From: Boqun Feng <boqun.feng@gmail.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
lkmm@lists.linux.dev, Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
Waiman Long <longman@redhat.com>,
Davidlohr Bueso <dave@stgolabs.net>,
"Paul E. McKenney" <paulmck@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
Joel Fernandes <joelagnelf@nvidia.com>,
Uladzislau Rezki <urezki@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Zqiang <qiang.zhang@linux.dev>, Breno Leitao <leitao@debian.org>,
aeh@meta.com, netdev@vger.kernel.org, edumazet@google.com,
jhs@mojatatu.com, kernel-team@meta.com,
Erik Lundgren <elundgren@meta.com>
Subject: Re: [PATCH 4/8] shazptr: Avoid synchronize_shaptr() busy waiting
Date: Wed, 25 Jun 2025 08:24:53 -0700 [thread overview]
Message-ID: <aFwUxblhRjh24JF1@Mac.home> (raw)
In-Reply-To: <aFv_9f9w_HdTj9Xj@localhost.localdomain>
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
next prev parent reply other threads:[~2025-06-25 15:24 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-06-25 3:10 [PATCH 0/8] Introduce simple hazard pointers for lockdep Boqun Feng
2025-06-25 3:10 ` [PATCH 1/8] Introduce simple hazard pointers Boqun Feng
2025-06-25 10:00 ` Peter Zijlstra
2025-06-25 14:25 ` Mathieu Desnoyers
2025-06-25 15:05 ` Boqun Feng
2025-06-25 15:52 ` Waiman Long
2025-06-25 16:09 ` Boqun Feng
2025-06-25 17:47 ` Waiman Long
2025-06-25 3:10 ` [PATCH 2/8] shazptr: Add refscale test Boqun Feng
2025-06-25 10:02 ` Peter Zijlstra
2025-06-25 3:10 ` [PATCH 3/8] shazptr: Add refscale test for wildcard Boqun Feng
2025-06-25 10:03 ` Peter Zijlstra
2025-06-25 3:10 ` [PATCH 4/8] shazptr: Avoid synchronize_shaptr() busy waiting Boqun Feng
2025-06-25 11:40 ` Peter Zijlstra
2025-06-25 11:56 ` Peter Zijlstra
2025-06-25 13:56 ` Frederic Weisbecker
2025-06-25 15:24 ` Boqun Feng [this message]
2025-06-26 13:45 ` Frederic Weisbecker
2025-06-25 3:10 ` [PATCH 5/8] shazptr: Allow skip self scan in synchronize_shaptr() Boqun Feng
2025-06-25 3:10 ` [PATCH 6/8] rcuscale: Allow rcu_scale_ops::get_gp_seq to be NULL Boqun Feng
2025-06-25 3:11 ` [PATCH 7/8] rcuscale: Add tests for simple hazard pointers Boqun Feng
2025-06-25 3:11 ` [PATCH 8/8] locking/lockdep: Use shazptr to protect the key hashlist Boqun Feng
2025-06-25 11:59 ` Peter Zijlstra
2025-06-25 14:18 ` Boqun Feng
2025-07-10 14:06 ` Breno Leitao
2025-07-11 2:31 ` Boqun Feng
2025-06-25 12:05 ` [PATCH 0/8] Introduce simple hazard pointers for lockdep Christoph Hellwig
2025-06-25 14:08 ` Boqun Feng
2025-06-26 10:16 ` Christoph Hellwig
2025-06-26 13:45 ` Mathieu Desnoyers
2025-06-26 15:47 ` Boqun Feng
2025-06-27 2:56 ` Paul E. McKenney
2025-06-25 12:25 ` Mathieu Desnoyers
2025-06-25 13:21 ` Boqun Feng
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