Re: [PATCH 15/17] powerpc/qspinlock: reduce remote node steal spins

[Jordan Niethe <jniethe5@gmail.com>]

On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
[resend as utf-8, not utf-7]
> Allow for a reduction in the number of times a CPU from a different
> node than the owner can attempt to steal the lock before queueing.
> This could bias the transfer behaviour of the lock across the
> machine and reduce NUMA crossings.
> ---
>  arch/powerpc/lib/qspinlock.c | 34 +++++++++++++++++++++++++++++++---
>  1 file changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> index d4594c701f7d..24f68bd71e2b 100644
> --- a/arch/powerpc/lib/qspinlock.c
> +++ b/arch/powerpc/lib/qspinlock.c
> @@ -4,6 +4,7 @@
>  #include <linux/export.h>
>  #include <linux/percpu.h>
>  #include <linux/smp.h>
> +#include <linux/topology.h>
>  #include <asm/qspinlock.h>
>  #include <asm/paravirt.h>
>  
> @@ -24,6 +25,7 @@ struct qnodes {
>  
>  /* Tuning parameters */
>  static int STEAL_SPINS __read_mostly = (1<<5);
> +static int REMOTE_STEAL_SPINS __read_mostly = (1<<2);
>  #if _Q_SPIN_TRY_LOCK_STEAL == 1
>  static const bool MAYBE_STEALERS = true;
>  #else
> @@ -39,9 +41,13 @@ static bool pv_prod_head __read_mostly = false;
>  
>  static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
>  
> -static __always_inline int get_steal_spins(bool paravirt)
> +static __always_inline int get_steal_spins(bool paravirt, bool remote)
>  {
> -	return STEAL_SPINS;
> +	if (remote) {
> +		return REMOTE_STEAL_SPINS;
> +	} else {
> +		return STEAL_SPINS;
> +	}
>  }
>  
>  static __always_inline int get_head_spins(bool paravirt)
> @@ -380,8 +386,13 @@ static __always_inline bool try_to_steal_lock(struct qspinlock *lock, bool parav
>  
>  		iters++;
>  
> -		if (iters >= get_steal_spins(paravirt))
> +		if (iters >= get_steal_spins(paravirt, false))
>  			break;
> +		if (iters >= get_steal_spins(paravirt, true)) {

There's no indication of what true and false mean here which is hard to read.
To me it feels like two separate functions would be more clear.


> +			int cpu = get_owner_cpu(val);
> +			if (numa_node_id() != cpu_to_node(cpu))

What about using node_distance() instead?


> +				break;
> +		}
>  	}
>  	spin_end();
>  
> @@ -588,6 +599,22 @@ static int steal_spins_get(void *data, u64 *val)
>  
>  DEFINE_SIMPLE_ATTRIBUTE(fops_steal_spins, steal_spins_get, steal_spins_set, "%llu\n");
>  
> +static int remote_steal_spins_set(void *data, u64 val)
> +{
> +	REMOTE_STEAL_SPINS = val;

REMOTE_STEAL_SPINS is int not u64.

> +
> +	return 0;
> +}
> +
> +static int remote_steal_spins_get(void *data, u64 *val)
> +{
> +	*val = REMOTE_STEAL_SPINS;
> +
> +	return 0;
> +}
> +
> +DEFINE_SIMPLE_ATTRIBUTE(fops_remote_steal_spins, remote_steal_spins_get, remote_steal_spins_set, "%llu\n");
> +
>  static int head_spins_set(void *data, u64 val)
>  {
>  	HEAD_SPINS = val;
> @@ -687,6 +714,7 @@ DEFINE_SIMPLE_ATTRIBUTE(fops_pv_prod_head, pv_prod_head_get, pv_prod_head_set, "
>  static __init int spinlock_debugfs_init(void)
>  {
>  	debugfs_create_file("qspl_steal_spins", 0600, arch_debugfs_dir, NULL, &fops_steal_spins);
> +	debugfs_create_file("qspl_remote_steal_spins", 0600, arch_debugfs_dir, NULL, &fops_remote_steal_spins);
>  	debugfs_create_file("qspl_head_spins", 0600, arch_debugfs_dir, NULL, &fops_head_spins);
>  	if (is_shared_processor()) {
>  		debugfs_create_file("qspl_pv_yield_owner", 0600, arch_debugfs_dir, NULL, &fops_pv_yield_owner);