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

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Date:	Fri, 26 Feb 2016 17:29:55 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	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 Fri, 26 Feb 2016, Peter Zijlstra wrote:
> On Thu, Feb 25, 2016 at 05:17:51PM +0000, Mathieu Desnoyers wrote:
> > ----- On Feb 25, 2016, at 12:04 PM, Peter Zijlstra peterz@...radead.org wrote:
> > 
> > > 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.
> > 
> > If we plan things well, we could have both the cpu number and the
> > seqnum in the same cache line, registered by two different system
> > calls. It's up to user-space to organize those two variables
> > to fit within the same cache-line.
> 
> I feel this is more fragile than needed. Why not do a single systemcall
> that does both?

Right. There is no point in having two calls and two update mechanisms for a
very similar purpose.

So let userspace have one struct where cpu/seq and whatever is required for
rseq is located and flag at register time which parts of the struct need to be
updated.

Thanks,

	tglx