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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: rcu@vger.kernel.org,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Kernel Team" <kernel-team@fb.com>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Lai Jiangshan" <jiangshanlai@gmail.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
	"Josh Triplett" <josh@joshtriplett.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"David Howells" <dhowells@redhat.com>,
	"Eric Dumazet" <edumazet@google.com>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	"Oleg Nesterov" <oleg@redhat.com>,
	"Joel Fernandes" <joel@joelfernandes.org>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates
Date: Wed, 28 Jul 2021 11:46:05 -0700	[thread overview]
Message-ID: <20210728184605.GI4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <CAHk-=wjwZzi=Lqsre8C8C4YJiVQNdNnco7jkLq=ohePSPA9JCA@mail.gmail.com>

On Wed, Jul 28, 2021 at 10:58:08AM -0700, Linus Torvalds wrote:
> On Wed, Jul 28, 2021 at 10:37 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > +/*
> > + * Increment the current CPU's rcu_data structure's ->dynticks field
> > + * with ordering.  Return the new value.
> > + */
> > +static noinstr unsigned long rcu_dynticks_inc(int incby)
> > +{
> > +       return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
> > +}
> 
> Maybe inline?

I bet the compiler does this already, but why give it the choice?
I will add inline.

> But more I reacted to how we sadly don't have percpu atomics. They'd
> be fairly easy to add on x86, but I guess it's not a huge deal.
> 
> And hey, if this is pretty much the only place that would use them, I
> guess that's not much of an issue.

I did a git grep for "atomic_.*this_cpu_ptr" in -rcu, and came up with
this:

	arch/s390/kernel/time.c:	atomic_t *sw_ptr = this_cpu_ptr(&clock_sync_word);
	arch/s390/kernel/time.c:	atomic_t *sw_ptr = this_cpu_ptr(&clock_sync_word);
	arch/x86/xen/spinlock.c:	atomic_t *nest_cnt = this_cpu_ptr(&xen_qlock_wait_nest);
	drivers/irqchip/irq-apple-aic.c:	atomic_andnot(irq_bit, this_cpu_ptr(&aic_vipi_enable));
	drivers/irqchip/irq-apple-aic.c:	atomic_or(irq_bit, this_cpu_ptr(&aic_vipi_enable));
	drivers/irqchip/irq-apple-aic.c:	if (atomic_read(this_cpu_ptr(&aic_vipi_flag)) & irq_bit)
	drivers/irqchip/irq-apple-aic.c:	enabled = atomic_read(this_cpu_ptr(&aic_vipi_enable));
	drivers/irqchip/irq-apple-aic.c:	firing = atomic_fetch_andnot(enabled, this_cpu_ptr(&aic_vipi_flag)) & enabled;
	kernel/events/core.c:	if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
	kernel/events/core.c:	if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
	kernel/rcu/rcuscale.c:	atomic_dec(this_cpu_ptr(&n_async_inflight));
	kernel/rcu/rcuscale.c:			if (rhp && atomic_read(this_cpu_ptr(&n_async_inflight)) < gp_async_max) {
	kernel/rcu/rcuscale.c:				atomic_inc(this_cpu_ptr(&n_async_inflight));
	kernel/rcu/tree.c:	return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
	kernel/rcu/tree.c:	return !(atomic_read(this_cpu_ptr(&rcu_data.dynticks)) & 0x1);
	kernel/trace/ring_buffer.c:	if (atomic_inc_return(this_cpu_ptr(&checking)) != 1)
	kernel/trace/ring_buffer.c:	atomic_dec(this_cpu_ptr(&checking));

Given that this mostly moves text from the first argument to the function
name, I suspect that we would need quite a few instances of any given
atomic operation to make the added lines for all the definitions and
the documentation worth it.  Maybe for atomic_read()?

	/**
	 * atomic_read_this_cpu - atomically read from this CPU's variable
	 * @p: Pointer to per-CPU variable.
	 *
	 * Do an atomic_read() from the current CPU's instance of the
	 * specified per-CPU variable.
	 */
	#define atomic_read_this_cpu(p) atomic_read(this_cpu_ptr(p))

But atomic_read_this_cpu(&rcu_data.dynticks) isn't all that much shorter
than atomic_read(this_cpu_ptr(&rcu_data.dynticks)).

