linux-kernel - Re: [PATCH RFC tip/core/rcu] SRCU rewrite

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Message-ID: <1857730044.1901.1479218090893.JavaMail.zimbra@efficios.com>
Date:   Tue, 15 Nov 2016 13:54:50 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        dvhart <dvhart@...ux.intel.com>, fweisbec <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        bobby prani <bobby.prani@...il.com>, ldr709 <ldr709@...il.com>
Subject: Re: [PATCH RFC tip/core/rcu] SRCU rewrite

----- On Nov 15, 2016, at 2:51 AM, Peter Zijlstra peterz@...radead.org wrote:

> On Mon, Nov 14, 2016 at 10:36:36AM -0800, Paul E. McKenney wrote:
>> SRCU uses two per-cpu counters: a nesting counter to count the number of
>> active critical sections, and a sequence counter to ensure that the nesting
>> counters don't change while they are being added together in
>> srcu_readers_active_idx_check().
>> 
>> This patch instead uses per-cpu lock and unlock counters. Because the both
>> counters only increase and srcu_readers_active_idx_check() reads the unlock
>> counter before the lock counter, this achieves the same end without having
>> to increment two different counters in srcu_read_lock(). This also saves a
>> smp_mb() in srcu_readers_active_idx_check().
> 
> A very small improvement... I feel SRCU has much bigger issues :/

Do you have specific issues in mind ?

Thanks,

Mathieu

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