From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6B29AC43381 for ; Mon, 25 Mar 2019 14:07:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 46FA22087E for ; Mon, 25 Mar 2019 14:07:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729129AbfCYOHJ (ORCPT ); Mon, 25 Mar 2019 10:07:09 -0400 Received: from ms01.santannapisa.it ([193.205.80.98]:43764 "EHLO mail.santannapisa.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726059AbfCYOHJ (ORCPT ); Mon, 25 Mar 2019 10:07:09 -0400 Received: from [10.30.3.184] (account l.abeni@santannapisa.it HELO luca64) by santannapisa.it (CommuniGate Pro SMTP 6.1.11) with ESMTPSA id 137597243; Mon, 25 Mar 2019 15:07:08 +0100 Date: Mon, 25 Mar 2019 15:07:03 +0100 From: luca abeni To: Boris Ostrovsky Cc: Dario Faggioli , thibodux@gmail.com, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, oleksandr_andrushchenko@epam.com, tglx@linutronix.de, jgross@suse.com, ryan.thibodeaux@starlab.io Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option Message-ID: <20190325150703.3774b92d@luca64> In-Reply-To: <69e40698-f7ae-11c3-e4b7-dda4f1fadcf6@oracle.com> References: <1553279397-130201-1-git-send-email-ryan.thibodeaux@starlab.io> <52bfeae7c256faec444b69efe58d363ad60c3fc5.camel@suse.com> <20190323114151.5cebf31b@sweethome> <20190325130530.56603806@luca64> <69e40698-f7ae-11c3-e4b7-dda4f1fadcf6@oracle.com> Organization: Scuola Superiore S. Anna X-Mailer: Claws Mail 3.17.1 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Mon, 25 Mar 2019 09:43:20 -0400 Boris Ostrovsky wrote: [...] > > http://retis.santannapisa.it/luca/XenTimers/ > > (there also is a link to the scripts to be used for reproducing the > > results). The latencies have been measured by running cyclictest in > > the guest (see the scripts for details). > > > > The picture shows the latencies measured with an unpatched guest > > kernel and with a guest kernel having TIMER_SLOP set to 1000 > > (arbitrary small value :). > > All the experiments have been performed booting the hypervisor with > > a small timer_slop (the hypervisor's one) value. So, they show that > > decreasing the hypervisor's timer_slop is not enough to measure low > > latencies with cyclictest. > > > > I have a couple of questions: > * Does it make sense to make this a tunable for other clockevent > devices as well? > * This patch adjusts min value. Could max value (ever) need a similar > adjustment? Sorry, I do not know much about clockevent devices, so I have no answers to these questions... What I can say is that when I repeated the cyclictest experiments on VMs using a different clockevent device (lapic) I did not measure large latencies. So, I guess the "lapic" clockevent device already defaults to a smaller min value (not sure about other clockevent devices, I do not know how to test them). Luca