From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from host.buserror.net (host.buserror.net [209.198.135.123]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 71D4B1A05A9 for ; Tue, 23 Feb 2016 13:15:24 +1100 (AEDT) Message-ID: <1456193718.2463.142.camel@buserror.net> From: Scott Wood To: Michael Ellerman , Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Date: Mon, 22 Feb 2016 20:15:18 -0600 In-Reply-To: <1456193080.16667.1.camel@ellerman.id.au> References: <20160211161650.1F12C1A2400@localhost.localdomain> <1455657664.2463.68.camel@buserror.net> <1456193080.16667.1.camel@ellerman.id.au> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Subject: Re: [PATCH v5] powerpc32: provide VIRT_CPU_ACCOUNTING List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, 2016-02-23 at 13:04 +1100, Michael Ellerman wrote: > On Tue, 2016-02-16 at 15:21 -0600, Scott Wood wrote: > > > On Thu, 2016-02-11 at 17:16 +0100, Christophe Leroy wrote: > > > > This patch provides VIRT_CPU_ACCOUTING to PPC32 architecture. > > > PPC32 doesn't have the PACA structure, so we use the task_info > > > structure to store the accounting data. > > > > > > In order to reuse on PPC32 the PPC64 functions, all u64 data has > > > been replaced by 'unsigned long' so that it is u32 on PPC32 and > > > u64 on PPC64 > > > > > > Signed-off-by: Christophe Leroy > > > --- > > > Changes in v3: unlike previous version of the patch that was inspired > > > from IA64 architecture, this new version tries to reuse as much as > > > possible the PPC64 implementation. > > > > > > PPC32 doesn't have PACA and past discusion on v2 version has shown > > > that it is not worth implementing a PACA in PPC32 architecture > > > (see below benh opinion) > > > > > > benh: PACA is actually a data structure and you really really don't want > > > it > > > on ppc32 :-) Having a register point to current works, having a register > > > point to per-cpu data instead works too (ie, change what we do today), > > > but don't introduce a PACA *please* :-) > > > > And Ben never replied to my reply at the time: > > > > "What is special about 64-bit that warrants doing things differently from > > 32 > > -bit? > > Nothing. It's just historical cruft. But we're not realistically going to > get > rid of it anytime soon on 64-bit. I wasn't suggesting getting rid of it on 64-bit, but rather adding it on 32 -bit, to hold things that are used by both. I was confused by the vehemence of Ben's objection. > > What is the difference between PACA and "per-cpu data", other than the > > obscure name?" > > Not much. The pacas are allocated differently to per-cpu data, they're > available earlier in boot etc. Ah, I was thinking of the general concept of per-cpu data, not the specific mechanism that Linux implements in percpu.h etc. > What we'd like is to have r13 point to the > per-cpu data area, and then the contents of the paca could just be regular > per-cpu data. But like I said above that's a big change. That change seems orthogonal to the question of making the mechanism available on 32-bit to ease unification of code which uses it. -Scott