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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 6C05BC5519F for ; Mon, 30 Nov 2020 09:26:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ED2632084C for ; Mon, 30 Nov 2020 09:26:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="kbQGkp1u" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728141AbgK3J0c (ORCPT ); Mon, 30 Nov 2020 04:26:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56696 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726249AbgK3J0b (ORCPT ); Mon, 30 Nov 2020 04:26:31 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 40A3BC0613D2; Mon, 30 Nov 2020 01:25:51 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=4dY4EOkG53RHP6M1jV6qxthiECmU22437CkB/VPnVPw=; b=kbQGkp1u8lv60ej2mi34V4mTz0 N8a3Ez89RDu9lKYw76gAQyyX4evCA66FdyQJveREpKWf3AV58pF4mJjBEPUd7MQos+KYNuk8RDGEN sh+Zc+1F45nsR3AapUUzVrd1wQkIKgS99MhYVVkhjfPOHphpcbIBCgYFiPh557zukzfsSuu5fuBGP tVD5hdB91z5EfXbvmHYzNrB/zkTO/Rj+qGDA7wEC3F3Mynk1troqL16mzp01WYZxOPKo/f2GikU41 jAHYbokoPtpCzotPYSODIsX08cc+6ZQQ/LUA7JxAacs4u8VFci8A+OqeitAohkQmbvqFrxeY466Bn tiJGqL4Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kjfQy-0007TZ-F6; Mon, 30 Nov 2020 09:25:24 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 3C56F301179; Mon, 30 Nov 2020 10:25:23 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 1B33020107BE2; Mon, 30 Nov 2020 10:25:23 +0100 (CET) Date: Mon, 30 Nov 2020 10:25:23 +0100 From: Peter Zijlstra To: Andy Lutomirski Cc: Nicholas Piggin , LKML , X86 ML , Mathieu Desnoyers , Arnd Bergmann , linux-arch , linuxppc-dev , Linux-MM , Anton Blanchard Subject: Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Message-ID: <20201130092523.GK2414@hirez.programming.kicks-ass.net> References: <20201128160141.1003903-1-npiggin@gmail.com> <20201128160141.1003903-7-npiggin@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Nov 29, 2020 at 12:16:26PM -0800, Andy Lutomirski wrote: > On Sat, Nov 28, 2020 at 7:54 PM Andy Lutomirski wrote: > > > > On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin wrote: > > > > > > On big systems, the mm refcount can become highly contented when doing > > > a lot of context switching with threaded applications (particularly > > > switching between the idle thread and an application thread). > > > > > > Abandoning lazy tlb slows switching down quite a bit in the important > > > user->idle->user cases, so so instead implement a non-refcounted scheme > > > that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down > > > any remaining lazy ones. > > > > > > Shootdown IPIs are some concern, but they have not been observed to be > > > a big problem with this scheme (the powerpc implementation generated > > > 314 additional interrupts on a 144 CPU system during a kernel compile). > > > There are a number of strategies that could be employed to reduce IPIs > > > if they turn out to be a problem for some workload. > > > > I'm still wondering whether we can do even better. > > > > Hold on a sec.. __mmput() unmaps VMAs, frees pagetables, and flushes > the TLB. On x86, this will shoot down all lazies as long as even a > single pagetable was freed. (Or at least it will if we don't have a > serious bug, but the code seems okay. We'll hit pmd_free_tlb, which > sets tlb->freed_tables, which will trigger the IPI.) So, on > architectures like x86, the shootdown approach should be free. The > only way it ought to have any excess IPIs is if we have CPUs in > mm_cpumask() that don't need IPI to free pagetables, which could > happen on paravirt. > > Can you try to figure out why you saw any increase in IPIs? It would > be nice if we can make the new code unconditional. Power doesn't do IPI based TLBI.