linux-kernel - RE: [RFC PATCH] thread_local_abi system call: caching current CPU number (x86)

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Message-ID: <5CDDBDF2D36D9F43B9F5E99003F6A0D48D616ED9@PRN-MBX02-1.TheFacebook.com>
Date:	Fri, 17 Jul 2015 10:49:19 +0000
From:	Ben Maurer <bmaurer@...com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Paul Turner <pjt@...gle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Andrew Hunter" <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: RE: [RFC PATCH] thread_local_abi system call: caching current CPU
 number (x86)

Mathieu Desnoyers wrote:
> Expose a new system call allowing threads to register a userspace memory
> area where to store the current CPU number. Scheduler migration sets the

I really like that this approach makes it easier to add a per-thread interaction between userspace and the kernel in the future.

>+       if (!tlap || t->thread_local_abi_len <
>+                       offsetof(struct thread_local_abi, cpu)
>+                       + sizeof(tlap->cpu))

Could you save a branch here by enforcing that thread_local_abi_len = 0 if thread_local_abi = null?

-b--
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