linux-kernel - Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

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Date:   Mon, 6 Jan 2020 10:33:27 -0500
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 v8 4/5] locking/qspinlock: Introduce starvation avoidance
 into CNA

On 12/30/19 2:40 PM, Alex Kogan wrote:
> Keep track of the number of intra-node lock handoffs, and force
> inter-node handoff once this number reaches a preset threshold.
> The default value for the threshold can be overridden with
> the new kernel boot command-line option "numa_spinlock_threshold".
>
> Signed-off-by: Alex Kogan <alex.kogan@...cle.com>
> Reviewed-by: Steve Sistare <steven.sistare@...cle.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  8 ++++
>  kernel/locking/qspinlock.c                    |  3 ++
>  kernel/locking/qspinlock_cna.h                | 41 ++++++++++++++++++-
>  3 files changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b68cb80e477f..30d79819a3b0 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3200,6 +3200,14 @@
>  			Not specifying this option is equivalent to
>  			numa_spinlock=auto.
>  
> +	numa_spinlock_threshold=	[NUMA, PV_OPS]
> +			Set the threshold for the number of intra-node
> +			lock hand-offs before the NUMA-aware spinlock
> +			is forced to be passed to a thread on another NUMA node.
> +			Valid values are in the [0..31] range. Smaller values
> +			result in a more fair, but less performant spinlock, and
> +			vice versa. The default value is 16.
> +
>  	cpu0_hotplug	[X86] Turn on CPU0 hotplug feature when
>  			CONFIG_BOOTPARAM_HOTPLUG_CPU0 is off.
>  			Some features depend on CPU0. Known dependencies are:
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 609980a53841..e382d8946ccc 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -597,6 +597,9 @@ EXPORT_SYMBOL(queued_spin_lock_slowpath);
>  #if !defined(_GEN_CNA_LOCK_SLOWPATH) && defined(CONFIG_NUMA_AWARE_SPINLOCKS)
>  #define _GEN_CNA_LOCK_SLOWPATH
>  
> +#undef pv_init_node
> +#define pv_init_node			cna_init_node
> +
>  #undef pv_wait_head_or_lock
>  #define pv_wait_head_or_lock		cna_pre_scan
>  
> diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
> index 3c99a4a6184b..30feff02865d 100644
> --- a/kernel/locking/qspinlock_cna.h
> +++ b/kernel/locking/qspinlock_cna.h
> @@ -51,13 +51,25 @@ struct cna_node {
>  	int			numa_node;
>  	u32			encoded_tail;
>  	u32			pre_scan_result; /* encoded tail or enum val */
> +	u32			intra_count;
>  };
>  
>  enum {
>  	LOCAL_WAITER_FOUND = 2,	/* 0 and 1 are reserved for @locked */
> +	FLUSH_SECONDARY_QUEUE = 3,
>  	MIN_ENCODED_TAIL
>  };
>  
> +/*
> + * Controls the threshold for the number of intra-node lock hand-offs before
> + * the NUMA-aware variant of spinlock is forced to be passed to a thread on
> + * another NUMA node. By default, the chosen value provides reasonable
> + * long-term fairness without sacrificing performance compared to a lock
> + * that does not have any fairness guarantees. The default setting can
> + * be changed with the "numa_spinlock_threshold" boot option.
> + */
> +int intra_node_handoff_threshold __ro_after_init = 1 << 16;

Another minor nit. (1 << 31) is negative for an int value. I will
suggest that you either make intra_node_handoff_threshold an unsigned
int or limit the upper bound to 30.

Cheers,
Longman