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

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Date:	Tue, 1 Mar 2016 22:32:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ben Maurer <bmaurer@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	linux-api <linux-api@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Dave Watson <davejwatson@...com>,
	rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Chris Lameter <cl@...ux.com>, Andi Kleen <andi@...stfloor.org>,
	Josh Triplett <josh@...htriplett.org>,
	Paul Turner <pjt@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Andrew Hunter <ahh@...gle.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
 running thread

On Tue, Mar 01, 2016 at 08:23:12PM +0000, Mathieu Desnoyers wrote:
> I think it's important that user-space fast-paths can quickly
> detect whether the feature is enabled without having to rely on
> always reading a separate cache-line. I've put together an ABI
> proposal that take into account the feedback received so far.

Nah, adding detectoring code to fast paths is silly, makes them less
fast. Doesn't userspace have self modifying code? I know that at least
glibc does linker trickery to call different functions depending on
runtime context.

> struct thread_local_abi {
>         /*
>          * Thread-local ABI cpu_id field.
>          * Updated by the kernel, and read by user-space with
>          * single-copy atomicity semantics. Aligned on 32-bit.
>          * Values:
>          * >= 0: CPU number of running thread.
>          * -1 (initial value): means the cpu_id feature is inactive.
>          * -2: cpu_id feature is not available.
>          */
>         int32_t cpu_id;
> 
>         /*
>          * Thread-local ABI rseq_seqnum field.
>          * Updated by the kernel, and read by user-space with
>          * single-copy atomicity semantics. Aligned on 32-bit.
>          * Values:
>          * >= 0: current seqnum for this thread (feature is active).
>          * -1 (initial value): means the rseq feature is inactive.
>          * -2: rseq feature is not available.
>          */
>         int32_t rseq_seqnum;

So I really hate that, that makes we have to check for these special
values whenever we increment the seq count and cannot have it wrap
naturally.