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

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Message-ID: <aFwUxblhRjh24JF1@Mac.home>
Date: Wed, 25 Jun 2025 08:24:53 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Frederic Weisbecker <frederic@...nel.org>
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

On Wed, Jun 25, 2025 at 03:56:05PM +0200, Frederic Weisbecker wrote:
> 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).
> 

synchronize_shazptr_normal() cannot wait for a whole grace period,
because the point of hazard pointers is to avoid waiting for unrelated
readers.

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

If we are talking about {k,v}malloc allocation:
synchronize_shazptr_normal() would mostly be used in cleanup/free path
similar to synchronize_rcu(), therefor I would like to avoid "allocating
memory to free memory".

> expectations?)
> 
> One possibility is to have the ptr contained in:
> 
> struct hazptr {
>        void *ptr;
>        struct cpumask scan_mask
> };
> 

You mean updaters passing a `struct hazptr *` into
synchronize_shazptr_normal()? That may be a good idea, if multiple
updaters can share the same `struct hazptr *`, we can add that later,
but...

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

.. this is a bad idea, sure, we can always burn some CPU time to scan,
but local optimization doesn't mean global optimization, if in the
future, we have a lots of synchronize_shazptr_normal()s happening at
the same time, the self busy-waiting scan would become problematic.

Regards,
Boqun

> 
> But I'm sure there are good reasons for now doing that :-)
> 
> Thanks.
> 
> -- 
> Frederic Weisbecker
> SUSE Labs