Re: [PATCH] locking: Generic ticket lock

[Stafford Horne]

On Thu, Oct 21, 2021 at 03:12:25PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 21, 2021 at 03:05:15PM +0200, Peter Zijlstra wrote:
> > 
> > There's currently a number of architectures that want/have graduated
> > from test-and-set locks and are looking at qspinlock.
> > 
> > *HOWEVER* qspinlock is very complicated and requires a lot of an
> > architecture to actually work correctly. Specifically it requires
> > forward progress between a fair number of atomic primitives, including
> > an xchg16 operation, which I've seen a fair number of fundamentally
> > broken implementations of in the tree (specifically for qspinlock no
> > less).
> > 
> > The benefit of qspinlock over ticket lock is also non-obvious, esp.
> > at low contention (the vast majority of cases in the kernel), and it
> > takes a fairly large number of CPUs (typically also NUMA) to make
> > qspinlock beat ticket locks.
> > 
> > Esp. things like ARM64's WFE can move the balance a lot in favour of
> > simpler locks by reducing the cacheline pressure due to waiters (see
> > their smp_cond_load_acquire() implementation for details).
> > 
> > Unless you've audited qspinlock for your architecture and found it
> > sound *and* can show actual benefit, simpler is better.

For OpenRISC originally we had a custom ticket locking mechanism, but it was
suggested to use qspinlocks as the genric implementation meant less code.

Changed here:

	https://yhbt.net/lore/all/86vaix5fmr.fsf@xxxxxxx/T/

I think moving to qspinlocks was suggested by you.  But now that we have this
generic infrastructure, I am good to switch.

> > Therefore provide ticket locks, which depend on a single atomic
> > operation (fetch_add) while still providing fairness.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> >  include/asm-generic/qspinlock.h         |   30 +++++++++
> >  include/asm-generic/ticket_lock_types.h |   11 +++
> >  include/asm-generic/ticket_lock.h       |   97 ++++++++++++++++++++++++++++++++
> >  3 files changed, 138 insertions(+)
> 
> A few notes...
> 
> > + * It relies on smp_store_release() + atomic_*_acquire() to be RCsc (or no
> > + * weaker than RCtso if you're Power, also see smp_mb__after_unlock_lock()),
> 
> This should hold true to RISC-V in its current form, AFAICT
> atomic_fetch_add ends up using AMOADD, and therefore the argument made
> in the unlock+lock thread [1], gives that this results in RW,RW
> ordering.
> 
> [1] https://lore.kernel.org/lkml/5412ab37-2979-5717-4951-6a61366df0f2@xxxxxxxxxx/
> 
> 
> I've compile tested on openrisc/simple_smp_defconfig using the below.
> 
> --- a/arch/openrisc/Kconfig
> +++ b/arch/openrisc/Kconfig
> @@ -30,7 +30,6 @@ config OPENRISC
>  	select HAVE_DEBUG_STACKOVERFLOW
>  	select OR1K_PIC
>  	select CPU_NO_EFFICIENT_FFS if !OPENRISC_HAVE_INST_FF1
> -	select ARCH_USE_QUEUED_SPINLOCKS
>  	select ARCH_USE_QUEUED_RWLOCKS
>  	select OMPIC if SMP
>  	select ARCH_WANT_FRAME_POINTERS
> --- a/arch/openrisc/include/asm/Kbuild
> +++ b/arch/openrisc/include/asm/Kbuild
> @@ -1,9 +1,8 @@
>  # SPDX-License-Identifier: GPL-2.0
>  generic-y += extable.h
>  generic-y += kvm_para.h
> -generic-y += mcs_spinlock.h
> -generic-y += qspinlock_types.h
> -generic-y += qspinlock.h
> +generic-y += ticket_lock_types.h
> +generic-y += ticket_lock.h
>  generic-y += qrwlock_types.h
>  generic-y += qrwlock.h
>  generic-y += user.h
> --- a/arch/openrisc/include/asm/spinlock.h
> +++ b/arch/openrisc/include/asm/spinlock.h
> @@ -15,7 +15,7 @@
>  #ifndef __ASM_OPENRISC_SPINLOCK_H
>  #define __ASM_OPENRISC_SPINLOCK_H
>  
> -#include <asm/qspinlock.h>
> +#include <asm/ticket_lock.h>
>  
>  #include <asm/qrwlock.h>
>  
> --- a/arch/openrisc/include/asm/spinlock_types.h
> +++ b/arch/openrisc/include/asm/spinlock_types.h
> @@ -1,7 +1,7 @@
>  #ifndef _ASM_OPENRISC_SPINLOCK_TYPES_H
>  #define _ASM_OPENRISC_SPINLOCK_TYPES_H
>  
> -#include <asm/qspinlock_types.h>
> +#include <asm/ticket_lock_types.h>
>  #include <asm/qrwlock_types.h>
>  
>  #endif /* _ASM_OPENRISC_SPINLOCK_TYPES_H */

This looks good to me.  Do you want to commit along with the
generic ticket lock patch?  Otherwise I can queue it after it is
upstreamed.  Another option is I can help merge the generic ticket
lock code via the OpenRISC branch.

Let me know what works.

-Stafford