linux-kernel - Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170413182939.jobaxup4k7n3qbhf@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Apr 2017 20:29:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text
 more explicit about skew_tick

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote:

> > And I have vague memories of it actually causing lock contention, but
> > I've forgotten how that worked.
> 
> That is a new one on me.  I can easily see how not skewing ticks could
> cause serious lock contention, but am missing how skewed ticks would
> do so.

It could've been something like cacheline bouncing. Where with a
synchronized tick, the (global) cacheline would get used by all CPUs on
a node before heading out to the next node etc.. Where with a skewed
tick, it would forever bounce around.