linux-kernel - Re: [RFC PATCH 0/3] Implement getcpu

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Message-ID: <20160111230306.GC28717@cloud>
Date:	Mon, 11 Jan 2016 15:03:06 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	"Seymour, Shane M" <shane.seymour@....com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH 0/3] Implement getcpu_cache system call

On Mon, Jan 11, 2016 at 10:38:28PM +0000, Seymour, Shane M wrote:
> I have some concerns and suggestions for you about this.
> 
> What's to stop someone in user space from requesting an arbitrarily large number of CPU # cache locations that the kernel needs to allocate memory to track and each time the task migrates to a new CPU it needs to update them all? Could you use it to dramatically slow down a system/task switching? Should there be a ulimit type value or a sysctl setting to limit the number that you're allowed to register per-task?

The documented behavior of the syscall allows only one location per
thread, so the kernel can track that one and only address rather easily
in the task_struct.  Allowing dynamic allocation definitely doesn't seem
like a good idea.

- Josh Triplett