linux-kernel - Re: Some observations (results) on BPF acquire and release

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Message-ID: <727cbf48-0135-de6b-6d05-a6f8f7b4b853@huaweicloud.com>
Date: Fri, 25 Oct 2024 17:00:20 +0200
From: Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: puranjay@...nel.org, paulmck@...nel.org, bpf@...r.kernel.org,
 lkmm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: Some observations (results) on BPF acquire and release

On 10/25/2024 4:27 PM, Andrea Parri wrote:
>> I am particularly interested in tests using lwarx and stwcx instructions
>> (this is what I understood would be used if one follows [1] to compile the
>> tests in this thread).
>>
>> I have not yet check the cambridge website, but due to the timeline, I don't
>> expect to find tests with those instructions. The same is true with [2].
>>
>> I have limited experience with diy7, but I remember that it had some
>> limitations to generate RMW instructions, at least for C [3].
> 
> Oh, I'm sure there are, though I'd also not consider myself the 'expert'
> when it comes to diy7 internals.  ;-)  Here's an example use of diy7 /
> diyone7 generating lwarx and stwcx and reflecting the previous pattern:
> 
> $ diyone7 -arch PPC LwSyncdWW Coe SyncdWRPA SyncdRRAP Fre
> PPC A
> "LwSyncdWW Coe SyncdWRNaA SyncdRRANa Fre"
> Generator=diyone7 (version 7.57+1)
> Prefetch=0:x=F,0:y=W,1:y=F,1:x=T
> Com=Co Fr
> Orig=LwSyncdWW Coe SyncdWRNaA SyncdRRANa Fre
> {
> 0:r2=x; 0:r4=y;
> 1:r2=y; 1:r3=z; 1:r6=x;
> }
>   P0           | P1              ;
>   li r1,1      | li r1,2         ;
>   stw r1,0(r2) | stw r1,0(r2)    ;
>   lwsync       | sync            ;
>   li r3,1      | Loop00:         ;
>   stw r3,0(r4) | lwarx r4,r0,r3  ;
>                | stwcx. r4,r0,r3 ;
>                | bne  Loop00     ;
>                | sync            ;
>                | lwz r5,0(r6)    ;
> exists ([y]=2 /\ 1:r5=0)

That is exactly what I was looking for. Thanks Andrea!

Hernan

> 
> But again, I'd probably have to defer to proper herdtools7 developers
> and maintainers for any diy7 bug or misbehavior you'd have to discover.
> 
>    Andrea
> 
> 
>>
>> Hernan
>>
>> [1] https://github.com/torvalds/linux/blob/master/arch/powerpc/net/bpf_jit_comp32.c
>> [2] https://github.com/herd/herdtools7/tree/master/catalogue/herding-cats/ppc/tests/campaign
>> [3] https://github.com/herd/herdtools7/issues/905
>>