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Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for
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To:     Will Deacon <will@kernel.org>, peterz@infradead.org
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <8532332b-85dd-661b-cf72-81a8ceb70747@redhat.com>
Date:   Fri, 24 Jul 2020 15:10:59 -0400
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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.

>
> 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.

Cheers,
Longman