linux-kernel - Re: [PATCH V7 4/5] asm-generic: spinlock: Add combo spinlock (ticket & queued)

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Date:   Wed, 29 Jun 2022 09:17:30 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V7 4/5] asm-generic: spinlock: Add combo spinlock (ticket
 & queued)

On Wed, Jun 29, 2022 at 2:13 AM Waiman Long <longman@...hat.com> wrote:
>
> On 6/28/22 04:17, guoren@...nel.org wrote:
> > From: Guo Ren <guoren@...ux.alibaba.com>
> >
> > Some architecture has a flexible requirement on the type of spinlock.
> > Some LL/SC architectures of ISA don't force micro-arch to give a strong
> > forward guarantee. Thus different kinds of memory model micro-arch would
> > come out in one ISA. The ticket lock is suitable for exclusive monitor
> > designed LL/SC micro-arch with limited cores and "!NUMA". The
> > queue-spinlock could deal with NUMA/large-scale scenarios with a strong
> > forward guarantee designed LL/SC micro-arch.
> >
> > So, make the spinlock a combo with feature.
> >
> > Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@...nel.org>
> > Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Palmer Dabbelt <palmer@...osinc.com>
> > ---
> >   include/asm-generic/spinlock.h | 43 ++++++++++++++++++++++++++++++++--
> >   kernel/locking/qspinlock.c     |  2 ++
> >   2 files changed, 43 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> > index f41dc7c2b900..a9b43089bf99 100644
> > --- a/include/asm-generic/spinlock.h
> > +++ b/include/asm-generic/spinlock.h
> > @@ -28,34 +28,73 @@
> >   #define __ASM_GENERIC_SPINLOCK_H
> >
> >   #include <asm-generic/ticket_spinlock.h>
> > +#ifdef CONFIG_ARCH_USE_QUEUED_SPINLOCKS
> > +#include <linux/jump_label.h>
> > +#include <asm-generic/qspinlock.h>
> > +
> > +DECLARE_STATIC_KEY_TRUE(use_qspinlock_key);
> > +#endif
> > +
> > +#undef arch_spin_is_locked
> > +#undef arch_spin_is_contended
> > +#undef arch_spin_value_unlocked
> > +#undef arch_spin_lock
> > +#undef arch_spin_trylock
> > +#undef arch_spin_unlock
> >
> >   static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
> >   {
> > -     ticket_spin_lock(lock);
> > +#ifdef CONFIG_ARCH_USE_QUEUED_SPINLOCKS
> > +     if (static_branch_likely(&use_qspinlock_key))
> > +             queued_spin_lock(lock);
> > +     else
> > +#endif
> > +             ticket_spin_lock(lock);
> >   }
>
> Why do you use a static key to control whether to use qspinlock or
> ticket lock? In the next patch, you have
>
> +#if !defined(CONFIG_NUMA) && defined(CONFIG_QUEUED_SPINLOCKS)
> +       static_branch_disable(&use_qspinlock_key);
> +#endif
>
> So the current config setting determines if qspinlock will be used, not
> some boot time parameter that user needs to specify. This patch will
> just add useless code to lock/unlock sites. I don't see any benefit of
> doing that.
This is a startup patch for riscv. next, we could let vendors make choices.
I'm not sure they like cmdline or vendor-specific errata style.

Eventually, we would let one riscv Image support all machines, some
use ticket-lock, and some use qspinlock.

>
> Cheers,
> Longman
>



--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/