linux-kernel - Re: [PATCH v11 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Message-ID: <05a65878-d24c-0f8e-c271-24ebc729d7e3@redhat.com>
Date:   Tue, 15 Sep 2020 22:40:11 -0400
From:   Waiman Long <longman@...hat.com>
To:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        peterz@...radead.org, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com
Subject: Re: [PATCH v11 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On 9/15/20 2:05 PM, Alex Kogan wrote:
> In CNA, spinning threads are organized in two queues, a primary queue for
> threads running on the same node as the current lock holder, and a
> secondary queue for threads running on other nodes. After acquiring the
> MCS lock and before acquiring the spinlock, the MCS lock
> holder checks whether the next waiter in the primary queue (if exists) is
> running on the same NUMA node. If it is not, that waiter is detached from
> the main queue and moved into the tail of the secondary queue. This way,
> we gradually filter the primary queue, leaving only waiters running on
> the same preferred NUMA node. For more details, see
> https://arxiv.org/abs/1810.05600.
>
> Note that this variant of CNA may introduce starvation by continuously
> passing the lock between waiters in the main queue. This issue will be
> addressed later in the series.
>
> Enabling CNA is controlled via a new configuration option
> (NUMA_AWARE_SPINLOCKS). By default, the CNA variant is patched in at the
> boot time only if we run on a multi-node machine in native environment and
> the new config is enabled. (For the time being, the patching requires
> CONFIG_PARAVIRT_SPINLOCKS to be enabled as well. However, this should be
> resolved once static_call() is available.) This default behavior can be
> overridden with the new kernel boot command-line option
> "numa_spinlock=on/off" (default is "auto").
>
> Signed-off-by: Alex Kogan <alex.kogan@...cle.com>
> Reviewed-by: Steve Sistare <steven.sistare@...cle.com>
> Reviewed-by: Waiman Long <longman@...hat.com>
> ---
>   .../admin-guide/kernel-parameters.txt         |  10 +
>   arch/x86/Kconfig                              |  20 ++
>   arch/x86/include/asm/qspinlock.h              |   4 +
>   arch/x86/kernel/alternative.c                 |   4 +
>   kernel/locking/mcs_spinlock.h                 |   2 +-
>   kernel/locking/qspinlock.c                    |  42 ++-
>   kernel/locking/qspinlock_cna.h                | 336 ++++++++++++++++++
>   7 files changed, 413 insertions(+), 5 deletions(-)
>   create mode 100644 kernel/locking/qspinlock_cna.h
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a1068742a6df..51ce050f8701 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3353,6 +3353,16 @@
>   
>   	nox2apic	[X86-64,APIC] Do not enable x2APIC mode.
>   
> +	numa_spinlock=	[NUMA, PV_OPS] Select the NUMA-aware variant
> +			of spinlock. The options are:
> +			auto - Enable this variant if running on a multi-node
> +			machine in native environment.
> +			on  - Unconditionally enable this variant.
> +			off - Unconditionally disable this variant.
> +
> +			Not specifying this option is equivalent to
> +			numa_spinlock=auto.
> +
>   	cpu0_hotplug	[X86] Turn on CPU0 hotplug feature when
>   			CONFIG_BOOTPARAM_HOTPLUG_CPU0 is off.
>   			Some features depend on CPU0. Known dependencies are:

You will have to move down this hunk according to alphabetic order. 
Other than that this patch looks good to me.

Cheers,
Longman