linux-kernel - Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU number of running thread

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Message-ID: <20160407202232.GF9407@two.firstfloor.org>
Date:	Thu, 7 Apr 2016 13:22:32 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Florian Weimer <fweimer@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <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>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU
 number of running thread

> One basic use of cpu id cache is to speed up the sched_getcpu(3)
> implementation in glibc. This is why I'm proposing it as a stand-alone

I don't think rseq is needed for faster getcpu.

User space has to be able handle stale return values anyways, as it
has no way to lock itself to a cpu while it is using the return value.
So it can be only a hint.

The original version of getcpu just had a jiffies based cache. The CPU
value was valid up to a jiffie (the next time jiffie changes), and then it
gets looked up again.

Processes are unlikely to switch CPUs more often than a jiffie, so it's
good enough as a hint.

This doesn't need any new kernel interfaces at all because jiffies is already
exported to the vdso.

It just needs a new entry point into the vdso that handles the jiffie
check.

-Andi