linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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]

Date:	Thu, 10 Oct 2013 16:57:38 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2


* Peter Zijlstra <peterz@...radead.org> wrote:

> But the thing is; our sense of NR_CPUS has shifted, where it used to be 
> ok to do something like:
> 
>   for_each_cpu()
> 
> With preemption disabled; it gets to be less and less sane to do so, 
> simply because 'common' hardware has 256+ CPUs these days. If we cannot 
> rely on preempt disable to exclude hotplug, we must use 
> get_online_cpus(), but get_online_cpus() is global state and thus cannot 
> be used at any sort of frequency.

So ... why not make it _really_ cheap, i.e. the read lock costing nothing, 
and tie CPU hotplug to freezing all tasks in the system?

Actual CPU hot unplugging and repluggin is _ridiculously_ rare in a 
system, I don't understand how we tolerate _any_ overhead from this utter 
slowpath.

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/