linux-kernel - Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

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Message-ID: <20160225170416.GV6356@twins.programming.kicks-ass.net>
Date:	Thu, 25 Feb 2016 18:04:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.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>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
 running thread

On Thu, Feb 25, 2016 at 04:55:26PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 25, 2016, at 4:56 AM, Peter Zijlstra peterz@...radead.org wrote:
> The restartable sequences are intrinsically designed to work
> on per-cpu data, so they need to fetch the current CPU number
> within the rseq critical section. This is where the getcpu_cache
> system call becomes very useful when combined with rseq:
> getcpu_cache allows reading the current CPU number in a
> fraction of cycle.

Yes yes, I know how restartable sequences work.

But what I worry about is that they want a cpu number and a sequence
number, and for performance it would be very good if those live in the
same cacheline.

That means either getcpu needs to grow a seq number, or restartable
sequences need to _also_ provide the cpu number.