From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S932134AbdA3OfB (ORCPT <rfc822;w@1wt.eu>);
        Mon, 30 Jan 2017 09:35:01 -0500
Received: from mx1.redhat.com ([209.132.183.28]:43554 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S932074AbdA3Oex (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 30 Jan 2017 09:34:53 -0500
Date: Mon, 30 Jan 2017 15:32:24 +0100
From: Stanislaw Gruszka <sgruszka@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Tony Luck <tony.luck@intel.com>, Wanpeng Li <wanpeng.li@hotmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Mackerras <paulus@samba.org>, Fenghua Yu <fenghua.yu@intel.com>,
        Rik van Riel <riel@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [GIT PULL] cputime: Convert core use of cputime_t to nsecs
Message-ID: <20170130143223.GC2669@redhat.com>
References: <1485751603-15913-1-git-send-email-fweisbec@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1485751603-15913-1-git-send-email-fweisbec@gmail.com>
User-Agent: Mutt/1.5.20 (2009-12-10)
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Mon, 30 Jan 2017 14:34:54 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Jan 30, 2017 at 05:46:43AM +0100, Frederic Weisbecker wrote:
> Now lets admit one drawback: s390 and powerpc with
> CONFIG_VIRT_CPU_ACCOUNTING_NATIVE have new cputime_t to nsecs conversion
> on cputime accounting path. But this should be leveraged by the recent
> changes which delay the cputime accounting to tick and context switch. 

I think it would be worth to mention that there are other drawbacks on
32bit architectures that use cputime-jiffies currently, like:
- cache utilization will be worse
- conversion cputime_to_jiffies() and cputime_to_clock_t() (with HZ == USER_HS)
  will no longer be an no-op
- to keep values consistent will need to add protection of u64 store/load,
  which will create additional performance costs

Stanislaw