can use all the remaining bits in
>> the 16-bit locked_pending. Reserving 1 bit for locked and 1 bit for pending,
>> there are 14 bits left. So as long as NR_CPUS < 16k (requirement for 16-bit
>> locked_pending), we can put all possible cpu numbers into the lock. We can
>> also just use smp_processor_id() without additional percpu data.
> That sounds horrific, wouldn't that destroy the whole point of using a
> byte for pending?
You are right. I realized that later on and had sent a follow-up mail to 
correct that.
>>> Also, you're talking ~1% gains here. I think our collective time would
>>> be better spent off reviewing the CNA series and trying to make it more
>>> deterministic.
>> I thought you guys are not interested in CNA. I do want to get CNA merged,
>> if possible. Let review the current version again and see if there are ways
>> we can further improve it.
> It's not a lack of interrest. We were struggling with the fairness
> issues and the complexity of the thing. I forgot the current state of
> matters, but at one point UNLOCK was O(n) in waiters, which is, of
> course, 'unfortunate'.
>
> I'll have to look up whatever notes remain, but the basic idea of
> keeping remote nodes on a secondary list is obviously breaking all sorts
> of fairness. After that they pile on a bunch of hacks to fix the worst
> of them, but it feels exactly like that, a bunch of hacks.
>
> One of the things I suppose we ought to do is see if some of the ideas
> of phase-fair locks can be applied to this.
That could be a possible solution to ensure better fairness.
>
> That coupled with a chronic lack of time for anything :-(
>
That is always true and I feel this way too:-)

Cheers,
Longman