linux-kernel - Re: [PATCH 0/5] Generic Ticket Spinlocks

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Message-ID: <7dca0cb8-f0aa-a4cf-7f6f-0e4025527f5d@conchuod.ie>
Date:   Tue, 22 Mar 2022 20:19:12 +0000
From:   Conor Dooley <mail@...chuod.ie>
To:     Palmer Dabbelt <palmer@...osinc.com>
Cc:     linux-riscv@...ts.infradead.org, jonas@...thpole.se,
        stefan.kristiansson@...nalahti.fi, shorne@...il.com,
        mingo@...hat.com, Will Deacon <will@...nel.org>,
        longman@...hat.com, boqun.feng@...il.com,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, Arnd Bergmann <arnd@...db.de>,
        jszhang@...nel.org, wangkefeng.wang@...wei.com,
        openrisc@...ts.librecores.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, peterz@...radead.org
Subject: Re: [PATCH 0/5] Generic Ticket Spinlocks

On 22/03/2022 20:02, Palmer Dabbelt wrote:
> On Tue, 22 Mar 2022 11:18:18 PDT (-0700), mail@...chuod.ie wrote:
>> On 16/03/2022 23:25, Palmer Dabbelt wrote:
>>> Peter sent an RFC out about a year ago
>>> <https://lore.kernel.org/lkml/YHbBBuVFNnI4kjj3@hirez.programming.kicks-ass.net/>, 
>>>
>>> but after a spirited discussion it looks like we lost track of things.
>>> IIRC there was broad consensus on this being the way to go, but there
>>> was a lot of discussion so I wasn't sure.  Given that it's been a year,
>>> I figured it'd be best to just send this out again formatted a bit more
>>> explicitly as a patch.
>>>
>>> This has had almost no testing (just a build test on RISC-V defconfig),
>>> but I wanted to send it out largely as-is because I didn't have a SOB
>>> from Peter on the code.  I had sent around something sort of similar in
>>> spirit, but this looks completely re-written.  Just to play it safe I
>>> wanted to send out almost exactly as it was posted.  I'd probably rename
>>> this tspinlock and tspinlock_types, as the mis-match kind of makes my
>>> eyes go funny, but I don't really care that much.  I'll also go through
>>> the other ports and see if there's any more candidates, I seem to
>>> remember there having been more than just OpenRISC but it's been a
>>> while.
>>>
>>> I'm in no big rush for this and given the complex HW dependencies I
>>> think it's best to target it for 5.19, that'd give us a full merge
>>> window for folks to test/benchmark it on their systems to make sure it's
>>> OK.
>>
>> Is there a specific way you have been testing/benching things, or is it
>> just a case of test what we ourselves care about?
> 
> I do a bunch of functional testing in QEMU (it's all in my 
> riscv-systems-ci repo, but that's not really fit for human consumption 
> so I don't tell folks to use it).  That's pretty much useless for 
> something like this: sure it'd find something just straight-up broken in 
> the lock implementation, but the stuff I'm really worried about here 
> would be poor interactions with hardware that wasn't designed/tested 
> against this flavor of locks.
> 
> I don't currently do any regular testing on HW, but there's a handful of 
> folks who do.  If you've got HW you care about then the best bet is to 
> give this a shot on it.  There's already been some boot test reports, so 
> it's at least mostly there (on RISC-V, last I saw it was breaking 
> OpenRISC so there's probably some lurking issue somewhere).  I was 
> hoping we'd get enough coverage that way to have confidence in this, but 
> if not then I've got a bunch of RISC-V hardware lying around that I can 
> spin up to fill the gaps.

Aye, I'll at the very least boot it on an Icicle (which should *finally* 
be able to boot a mainline kernel with 5.18), but I don't think that'll 
be a problem.

> As far as what workloads, I really don't know here.  At least on RISC-V, 
> I think any lock microbenchmarks would be essentially meaningless: this 
> is fair, so even if lock/unlock is a bit slower that's probably a win 
> for real workloads.  That said, I'm not sure any of the existing 
> hardware runs any workloads that I'm personally interested in so unless 
> this is some massive hit to just general system responsiveness or 
> make/GCC then I'm probably not going to find anything.

There's a couple benchmarks we've been looking at, although I'm not sure 
that they are "real" workloads. If they encounter any meaningful 
difference I'll let you know I guess.


> Happy to hear if anyone has ideas, though.

Me too!