							Thanx, Paul

  parent reply	other threads:[~2021-07-28 18:46 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-21 20:20 [PATCH rcu 0/18] Miscellaneous fixes for v5.15 Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 01/18] rcu: Fix to include first blocked task in stall warning Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 02/18] rcu: Fix stall-warning deadlock due to non-release of rcu_node ->lock Paul E. McKenney
2021-08-03 14:24   ` Qais Yousef
2021-08-03 15:52     ` Paul E. McKenney
2021-08-03 16:12       ` Qais Yousef
2021-08-03 16:28         ` Paul E. McKenney
2021-08-03 16:33           ` Qais Yousef
2021-08-04 13:50           ` Qais Yousef
2021-08-04 22:33             ` Paul E. McKenney
2021-08-06  9:56               ` Qais Yousef
2021-08-06  9:57   ` Qais Yousef
2021-08-06 11:43     ` Paul E. McKenney
2021-08-06 12:33       ` Qais Yousef
2021-07-21 20:21 ` [PATCH rcu 03/18] rcu: Remove special bit at the bottom of the ->dynticks counter Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 04/18] rcu: Weaken ->dynticks accesses and updates Paul E. McKenney
2021-07-21 20:41   ` Linus Torvalds
2021-07-21 21:25     ` Paul E. McKenney
2021-07-28 17:37   ` [PATCH v2 " Paul E. McKenney
2021-07-28 17:58     ` Linus Torvalds
2021-07-28 18:12       ` Mathieu Desnoyers
2021-07-28 18:32         ` Linus Torvalds
2021-07-28 18:39           ` Mathieu Desnoyers
2021-07-28 18:46         ` Paul E. McKenney
2021-07-28 18:46       ` Paul E. McKenney [this message]
2021-07-28 18:57         ` Linus Torvalds
2021-07-28 18:23     ` Mathieu Desnoyers
2021-07-28 18:58       ` Paul E. McKenney
2021-07-28 19:45         ` Paul E. McKenney
2021-07-28 20:03           ` Mathieu Desnoyers
2021-07-28 20:28             ` Paul E. McKenney
2021-07-29 14:41               ` Mathieu Desnoyers
2021-07-29 15:57                 ` Paul E. McKenney
2021-07-29 17:41                   ` Mathieu Desnoyers
2021-07-29 18:05                     ` Paul E. McKenney
2021-07-29 18:42                       ` Mathieu Desnoyers
2021-07-28 20:37     ` Josh Triplett
2021-07-28 20:47       ` Paul E. McKenney
2021-07-28 22:23         ` Frederic Weisbecker
2021-07-29  1:07           ` Paul E. McKenney
2021-07-29  7:58   ` [PATCH " Boqun Feng
2021-07-29 10:53     ` Frederic Weisbecker
2021-07-30  5:56       ` Boqun Feng
2021-07-30 17:18         ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 05/18] rcu: Mark accesses to ->rcu_read_lock_nesting Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 06/18] rculist: Unify documentation about missing list_empty_rcu() Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 07/18] rcu/tree: Handle VM stoppage in stall detection Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 08/18] rcu: Do not disable GP stall detection in rcu_cpu_stall_reset() Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 09/18] rcu: Start timing stall repetitions after warning complete Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 10/18] srcutiny: Mark read-side data races Paul E. McKenney
2021-07-29  8:23   ` Boqun Feng
2021-07-29 13:36     ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 11/18] rcu: Mark lockless ->qsmask read in rcu_check_boost_fail() Paul E. McKenney
2021-07-29  8:54   ` Boqun Feng
2021-07-29 14:03     ` Paul E. McKenney
2021-07-30  2:28       ` Boqun Feng
2021-07-30  3:26         ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 12/18] rcu: Make rcu_gp_init() and rcu_gp_fqs_loop noinline to conserve stack Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 13/18] rcu: Remove trailing spaces and tabs Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 14/18] rcu: Mark accesses in tree_stall.h Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 15/18] rcu: Remove useless "ret" update in rcu_gp_fqs_loop() Paul E. McKenney
2021-08-03 16:48   ` Joe Perches
2021-08-03 17:10     ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 16/18] rcu: Use per_cpu_ptr to get the pointer of per_cpu variable Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 17/18] rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 18/18] rcu: Print human-readable message for schedule() in RCU reader Paul E. McKenney

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