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 2EB57C7619A for ; Wed, 5 Apr 2023 11:43:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237841AbjDELnT (ORCPT ); Wed, 5 Apr 2023 07:43:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54574 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237443AbjDELnQ (ORCPT ); Wed, 5 Apr 2023 07:43:16 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5BD0D3AA9; Wed, 5 Apr 2023 04:42:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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=DQKKqvngu6XrRzj+GHyns7zyGuc6jnX9ARbttsuRzGk=; b=idnRPt7gzQIUPNNaD/IneFbHZm Hm/M7lAn3X4M4YAYJsQgsjTurXZhVLP+iiZ9DLmmlDgK6Ty2FAY7CRFNNpqMni4zQbFYv3FtAENxu rNPAKqNkSHWeLDWiAjKwNyv5my/aHd3mnPLCSOHTHa9T7oJzr+m9d08KJcsOsdQQx5jQUsB9UhhSb RLcTiqExcEOHERblZmxKkZC4oD4bnhiuRkWSqwuebLcFLkMDvh1IOWxDvijroXsSnIf9/EcuRxJFg DXJ4LPSdwxCbUqwIBrua3JDo72zyVBMFh6D4PI6IsCPV+XocyHo3f1L25Z3MBwKSoBQ7MBQZtcEmr 6DcJWnTw==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1pk1WT-009tua-07; Wed, 05 Apr 2023 11:41:53 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id F3173300274; Wed, 5 Apr 2023 13:41:48 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 9417320B6A7D6; Wed, 5 Apr 2023 13:41:48 +0200 (CEST) Date: Wed, 5 Apr 2023 13:41:48 +0200 From: Peter Zijlstra To: Frederic Weisbecker Cc: Yair Podemsky , linux@armlinux.org.uk, mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, will@kernel.org, aneesh.kumar@linux.ibm.com, akpm@linux-foundation.org, arnd@arndb.de, keescook@chromium.org, paulmck@kernel.org, jpoimboe@kernel.org, samitolvanen@google.com, ardb@kernel.org, juerg.haefliger@canonical.com, rmk+kernel@armlinux.org.uk, geert+renesas@glider.be, tony@atomide.com, linus.walleij@linaro.org, sebastian.reichel@collabora.com, nick.hawkins@hpe.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, mtosatti@redhat.com, vschneid@redhat.com, dhildenb@redhat.com, alougovs@redhat.com Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI only to CPUs in kernel mode Message-ID: <20230405114148.GA351571@hirez.programming.kicks-ass.net> References: <20230404134224.137038-1-ypodemsk@redhat.com> <20230404134224.137038-4-ypodemsk@redhat.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-arch@vger.kernel.org On Wed, Apr 05, 2023 at 01:10:07PM +0200, Frederic Weisbecker wrote: > On Wed, Apr 05, 2023 at 12:44:04PM +0200, Frederic Weisbecker wrote: > > On Tue, Apr 04, 2023 at 04:42:24PM +0300, Yair Podemsky wrote: > > > + int state = atomic_read(&ct->state); > > > + /* will return true only for cpus in kernel space */ > > > + return state & CT_STATE_MASK == CONTEXT_KERNEL; > > > +} > > > > Also note that this doesn't stricly prevent userspace from being interrupted. > > You may well observe the CPU in kernel but it may receive the IPI later after > > switching to userspace. > > > > We could arrange for avoiding that with marking ct->state with a pending work bit > > to flush upon user entry/exit but that's a bit more overhead so I first need to > > know about your expectations here, ie: can you tolerate such an occasional > > interruption or not? > > Bah, actually what can we do to prevent from that racy IPI? Not much I fear... Yeah, so I don't think that's actually a problem. The premise is that *IFF* NOHZ_FULL stays in userspace, then it will never observe the IPI. If it violates this by doing syscalls or other kernel entries; it gets to keep the pieces.