Message-ID: <6f1bcc9b-1812-4e8c-9050-a750bfadd008@ghiti.fr>
Date: Thu, 15 Aug 2024 15:27:31 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Alexandre Ghiti <alexghiti@...osinc.com>,
 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 v4 13/13] riscv: Add qspinlock support

Hi Andrew,

On 01/08/2024 08:53, Alexandre Ghiti wrote:
> On Wed, Jul 31, 2024 at 5:29 PM Andrew Jones <ajones@...tanamicro.com> wrote:
>> On Wed, Jul 31, 2024 at 09:24:05AM 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                            | 29 +++++++++++++
>>>   arch/riscv/include/asm/Kbuild                 |  4 +-
>>>   arch/riscv/include/asm/spinlock.h             | 43 +++++++++++++++++++
>>>   arch/riscv/kernel/setup.c                     | 38 ++++++++++++++++
>>>   include/asm-generic/qspinlock.h               |  2 +
>>>   include/asm-generic/ticket_spinlock.h         |  2 +
>>>   7 files changed, 118 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..c9ff8081efc1 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
>> Why do we need this? Also, we presumably would prefer not to have it
>> when we end up using ticket spinlocks when combo spinlocks is selected.
>> Is there no way to avoid it?
> I'll let Andrea answer this as he asked for it.
>
>>>        select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
>>>        select BUILDTIME_TABLE_SORT if MMU
>>>        select CLINT_TIMER if RISCV_M_MODE
>>> @@ -488,6 +489,34 @@ 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.
>> Maybe some text recommending combo spinlocks here? As it stands it sounds
>> like enabling queued spinlocks is a bad idea for anybody that doesn't know
>> what platforms will run the kernel they're building, which is all distros.
> That's NONPORTABLE, so people enabling this config are supposed to
> know that right?
>
>>> +
>>> +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.
>> nit: Add a blank line here
> Done
>
>>> +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..503aef31db83
>>> --- /dev/null
>>> +++ b/arch/riscv/include/asm/spinlock.h
>>> @@ -0,0 +1,43 @@
>>> +/* 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/alternative.h>
>> We need asm/jump_label.h instead of asm/alternative.h, but...
>>
>>> +
>>> +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);                                  \
>>> +}
>> ...do you know what impact this inlined static key check has on the
>> kernel size?
> No, I'll check, thanks.


So I have just checked the size of the jump table section:

* defconfig:

- ticket: 26928 bytes
- combo: 28320 bytes

So that's a ~5% increase.

* ubuntu config

- ticket: 107840 bytes
- combo: 174752 bytes

And that's a ~62% increase.

This is the ELF size difference between ticket and combo spinlocks:

* ticket: 776915592 bytes
* combo: 786958968 bytes

So that's an increase of ~1.3% on the ELF.

And the .text section size:

* ticket: 12290960 bytes
* combo: 12366644 bytes

And that's a ~0.6% increase!

Finally, I'd say the impact is very limited :)

Thanks,

Alex


>
>> Actually, why not use ALTERNATIVE with any nonzero cpufeature value.
>> Then add code to riscv_cpufeature_patch_check() to return true when
>> qspinlocks should be enabled (based on the value of some global set
>> during riscv_spinlock_init)?
> As discussed with Guo in the previous iteration, the patching of the
> alternatives intervenes far after the first use of the spinlocks and
> the ticket spinlock implementation pollutes the spinlock value, so
> we'd have to unconditionally start with the qspinlock implementation
> and after switch to the ticket implementation if not supported by the
> platform. It works but it's dirty, I really don't like this hack.
>
> We could though:
> - add an initial value to the alternatives (not sure it's feasible though)
> - make the patching of alternatives happen sooner by parsing the isa
> string sooner, either in DT or ACPI (I have a working PoC for very
> early parsing of ACPI).
>
> I intend to do the latter as I think we should be aware of the
> extensions sooner in the boot process, so I'll change that to the
> alternatives when it's done. WDYT, any other idea?
>
>
>>> +
>>> +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..b811fa331982 100644
>>> --- a/arch/riscv/kernel/setup.c
>>> +++ b/arch/riscv/kernel/setup.c
>>> @@ -244,6 +244,43 @@ 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 {
>> else if (IS_ENABLED(CONFIG_RISCV_COMBO_SPINLOCKS))
>>
>>> +             static_branch_disable(&qspinlock_key);
>>> +             pr_info("Ticket spinlock: enabled\n");
>>> +
>> nit: remove this blank line
>>
>>> +             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 +334,7 @@ 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
>> I'm not sure what's better/worse, but instead of inventing this
>> __no_arch_spinlock_redefine thing we could just name all the functions
>> something like __arch_spin* and then add defines for both to asm/spinlock.h,
>> i.e.
>>
>> #define queued_spin_lock(l) __arch_spin_lock(l)
>> ...
>>
>> #define ticket_spin_lock(l) __arch_spin_lock(l)
>> ...
>>
>> Besides not having to touch asm-generic/qspinlock.h and
>> asm-generic/ticket_spinlock.h it allows one to find the implementations
>> a bit easier as following a tag to arch_spin_lock() will take them to
>> queued_spin_lock() which will then take them to
>> arch/riscv/include/asm/spinlock.h and there they'll figure out how
>> __arch_spin_lock() was defined.
>>
>>>   /*
>>>    * 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
>>>
>> Thanks,
>> drew
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