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From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: rcu <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 11:57:14 -0700	[thread overview]
Message-ID: <CAHk-=wj-5vtA_-9xs6zV0U20LRaRhVeE1Ky7X+9kHeucrb6_ig@mail.gmail.com> (raw)
In-Reply-To: <20210728184605.GI4397@paulmck-ThinkPad-P17-Gen-1>

On Wed, Jul 28, 2021 at 11:46 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> But atomic_read_this_cpu(&rcu_data.dynticks) isn't all that much shorter
> than atomic_read(this_cpu_ptr(&rcu_data.dynticks)).

It's not so much that it's shorter to write for a human, it's that we
could generate better code for it.

That atomic_read(this_cpu_ptr()) pattern generates code like

        movq    $rcu_data+288, %rax
        add %gs:this_cpu_off(%rip), %rax
        movl    (%rax), %eax

but it *could* just generate

        movl %gs:rcu_data+288, %rax

instead.

Similar patterns for the other per-cpu atomics, ie it would be
possible to just generate

        lock ; xaddl %gs:..., %rax

instead of generating the address by doing that "add %gs:this_cpu_off" thing..

But no, it doesn't look like there are enough users of this to matter.
We're just talking a few extra bytes, and a couple of extra
instructions (and possibly slightly higher register pressure, which
then generates more instructions).

The *expensive* part remains the SMP serialization of the "lock".

                Linus

  reply	other threads:[~2021-07-28 18:57 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
2021-07-28 18:57         ` Linus Torvalds [this message]
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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