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Message-ID: <CAHVXubgtw3rZq1+jNv2LrsQBhViu4Sm9Gw3B_7-XLzBw52x6oQ@mail.gmail.com>
Date: Wed, 28 Aug 2024 10:16:40 +0200
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Andrew Jones <ajones@...tanamicro.com>
Cc: Jonathan Corbet <corbet@....net>, Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>,
Krzysztof Kozlowski <krzk+dt@...nel.org>, Andrea Parri <parri.andrea@...il.com>,
Nathan Chancellor <nathan@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
Arnd Bergmann <arnd@...db.de>, Leonardo Bras <leobras@...hat.com>, Guo Ren <guoren@...nel.org>,
linux-doc@...r.kernel.org, devicetree@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
linux-arch@...r.kernel.org
Subject: Re: [PATCH v5 13/13] riscv: Add qspinlock support
Hi drew,
On Wed, Aug 21, 2024 at 4:51 PM Andrew Jones <ajones@...tanamicro.com> wrote:
>
> On Sun, Aug 18, 2024 at 08:35:38AM GMT, Alexandre Ghiti wrote:
> > In order to produce a generic kernel, a user can select
> > CONFIG_COMBO_SPINLOCKS which will fallback at runtime to the ticket
> > spinlock implementation if Zabha or Ziccrse are not present.
> >
> > Note that we can't use alternatives here because the discovery of
> > extensions is done too late and we need to start with the qspinlock
> > implementation because the ticket spinlock implementation would pollute
> > the spinlock value, so let's use static keys.
> >
> > This is largely based on Guo's work and Leonardo reviews at [1].
> >
> > Link: https://lore.kernel.org/linux-riscv/20231225125847.2778638-1-guoren@kernel.org/ [1]
> > Signed-off-by: Guo Ren <guoren@...nel.org>
> > Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> > ---
> > .../locking/queued-spinlocks/arch-support.txt | 2 +-
> > arch/riscv/Kconfig | 34 ++++++++++++++
> > arch/riscv/include/asm/Kbuild | 4 +-
> > arch/riscv/include/asm/spinlock.h | 47 +++++++++++++++++++
> > arch/riscv/kernel/setup.c | 37 +++++++++++++++
> > include/asm-generic/qspinlock.h | 2 +
> > include/asm-generic/ticket_spinlock.h | 2 +
> > 7 files changed, 126 insertions(+), 2 deletions(-)
> > create mode 100644 arch/riscv/include/asm/spinlock.h
> >
> > diff --git a/Documentation/features/locking/queued-spinlocks/arch-support.txt b/Documentation/features/locking/queued-spinlocks/arch-support.txt
> > index 22f2990392ff..cf26042480e2 100644
> > --- a/Documentation/features/locking/queued-spinlocks/arch-support.txt
> > +++ b/Documentation/features/locking/queued-spinlocks/arch-support.txt
> > @@ -20,7 +20,7 @@
> > | openrisc: | ok |
> > | parisc: | TODO |
> > | powerpc: | ok |
> > - | riscv: | TODO |
> > + | riscv: | ok |
> > | s390: | TODO |
> > | sh: | TODO |
> > | sparc: | ok |
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index ef55ab94027e..201d0669db7f 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -79,6 +79,7 @@ config RISCV
> > select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP
> > select ARCH_WANTS_NO_INSTR
> > select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
> > + select ARCH_WEAK_RELEASE_ACQUIRE if ARCH_USE_QUEUED_SPINLOCKS
>
> Hi Alex,
>
> Did you get a chance to experiment this as suggested by Andrea?
We talked about it and that will be a separate patch (not sure when
though, I don't feel really comfortable sending what Andrea
suggested).
>
> > select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
> > select BUILDTIME_TABLE_SORT if MMU
> > select CLINT_TIMER if RISCV_M_MODE
> > @@ -488,6 +489,39 @@ config NODES_SHIFT
> > Specify the maximum number of NUMA Nodes available on the target
> > system. Increases memory reserved to accommodate various tables.
> >
> > +choice
> > + prompt "RISC-V spinlock type"
> > + default RISCV_COMBO_SPINLOCKS
> > +
> > +config RISCV_TICKET_SPINLOCKS
> > + bool "Using ticket spinlock"
> > +
> > +config RISCV_QUEUED_SPINLOCKS
> > + bool "Using queued spinlock"
> > + depends on SMP && MMU && NONPORTABLE
> > + select ARCH_USE_QUEUED_SPINLOCKS
> > + help
> > + The queued spinlock implementation requires the forward progress
> > + guarantee of cmpxchg()/xchg() atomic operations: CAS with Zabha or
> > + LR/SC with Ziccrse provide such guarantee.
