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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 0216AC64E7C for ; Wed, 2 Dec 2020 12:46:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9C2A6221FE for ; Wed, 2 Dec 2020 12:46:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727689AbgLBMqR (ORCPT ); Wed, 2 Dec 2020 07:46:17 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54888 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726730AbgLBMqQ (ORCPT ); Wed, 2 Dec 2020 07:46:16 -0500 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3AB04C0613CF; Wed, 2 Dec 2020 04:45:36 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=h8OFn1f4rN5V534vygyf7uXjyFm0rtIR0TiLL9ElMpE=; b=holGE3HHKSUGIf0MMwr+/aM+qs FTNKPPXCxqVAY1s9pQKziuNBFrbP70W8+ZvdIMJroEiNwmrXozoIpAUZR/GV1yh3VESZ2MpDeAJRI RLkV3mo+oOqWE6hc7LgRr1jwJdcjnLfXpa9cty+bXSyzGJ3d9ruZPbDKSSPB2dFPb4sKj/dSDdB3d mvQJh7CkL1vepEOPZt2GA0Ly300hbL9J8oFFMGEk5GGrV202e3qSNXweEzPECETXURfWxBcGNFMi9 UNg2132ny9LyyXy8sG4nBgpRx6mYDp4i31b+pPxpPKfj+Ux54teqDFT81lWSGBsDsM/IQpdYzVv52 o8k/PYAQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kkRVa-000141-5Z; Wed, 02 Dec 2020 12:45:22 +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 A93A23035D4; Wed, 2 Dec 2020 13:45:19 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 82E5B2143477D; Wed, 2 Dec 2020 13:45:19 +0100 (CET) Date: Wed, 2 Dec 2020 13:45:19 +0100 From: Peter Zijlstra To: Nicholas Piggin Cc: linux-kernel@vger.kernel.org, x86@kernel.org, Mathieu Desnoyers , Arnd Bergmann , linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, Anton Blanchard Subject: Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Message-ID: <20201202124519.GP3092@hirez.programming.kicks-ass.net> References: <20201128160141.1003903-1-npiggin@gmail.com> <20201128160141.1003903-7-npiggin@gmail.com> <20201202111731.GA2414@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201202111731.GA2414@hirez.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 02, 2020 at 12:17:31PM +0100, Peter Zijlstra wrote: > So the obvious 'improvement' here would be something like: > > for_each_online_cpu(cpu) { > p = rcu_dereference(cpu_rq(cpu)->curr; > if (p->active_mm != mm) > continue; > __cpumask_set_cpu(cpu, tmpmask); > } > on_each_cpu_mask(tmpmask, ...); > > The remote CPU will never switch _to_ @mm, on account of it being quite > dead, but it is quite prone to false negatives. > > Consider that __schedule() sets rq->curr *before* context_switch(), this > means we'll see next->active_mm, even though prev->active_mm might still > be our @mm. > > Now, because we'll be removing the atomic ops from context_switch()'s > active_mm swizzling, I think we can change this to something like the > below. The hope being that the cost of the new barrier can be offset by > the loss of the atomics. > > Hmm ? > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 41404afb7f4c..2597c5c0ccb0 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -4509,7 +4509,6 @@ context_switch(struct rq *rq, struct task_struct *prev, > if (!next->mm) { // to kernel > enter_lazy_tlb(prev->active_mm, next); > > - next->active_mm = prev->active_mm; > if (prev->mm) // from user > mmgrab(prev->active_mm); > else > @@ -4524,6 +4523,7 @@ context_switch(struct rq *rq, struct task_struct *prev, > * case 'prev->active_mm == next->mm' through > * finish_task_switch()'s mmdrop(). > */ > + next->active_mm = next->mm; > switch_mm_irqs_off(prev->active_mm, next->mm, next); I think that next->active_mm store should be after switch_mm(), otherwise we still race. > > if (!prev->mm) { // from kernel > @@ -5713,11 +5713,9 @@ static void __sched notrace __schedule(bool preempt) > > if (likely(prev != next)) { > rq->nr_switches++; > - /* > - * RCU users of rcu_dereference(rq->curr) may not see > - * changes to task_struct made by pick_next_task(). > - */ > - RCU_INIT_POINTER(rq->curr, next); > + > + next->active_mm = prev->active_mm; > + rcu_assign_pointer(rq->curr, next); > /* > * The membarrier system call requires each architecture > * to have a full memory barrier after updating