linux-kernel - Re: [PATCH v15 3/6] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Thu, 3 Aug 2023 13:56:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        mingo@...hat.com, will.deacon@....com, arnd@...db.de,
        longman@...hat.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com
Subject: Re: [PATCH v15 3/6] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On Thu, Aug 03, 2023 at 06:28:51PM +0800, Guo Ren wrote:
> On Thu, Aug 3, 2023 at 4:50 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Wed, Aug 02, 2023 at 07:14:05PM -0400, Guo Ren wrote:
> >
> > > The pv_ops is belongs to x86 custom frame work, and it prevent other
> > > architectures connect to the CNA spinlock.
> >
> > static_call() exists as a arch neutral variant of this.
> Emm... we have used static_call() in the riscv queued_spin_lock_:
> https://lore.kernel.org/all/20230802164701.192791-20-guoren@kernel.org/

Yeah, I think I saw that land in the INBOX, just haven't had time to
look at it.

> But we met a compile problem:
> 
>   GEN     .vmlinux.objs
>   MODPOST Module.symvers
> ERROR: modpost: "__SCK__pv_queued_spin_unlock" [arch/riscv/kvm/kvm.ko]
> undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock"
> [kernel/locking/locktorture.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock" [mm/z3fold.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock"
> [fs/nfs_common/grace.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock" [fs/quota/quota_v1.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock" [fs/quota/quota_v2.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock"
> [fs/quota/quota_tree.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock" [fs/fuse/virtiofs.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock" [fs/dlm/dlm.ko] undefined!
> ERROR: modpost: "__SCK__pv_queued_spin_unlock" [fs/fscache/fscache.ko]
> undefined!
> WARNING: modpost: suppressed 839 unresolved symbol warnings because
> there were too many)
> /home/guoren/source/kernel/linux/scripts/Makefile.modpost:144: recipe
> for target 'Module.symvers' failed
> 
> Our solution is:
> EXPORT_SYMBOL(__SCK__pv_queued_spin_unlock);
> 
> What do you think about it?

Could be you're not using static_call_mod() to go with
EXPORT_STATIC_CALL_TRAMP()

> > > I'm working on riscv qspinlock on sg2042 64 cores 2/4 NUMA nodes
> > > platforms. Here are the patches about riscv CNA qspinlock:
> > > https://lore.kernel.org/linux-riscv/20230802164701.192791-19-guoren@kernel.org/
> > >
> > > What's the next plan for this patch series? I think the two-queue design
> > > has satisfied most platforms with two NUMA nodes.
> >
> > What has been your reason for working on CNA? What lock has been so
> > contended you need this?
> I wrote the reason here:
> https://lore.kernel.org/all/20230802164701.192791-1-guoren@kernel.org/
> 
> The target platform is: https://www.sophon.ai/
> 
> The two NUMA nodes platform has come out, so we want to measure the
> benefit of CNA qspinlock.

CNA should only show a benefit when there is strong inter-node
contention, and in that case it is typically best to fix the kernel side
locking.

Hence the question as to what lock prompted you to look at this.