From: David Laight <david.laight.linux@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Josef Bacik <josef@toxicpanda.com>,
Frederic Weisbecker <frederic@kernel.org>,
Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
Joel Fernandes <joelagnelf@nvidia.com>,
Boqun Feng <boqun@kernel.org>, Thomas Gleixner <tglx@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Mark Rutland <mark.rutland@arm.com>, Jiri Olsa <jolsa@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
x86@kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Puranjay Mohan <puranjay@kernel.org>,
Xu Kuohai <xukuohai@huaweicloud.com>,
Andy Lutomirski <luto@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Uladzislau Rezki <urezki@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Zqiang <qiang.zhang@linux.dev>, Juergen Gross <jgross@suse.com>,
Luis Chamberlain <mcgrof@kernel.org>,
Ihor Solodrai <ihor.solodrai@linux.dev>,
linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
xen-devel@lists.xenproject.org
Subject: Re: [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline
Date: Sun, 13 Sep 2026 12:28:59 +0100 [thread overview]
Message-ID: <20260913122859.4d5067cd@pumpkin> (raw)
In-Reply-To: <ffffa178-c475-4249-a609-c5d222704987@paulmck-laptop>
On Sat, 12 Sep 2026 15:31:24 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:
> On Sat, Sep 12, 2026 at 10:14:00PM +0100, David Laight wrote:
> > On Sat, 12 Sep 2026 11:03:34 -0700
> > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> >
> > > In the old kernels, yes, we have current->trc_reader_nesting++.
> > > In the newer kernels, Tasks Trace RCU is instead implemented in terms
> > > of SRCU-fast, which instead increments per-CPU counters. Which among
> > > other thins is a bit faster and does not need to hook into the scheduler.
> >
> > Isn't that rather architecture dependant?
> > It is fine on x86, but on arm incrementing a per-cpu variable is
> > significantly expensive.
>
> Last I heard, slow ARM increments of per-CPU variables were to be a
> transitory phenomemon. Plus changes late last year greatly sped up the
> per-CPU increment operations.
There are some unapplied patches to improve per-cpu operations for both
arm64 and s390.
Without those preemption has to be disabled (in current->xxx) which requires
a conditional call in the preempt enable path.
David
> Plus this change removed some hundreds
> of lines of RCU code.
>
> Thanx, Paul
>
next prev parent reply other threads:[~2026-09-13 11:29 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-11 14:08 [PATCH RFC v2 00/15] rcu-tasks: let preemption outside trampolines be a quiescent state Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 01/15] rcu-tasks: Add per-task trampoline nesting count Josef Bacik
2026-09-11 17:23 ` Paul E. McKenney
2026-09-11 14:08 ` [PATCH RFC v2 02/15] entry: Pass pt_regs to irqentry_exit_cond_resched() Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 03/15] rcu-tasks: Hold trampoline nesting across irq-exit preemption in trampoline text Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 04/15] kprobes: Let Tasks RCU recognise tasks preempted in an optprobe jump window Josef Bacik
2026-09-11 14:26 ` sashiko-bot
2026-09-11 17:27 ` Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 05/15] ftrace: Mark modules hosting direct-call trampolines for Tasks RCU Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 06/15] x86/ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 07/15] x86/kprobes: Maintain Tasks RCU trampoline nesting in the optprobe template Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline Josef Bacik
2026-09-12 3:27 ` Alexei Starovoitov
2026-09-12 5:10 ` Paul E. McKenney
2026-09-12 17:18 ` Alexei Starovoitov
2026-09-12 18:03 ` Paul E. McKenney
2026-09-12 19:40 ` Alexei Starovoitov
2026-09-12 22:28 ` Paul E. McKenney
2026-09-12 23:59 ` Alexei Starovoitov
2026-09-13 3:07 ` Paul E. McKenney
2026-09-12 21:14 ` David Laight
2026-09-12 22:31 ` Paul E. McKenney
2026-09-13 11:28 ` David Laight [this message]
2026-09-13 18:20 ` Paul E. McKenney
2026-09-11 14:08 ` [PATCH RFC v2 09/15] arm64: ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 10/15] bpf, arm64: Maintain Tasks RCU trampoline nesting in the BPF trampoline Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 11/15] samples: ftrace: Maintain Tasks RCU trampoline nesting in direct-call trampolines Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 12/15] rcutorture: Bracket Tasks RCU readers with trampoline nesting Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 13/15] rcu-tasks: Treat preemption outside trampolines as a quiescent state Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 14/15] rcu-tasks: Retire switched-out tasks with no trampoline nesting at scan time Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 15/15] rcu-tasks: Kick running holdouts through the scheduler Josef Bacik
2026-09-11 18:46 ` Paul E. McKenney
2026-09-13 7:13 ` [PATCH RFC v2 00/15] rcu-tasks: let preemption outside trampolines be a quiescent state Yafang Shao
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