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

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Message-ID: <aF1O4aH7m43xAbCZ@localhost.localdomain>
Date: Thu, 26 Jun 2025 15:45:05 +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 Wed, Jun 25, 2025 at 08:24:53AM -0700, Boqun Feng a écrit :
> 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.

Fair enough!

> 
> > 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".

Good point!

> 
> > 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.

Ok.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs