linux-kernel - Re: [RFC PATCH 7/8] rcuscale: Add tests for simple hazard pointers

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Message-ID: <dd9b0591-aea2-4a50-bf4e-276224f15f68@paulmck-laptop>
Date: Thu, 10 Jul 2025 18:03:16 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Breno Leitao <leitao@...ian.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, aeh@...a.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	edumazet@...gle.com, jhs@...atatu.com, kernel-team@...a.com,
	Erik Lundgren <elundgren@...a.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org
Subject: Re: [RFC PATCH 7/8] rcuscale: Add tests for simple hazard pointers

On Sun, Apr 13, 2025 at 11:00:54PM -0700, Boqun Feng wrote:
> Add two rcu_scale_ops to include tests from simple hazard pointers
> (shazptr). One is with evenly distributed readers, and the other is with
> all WILDCARD readers. This could show the best and worst case scenarios
> for the synchronization time of simple hazard pointers.
> 
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

Cute trick using the CPU number plus one as a stand-in for a pointer.  ;-)

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/rcu/rcuscale.c | 52 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 51 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
> index d9bff4b1928b..cab42bcc1d26 100644
> --- a/kernel/rcu/rcuscale.c
> +++ b/kernel/rcu/rcuscale.c
> @@ -32,6 +32,7 @@
>  #include <linux/freezer.h>
>  #include <linux/cpu.h>
>  #include <linux/delay.h>
> +#include <linux/shazptr.h>
>  #include <linux/stat.h>
>  #include <linux/srcu.h>
>  #include <linux/slab.h>
> @@ -429,6 +430,54 @@ static struct rcu_scale_ops tasks_tracing_ops = {
>  
>  #endif // #else // #ifdef CONFIG_TASKS_TRACE_RCU
>  
> +static int shazptr_scale_read_lock(void)
> +{
> +	long cpu = raw_smp_processor_id();
> +
> +	/* Use cpu + 1 as the key */
> +	guard(shazptr)((void *)(cpu + 1));
> +
> +	return 0;
> +}
> +
> +static int shazptr_scale_wc_read_lock(void)
> +{
> +	guard(shazptr)(SHAZPTR_WILDCARD);
> +
> +	return 0;
> +}
> +
> +
> +static void shazptr_scale_read_unlock(int idx)
> +{
> +	/* Do nothing, it's OK since readers are doing back-to-back lock+unlock*/
> +}
> +
> +static void shazptr_scale_sync(void)
> +{
> +	long cpu = raw_smp_processor_id();
> +
> +	synchronize_shazptr((void *)(cpu + 1));
> +}
> +
> +static struct rcu_scale_ops shazptr_ops = {
> +	.ptype		= RCU_FLAVOR,
> +	.readlock	= shazptr_scale_read_lock,
> +	.readunlock	= shazptr_scale_read_unlock,
> +	.sync		= shazptr_scale_sync,
> +	.exp_sync	= shazptr_scale_sync,
> +	.name		= "shazptr"
> +};
> +
> +static struct rcu_scale_ops shazptr_wc_ops = {
> +	.ptype		= RCU_FLAVOR,
> +	.readlock	= shazptr_scale_wc_read_lock,
> +	.readunlock	= shazptr_scale_read_unlock,
> +	.sync		= shazptr_scale_sync,
> +	.exp_sync	= shazptr_scale_sync,
> +	.name		= "shazptr_wildcard"
> +};
> +
>  static unsigned long rcuscale_seq_diff(unsigned long new, unsigned long old)
>  {
>  	if (!cur_ops->gp_diff)
> @@ -1090,7 +1139,8 @@ rcu_scale_init(void)
>  	long i;
>  	long j;
>  	static struct rcu_scale_ops *scale_ops[] = {
> -		&rcu_ops, &srcu_ops, &srcud_ops, TASKS_OPS TASKS_RUDE_OPS TASKS_TRACING_OPS
> +		&rcu_ops, &srcu_ops, &srcud_ops, &shazptr_ops, &shazptr_wc_ops,
> +		TASKS_OPS TASKS_RUDE_OPS TASKS_TRACING_OPS
>  	};
>  
>  	if (!torture_init_begin(scale_type, verbose))
> -- 
> 2.47.1
>