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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4B0B9C6FD1D for ; Thu, 23 Mar 2023 13:31:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231719AbjCWNb0 (ORCPT ); Thu, 23 Mar 2023 09:31:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40184 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231513AbjCWNbX (ORCPT ); Thu, 23 Mar 2023 09:31:23 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 77BB4184 for ; Thu, 23 Mar 2023 06:30:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1679578237; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=DIdz7SL32Ku/igrCLHEovkyNzN22Xp2j4+xaPGhABbc=; b=Xk9ploU/eTjBLdkodO0DN97P6SWhtgOXlUre/ofOGPhCF+98NYsbwCHdY5YNuxB61mEvMe MJHrFmSNrczV67NKn3QPM7CrGe7qlMmP2QxhEOmws8rSNFJKm0SWO7UJr2FzoZWvVtoC8D i52u5LFqn2Ac4VkUnqoMR/OzhpTV0Iw= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-640-e-TjctOwO9SsrvYNbcXA6Q-1; Thu, 23 Mar 2023 09:30:32 -0400 X-MC-Unique: e-TjctOwO9SsrvYNbcXA6Q-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 5FB688030D3; Thu, 23 Mar 2023 13:30:31 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id F2144440BC; Thu, 23 Mar 2023 13:30:30 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 514C0403C47C1; Thu, 23 Mar 2023 10:30:13 -0300 (-03) Date: Thu, 23 Mar 2023 10:30:13 -0300 From: Marcelo Tosatti To: Michal Hocko Cc: Christoph Lameter , Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Russell King , Huacai Chen , Heiko Carstens , x86@kernel.org, Vlastimil Babka Subject: Re: [PATCH v7 00/13] fold per-CPU vmstats remotely Message-ID: References: <20230320180332.102837832@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 23, 2023 at 01:17:32PM +0100, Michal Hocko wrote: > On Thu 23-03-23 07:52:22, Marcelo Tosatti wrote: > > On Thu, Mar 23, 2023 at 08:51:14AM +0100, Michal Hocko wrote: > > > On Wed 22-03-23 11:20:55, Marcelo Tosatti wrote: > > > > On Wed, Mar 22, 2023 at 02:35:20PM +0100, Michal Hocko wrote: > > > [...] > > > > > > "Performance details for the kworker interruption: > > > > > > > > > > > > oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000) > > > > > > oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ... > > > > > > oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ... > > > > > > kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ... > > > > > > > > > > > > The example above shows an additional 7us for the > > > > > > > > > > > > oslat -> kworker -> oslat > > > > > > > > > > > > switches. In the case of a virtualized CPU, and the vmstat_update > > > > > > interruption in the host (of a qemu-kvm vcpu), the latency penalty > > > > > > observed in the guest is higher than 50us, violating the acceptable > > > > > > latency threshold for certain applications." > > > > > > > > > > Yes, I have seen that but it doesn't really give a wider context to > > > > > understand why those numbers matter. > > > > > > > > OK. > > > > > > > > "In the case of RAN, a MAC scheduler with TTI=1ms, this causes >100us > > > > interruption observed in a guest (which is above the safety > > > > threshold for this application)." > > > > > > > > Is that OK? > > > > > > This might be a sufficient information for somebody familiar with the > > > matter (not me). So no, not enough. We need to hear a more complete > > > story. > > > > Michal, > > > > Please refer to > > https://www.diva-portal.org/smash/get/diva2:541460/FULLTEXT01.pdf > > > > 2.3 Channel Dependent Scheduling > > The purpose of scheduling is to decide which terminal will transmit data on which set > > of resource blocks with what transport format to use. The objective is to assign > > resources to the terminal such that the quality of service (QoS) requirement is fulfilled. > > Scheduling decision is taken every 1 ms by base station (termed as eNodeB) as the > > same length of Transmission Time Interval (TTI) in LTE system. > > > > In general: > > > > https://en.wikipedia.org/wiki/Real-time_computing > > Thank you, but not something I was really asking for (repeatedly). I am > pretty aware of what RT computing is about. I am not really interested > in a generic fluff. I am asking about specific usecases you have in mind > when pushing these changes. > > > For example, for the MAC scheduler processing must occur every 1ms, > > and a certain amount of computation takes place (and must finish before > > the next 1ms timeframe). A > 50us latency spike as observed by cyclictest > > is considered a "failure". > > OK, you are claiming that much but you are not really filling up other > holes in your story. Let me just outline few questions I have. Your > measurements talk about 7us overhead the vmstat processing might add. > This is really far from > 50us above. 7us in the host, for the following sched_switch events: oslat -> kworker kworker -> oslat However, if the impact is for a virtualized application: oslat, executing via qemu-vcpu process in the host. oslat executing qemu-vcpu VM-EXIT qemu-vcpu -> kworker kworker -> qemu-vcpu qemu-vcpu VM-ENTRY is much higher than the 7us (can be above 100us). > You suggest that this is an effect > of the workload running in a guest without more details. I am quite > surprised to hear about RT expectations inside a guest system TBH. https://www.youtube.com/watch?v=zIDwc6uDszY > All that being said, it would be really helpful if you were more > specific about the workload and why there is no other way but making > vmstat infrastructure more complex (it is quite complex on its own). The patchset is just changing vmstat_shepherd from happening locally to happening remotely. There are a number of algorithms in the kernel that deal with concurrent access already. What you think this particular patchset makes things complicated and what can be done to make it simpler?