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

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Date:	Tue, 5 Apr 2016 18:47:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Florian Weimer <fweimer@...hat.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.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@...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>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU
 number of running thread

On Tue, Apr 05, 2016 at 06:02:25PM +0200, Florian Weimer wrote:
> On 04/04/2016 10:48 PM, Mathieu Desnoyers wrote:
> 
> > Moreover, the feature set that the application knows about, glibc
> > knows about, and the kernel knows about are three different things.
> > My intent here is to have glibc stay out of the way as much as possible,
> > since this is really an interface between various applications/libraries
> > and the kernel.
> 
> Surely glibc can allocate the space based on what is advertised as
> needed by the kernel?  Why would it limit itself to what is supported by
> the kernel headers it is compiled against if the actual size can be
> queried from the kernel?

I guess the question is; can we do thread local variable arrays like:

  __thread uint32_t[x]; /* with x being a runtime constant */

Because then we can do:

  __thread struct thread_local_abi tla;

where sizeof(struct thread_local_abi) is a runtime variable.

Without that we cannot have this thread-local-abi structure be part of
the immediately addressable TLS space. That is, we then need a pointer
like:

  __thread struct thread_local_abi *tla;

and every usage will need the extra pointer deref.

Because ideally this structure would be part of the initial (glibc) TCB
with fixed offset etc.