> > +
> > + Select this if and only if Zabha or Ziccrse is available on your
> > + platform, RISCV_QUEUED_SPINLOCKS must not be selected for platforms
> > + without one of those extensions.
> > +
> > + If unsure, select RISCV_COMBO_SPINLOCKS, which will use qspinlocks
> > + when supported and otherwise ticket spinlocks.
> > +
> > +config RISCV_COMBO_SPINLOCKS
> > + bool "Using combo spinlock"
> > + depends on SMP && MMU
> > + select ARCH_USE_QUEUED_SPINLOCKS
> > + help
> > + Embed both queued spinlock and ticket lock so that the spinlock
> > + implementation can be chosen at runtime.
> > +
> > +endchoice
> > +
> > config RISCV_ALTERNATIVE
> > bool
> > depends on !XIP_KERNEL
> > diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
> > index 5c589770f2a8..1c2618c964f0 100644
> > --- a/arch/riscv/include/asm/Kbuild
> > +++ b/arch/riscv/include/asm/Kbuild
> > @@ -5,10 +5,12 @@ syscall-y += syscall_table_64.h
> > generic-y += early_ioremap.h
> > generic-y += flat.h
> > generic-y += kvm_para.h
> > +generic-y += mcs_spinlock.h
> > generic-y += parport.h
> > -generic-y += spinlock.h
> > generic-y += spinlock_types.h
> > +generic-y += ticket_spinlock.h
> > generic-y += qrwlock.h
> > generic-y += qrwlock_types.h
> > +generic-y += qspinlock.h
> > generic-y += user.h
> > generic-y += vmlinux.lds.h
> > diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
> > new file mode 100644
> > index 000000000000..e5121b89acea
> > --- /dev/null
> > +++ b/arch/riscv/include/asm/spinlock.h
> > @@ -0,0 +1,47 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef __ASM_RISCV_SPINLOCK_H
> > +#define __ASM_RISCV_SPINLOCK_H
> > +
> > +#ifdef CONFIG_RISCV_COMBO_SPINLOCKS
> > +#define _Q_PENDING_LOOPS (1 << 9)
> > +
> > +#define __no_arch_spinlock_redefine
> > +#include <asm/ticket_spinlock.h>
> > +#include <asm/qspinlock.h>
> > +#include <asm/jump_label.h>
> > +
> > +/*
> > + * TODO: Use an alternative instead of a static key when we are able to parse
> > + * the extensions string earlier in the boot process.
> > + */
> > +DECLARE_STATIC_KEY_TRUE(qspinlock_key);
> > +
> > +#define SPINLOCK_BASE_DECLARE(op, type, type_lock) \
> > +static __always_inline type arch_spin_##op(type_lock lock) \
> > +{ \
> > + if (static_branch_unlikely(&qspinlock_key)) \
> > + return queued_spin_##op(lock); \
> > + return ticket_spin_##op(lock); \
> > +}
>
> I guess there were still a couple questions on the kernel size impact of
> this.
