linux-kernel - Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of running thread

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Message-ID: <20160105174017.GY19062@n2100.arm.linux.org.uk>
Date:	Tue, 5 Jan 2016 17:40:18 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Will Deacon <will.deacon@....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-api <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>,
	rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of
 running thread

On Tue, Jan 05, 2016 at 05:31:45PM +0000, Mathieu Desnoyers wrote:
> For instance, an application could create a linked list or hash map
> of thread control structures, which could contain the current CPU
> number of each thread. A dispatch thread could then traverse or
> lookup this structure to see on which CPU each thread is running and
> do work queue dispatch or scheduling decisions accordingly.

So, what happens if the linked list is walked from thread X, and we
discover that thread Y is allegedly running on CPU1.  We decide that
we want to dispatch some work on that thread due to it being on CPU1,
so we send an event to thread Y.

Thread Y becomes runnable, and the scheduler decides to schedule the
thread on CPU3 instead of CPU1.

My point is that the above idea is inherently racy.  The only case
where it isn't racy is when thread Y is bound to CPU1, and so can't
move - but then you'd know that thread Y is on CPU1 and there
wouldn't be a need for the inherent complexity suggested above.

The behaviour I've seen on ARM from the scheduler (on a quad CPU
platform, observing the system activity with top reporting the last
CPU number used by each thread) is that threads often migrate
between CPUs - especially in the case of (eg) one or two threads
running in a quad-CPU system.

Given that, I'm really not sure what the use of reading and making
decisions on the current CPU number would be within a program -
unless the thread is bound to a particular CPU or group of CPUs,
it seems that you can't rely on being on the reported CPU by the
time the system call returns.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
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