From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (ozlabs.org [103.22.144.67]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id E5D521A0591 for ; Tue, 23 Feb 2016 14:25:45 +1100 (AEDT) Message-ID: <1456197945.12877.1.camel@ellerman.id.au> Subject: Re: [PATCH v5] powerpc32: provide VIRT_CPU_ACCOUNTING From: Michael Ellerman To: Scott Wood , Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Date: Tue, 23 Feb 2016 14:25:45 +1100 In-Reply-To: <1456193718.2463.142.camel@buserror.net> References: <20160211161650.1F12C1A2400@localhost.localdomain> <1455657664.2463.68.camel@buserror.net> <1456193080.16667.1.camel@ellerman.id.au> <1456193718.2463.142.camel@buserror.net> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Mon, 2016-02-22 at 20:15 -0600, Scott Wood wrote: > 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 been replaced by 'unsigned long' so that it is u32 on PPC32 and 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. OK right. I think he's just saying we'd like to (eventually) get rid of it on 64-bit, so adding it on 32-bit would be a step backward. > > > 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. Oh ok, in that case no it's not special at all. > > 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. That's true. Though in this case I think you actually do want to store those values in the thread_info. If you look at eg. vtime_delta() where we use those values, it's passed a task struct. So your suggestion to define a common struct that is shared between the 32-bit thread_info and the 64-bit paca would be a good solution I think. cheers