Messages in this thread |  | | Subject | Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR | From | Waiman Long <> | 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
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