Date: Thu, 7 Apr 2016 13:22:32 -0700
From: Andi Kleen <andi@firstfloor.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Florian Weimer <fweimer@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>, Paul Turner <pjt@google.com>,
        Andrew Hunter <ahh@google.com>, Andy Lutomirski <luto@amacapital.net>,
        Andi Kleen <andi@firstfloor.org>, Dave Watson <davejwatson@fb.com>,
        Chris Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU
 number of running thread
Message-ID: <20160407202232.GF9407@two.firstfloor.org>
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> 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