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Mon, 25 Mar 2019 18:31:53 GMT Received: from abhmp0016.oracle.com (abhmp0016.oracle.com [141.146.116.22]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x2PIVqKh024393; Mon, 25 Mar 2019 18:31:52 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 25 Mar 2019 11:31:52 -0700 Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option To: Ryan Thibodeaux Cc: luca abeni , Dario Faggioli , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, oleksandr_andrushchenko@epam.com, tglx@linutronix.de, jgross@suse.com, ryan.thibodeaux@starlab.io 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> <20190325141138.GA44413@centos-dev.localdomain> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190325141138.GA44413@centos-dev.localdomain> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9206 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=877 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1903250132 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/25/19 10:11 AM, Ryan Thibodeaux wrote: > On Mon, Mar 25, 2019 at 09:43:20AM -0400, Boris Ostrovsky wrote: >> On 3/25/19 8:05 AM, luca abeni wrote: >>> Hi all, >>> >>> On Sat, 23 Mar 2019 11:41:51 +0100 >>> luca abeni wrote: >>> [...] >>>>>> Is there any data that shows effects of using this new parameter? >>>>>> >>>>> Yes, I've done some research and experiments on this. I did it >>>>> together with a friend, which I'm Cc-ing, as I'm not sure we're >>>>> ready/capable to share the results, yet (Luca?). >>>> I think we can easily share the experimental data (cyclictest output >>>> and plots). >>>> >>>> Moreover, we can share the scripts and tools for running the >>>> experiments (so, everyone can easily reproduce the numbers by simply >>>> typing "make" and waiting for some time :) >>>> >>>> >>>> I'll try to package the results and the scripts/tools this evening, >>>> and I'll send them. >>> Sorry for the delay. I put some quick results here: >>> 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? > I gather that would be on a case-by-case basis for very specific > ones. > > For many timers in the kernel, the minimums are determined by the > actual hardware backing the timer, and the minimum can be > adjusted by the clockevent code. This case is special since it > is entirely a software-based implementation in the kernel, where > the actual timer implementation is in the Xen hypervisor. > >> * This patch adjusts min value. Could max value (ever) need a similar >> adjustment? > I cannot think of such a case where that would be helpful, but I > cannot rule that out or speak as an authority. I am asking mostly because you are introducing new interface and I don't want it to change in the future. I suppose if later we decide to add control for the max value we could just expand your current proposal to xen_timer_slop=[min],[max] and keep it to be back-compatible. For the patch: Reviewed-by: Boris Ostrovsky