netdev - Re: [PATCH 4/8] shazptr: Avoid synchronize

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Message-ID: <aFv_9f9w_HdTj9Xj@localhost.localdomain>
Date: Wed, 25 Jun 2025 15:56:05 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org, lkmm@...ts.linux.dev,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>, Breno Leitao <leitao@...ian.org>,
	aeh@...a.com, netdev@...r.kernel.org, edumazet@...gle.com,
	jhs@...atatu.com, kernel-team@...a.com,
	Erik Lundgren <elundgren@...a.com>
Subject: Re: [PATCH 4/8] shazptr: Avoid synchronize_shaptr() busy waiting

Le Tue, Jun 24, 2025 at 08:10:57PM -0700, Boqun Feng a écrit :
> +static void synchronize_shazptr_normal(void *ptr)
> +{
> +	int cpu;
> +	unsigned long blocking_grp_mask = 0;
> +
> +	smp_mb(); /* Synchronize with the smp_mb() in shazptr_acquire(). */
> +
> +	for_each_possible_cpu(cpu) {
> +		void **slot = per_cpu_ptr(&shazptr_slots, cpu);
> +		void *val;
> +
> +		/* Pair with smp_store_release() in shazptr_clear(). */
> +		val = smp_load_acquire(slot);
> +
> +		if (val == ptr || val == SHAZPTR_WILDCARD)
> +			blocking_grp_mask |= 1UL << (cpu / shazptr_scan.cpu_grp_size);
> +	}
> +
> +	/* Found blocking slots, prepare to wait. */
> +	if (blocking_grp_mask) {

synchronize_rcu() here would be enough since all users have preemption disabled.
But I guess this defeats the performance purpose? (If so this might need a
comment somewhere).

I guess blocking_grp_mask is to avoid allocating a cpumask (again for
performance purpose? So I guess synchronize_shazptr_normal() has some perf
expectations?)

One possibility is to have the ptr contained in:

struct hazptr {
       void *ptr;
       struct cpumask scan_mask
};

And then the caller could simply scan itself those remaining CPUs without
relying on the kthread.

But I'm sure there are good reasons for now doing that :-)

Thanks.

-- 
Frederic Weisbecker
SUSE Labs