>
> > +
> > +SPINLOCK_BASE_DECLARE(lock, void, arch_spinlock_t *)
> > +SPINLOCK_BASE_DECLARE(unlock, void, arch_spinlock_t *)
> > +SPINLOCK_BASE_DECLARE(is_locked, int, arch_spinlock_t *)
> > +SPINLOCK_BASE_DECLARE(is_contended, int, arch_spinlock_t *)
> > +SPINLOCK_BASE_DECLARE(trylock, bool, arch_spinlock_t *)
> > +SPINLOCK_BASE_DECLARE(value_unlocked, int, arch_spinlock_t)
> > +
> > +#elif defined(CONFIG_RISCV_QUEUED_SPINLOCKS)
> > +
> > +#include <asm/qspinlock.h>
> > +
> > +#else
> > +
> > +#include <asm/ticket_spinlock.h>
> > +
> > +#endif
> > +
> > +#include <asm/qrwlock.h>
> > +
> > +#endif /* __ASM_RISCV_SPINLOCK_H */
> > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > index a2cde65b69e9..438e4f6ad2ad 100644
> > --- a/arch/riscv/kernel/setup.c
> > +++ b/arch/riscv/kernel/setup.c
> > @@ -244,6 +244,42 @@ static void __init parse_dtb(void)
> > #endif
> > }
> >
> > +#if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
> > +DEFINE_STATIC_KEY_TRUE(qspinlock_key);
> > +EXPORT_SYMBOL(qspinlock_key);
> > +#endif
> > +
> > +static void __init riscv_spinlock_init(void)
> > +{
> > + char *using_ext = NULL;
> > +
> > + if (IS_ENABLED(CONFIG_RISCV_TICKET_SPINLOCKS)) {
> > + pr_info("Ticket spinlock: enabled\n");
> > + return;
> > + }
> > +
> > + if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) &&
> > + IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) &&
> > + riscv_isa_extension_available(NULL, ZABHA) &&
> > + riscv_isa_extension_available(NULL, ZACAS)) {
> > + using_ext = "using Zabha";
> > + } else if (riscv_isa_extension_available(NULL, ZICCRSE)) {
> > + using_ext = "using Ziccrse";
> > + }
> > +#if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
> > + else {
> > + static_branch_disable(&qspinlock_key);
> > + pr_info("Ticket spinlock: enabled\n");
> > + return;
> > + }
> > +#endif
> > +
> > + if (!using_ext)
> > + pr_err("Queued spinlock without Zabha or Ziccrse");
> > + else
> > + pr_info("Queued spinlock %s: enabled\n", using_ext);
> > +}
> > +
> > extern void __init init_rt_signal_env(void);
> >
> > void __init setup_arch(char **cmdline_p)
> > @@ -297,6 +333,7 @@ void __init setup_arch(char **cmdline_p)
> > riscv_set_dma_cache_alignment();
> >
> > riscv_user_isa_enable();
> > + riscv_spinlock_init();
> > }
> >
> > bool arch_cpu_is_hotpluggable(int cpu)
> > diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> > index 0655aa5b57b2..bf47cca2c375 100644
> > --- a/include/asm-generic/qspinlock.h
> > +++ b/include/asm-generic/qspinlock.h
> > @@ -136,6 +136,7 @@ static __always_inline bool virt_spin_lock(struct qspinlock *lock)
> > }
> > #endif
> >
> > +#ifndef __no_arch_spinlock_redefine
> > /*
> > * Remapping spinlock architecture specific functions to the corresponding
> > * queued spinlock functions.
> > @@ -146,5 +147,6 @@ static __always_inline bool virt_spin_lock(struct qspinlock *lock)
> > #define arch_spin_lock(l) queued_spin_lock(l)
> > #define arch_spin_trylock(l) queued_spin_trylock(l)
> > #define arch_spin_unlock(l) queued_spin_unlock(l)
> > +#endif
> >
> > #endif /* __ASM_GENERIC_QSPINLOCK_H */
> > diff --git a/include/asm-generic/ticket_spinlock.h b/include/asm-generic/ticket_spinlock.h
> > index cfcff22b37b3..325779970d8a 100644
> > --- a/include/asm-generic/ticket_spinlock.h
> > +++ b/include/asm-generic/ticket_spinlock.h
> > @@ -89,6 +89,7 @@ static __always_inline int ticket_spin_is_contended(arch_spinlock_t *lock)
> > return (s16)((val >> 16) - (val & 0xffff)) > 1;
> > }
> >
> > +#ifndef __no_arch_spinlock_redefine
> > /*
> > * Remapping spinlock architecture specific functions to the corresponding
> > * ticket spinlock functions.
> > @@ -99,5 +100,6 @@ static __always_inline int ticket_spin_is_contended(arch_spinlock_t *lock)
> > #define arch_spin_lock(l) ticket_spin_lock(l)
> > #define arch_spin_trylock(l) ticket_spin_trylock(l)
> > #define arch_spin_unlock(l) ticket_spin_unlock(l)
> > +#endif
> >
> > #endif /* __ASM_GENERIC_TICKET_SPINLOCK_H */
> > --
> > 2.39.2
> >
>
> The patch looks good to me, so
>
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
>
> It'd still be good to hear more about ARCH_WEAK_RELEASE_ACQUIRE and the
> kernel size though.
>
I sent the kernel size impact using -Os as asked and
ARCH_WEAK_RELEASE_ACQUIRE should be handled by Andrea.
Thanks for all the reviews drew, the patchset is way better now! I
won't respin a new version since there is only a minor comment change
requested in "riscv: Improve zacas fully-ordered cmpxchg()" unless you
insist.
Thanks again,
Alex
> Thanks,
> drew
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