linux-kernel - Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

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Message-ID: <1066702992.9268.1627497574534.JavaMail.zimbra@efficios.com>
Date:   Wed, 28 Jul 2021 14:39:34 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     paulmck <paulmck@...nel.org>, rcu <rcu@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and
 updates

----- On Jul 28, 2021, at 2:32 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Wed, Jul 28, 2021 at 11:12 AM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> Are the percpu atomics you have in mind different from what is found in
>> Documentation/core-api/this_cpu_ops.rst ?
>>
>> Namely this_cpu_add_return(pcp, val) in this case.
> 
> Nope.
> 
> Those are only "CPU-atomic", ie atomic wrt interrupts etc.
> 
> The RCU code wants SMP-atomic, and it's mainly that we *could* do the
> addressing more efficiently.

OK, so combining the addressing tricks of this_cpu operations with
smp-atomic operations (e.g. LOCK prefix on x86). It may indeed become
worthwhile given enough users, and fast enough atomic operations.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com