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

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Date:   Wed, 29 Jun 2022 10:29:50 +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 9:34 AM Waiman Long <longman@...hat.com> wrote:
>
> On 6/28/22 21:17, Guo Ren wrote:
> > 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.
>
> OK. Maybe you can postpone this combo spinlock until there is a good use
> case for it. Upstream usually don't accept patches that have no good use
> case yet.
>
> Cheers,
> Longman
>

I would add a cmdline to control the choice of qspinlock/ticket-lock
in the next version.

diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index b9b234157a66..5ade490c2f27 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -270,6 +270,10 @@ void __init setup_arch(char **cmdline_p)

        early_ioremap_setup();
        jump_label_init();
+
+#if !defined(CONFIG_NUMA) && defined(CONFIG_QUEUED_SPINLOCKS)
+       static_branch_disable(&use_qspinlock_key);
+#endif
        parse_early_param();

        efi_init();
@@ -295,10 +299,6 @@ void __init setup_arch(char **cmdline_p)
        setup_smp();
 #endif

-#if !defined(CONFIG_NUMA) && defined(CONFIG_QUEUED_SPINLOCKS)
-       static_branch_disable(&use_qspinlock_key);
-#endif
-
        riscv_fill_hwcap();
        apply_boot_alternatives();
 }
@@ -330,3 +330,13 @@ void free_initmem(void)

        free_initmem_default(POISON_FREE_INITMEM);
 }
+
+#ifdef CONFIG_QUEUED_SPINLOCKS
+static int __init disable_qspinlock(char *p)
+{
+       static_branch_disable(&use_qspinlock_key);
+       return 0;
+}
+
+early_param("disable_qspinlock", disable_qspinlock);
+#endif


-- 
Best Regards
 Guo Ren

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