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 33134C433F5 for ; Fri, 7 Jan 2022 13:04:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347377AbiAGNEY (ORCPT ); Fri, 7 Jan 2022 08:04:24 -0500 Received: from smtp-out2.suse.de ([195.135.220.29]:41722 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231883AbiAGNEW (ORCPT ); Fri, 7 Jan 2022 08:04:22 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 4D66B1F397; Fri, 7 Jan 2022 13:04:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1641560661; h=from:from:reply-to: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=ydS8jbmtTNSvhWKqliTJ11ZJ+8dm8meUoBr5M26AGBs=; b=qt9txL8Q9yT8aJ3LdX4wHHomcTGi8nLR32MAdumkrzI1zQINKKFXegLKTnGdo3jU9qnaAN v/zrgeWO21SkaEN7XP/vyhAUxp195PZ3aWs09oI5q8D1AMrUMMSWT9RMtlByTcErRyw52W zWNLk5dBfO3WW94F7cozZ4+el+0ae5E= Received: from suse.cz (unknown [10.100.216.66]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 30B1DA3B84; Fri, 7 Jan 2022 13:04:21 +0000 (UTC) Date: Fri, 7 Jan 2022 14:03:49 +0100 From: Petr Mladek To: David Vernet Cc: live-patching@vger.kernel.org, linux-kernel@vger.kernel.org, jpoimboe@redhat.com, jikos@kernel.org, mbenes@suse.cz, joe.lawrence@redhat.com Subject: Re: [PATCH] livepatch: Avoid CPU hogging with cond_resched Message-ID: References: <20211229215646.830451-1-void@manifault.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211229215646.830451-1-void@manifault.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 2021-12-29 13:56:47, David Vernet wrote: > When initializing a 'struct klp_object' in klp_init_object_loaded(), and > performing relocations in klp_resolve_symbols(), klp_find_object_symbol() > is invoked to look up the address of a symbol in an already-loaded module > (or vmlinux). This, in turn, calls kallsyms_on_each_symbol() or > module_kallsyms_on_each_symbol() to find the address of the symbol that is > being patched. > > It turns out that symbol lookups often take up the most CPU time when > enabling and disabling a patch, and may hog the CPU and cause other tasks > on that CPU's runqueue to starve -- even in paths where interrupts are > enabled. For example, under certain workloads, enabling a KLP patch with > many objects or functions may cause ksoftirqd to be starved, and thus for > interrupts to be backlogged and delayed. This may end up causing TCP > retransmits on the host where the KLP patch is being applied, and in > general, may cause any interrupts serviced by softirqd to be delayed while > the patch is being applied. > > So as to ensure that kallsyms_on_each_symbol() does not end up hogging the > CPU, this patch adds a call to cond_resched() in kallsyms_on_each_symbol() > and module_kallsyms_on_each_symbol(), which are invoked when doing a symbol > lookup in vmlinux and a module respectively. Without this patch, if a > live-patch is applied on a 36-core Intel host with heavy TCP traffic, a > ~10x spike is observed in TCP retransmits while the patch is being applied. > Additionally, collecting sched events with perf indicates that ksoftirqd is > awakened ~1.3 seconds before it's eventually scheduled. With the patch, no > increase in TCP retransmit events is observed, and ksoftirqd is scheduled > shortly after it's awakened. > > Signed-off-by: David Vernet OK, there was not any strong pushback. I have committed the patch into livepatch.git, branch for-5.17/kallsyms. Best Regards, Petr