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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: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: F386E20020 X-Stat-Signature: pjkg9czjxik5aepk9pf13b1ptme8zne1 X-Rspam-User: X-HE-Tag: 1680694950-843731 X-HE-Meta: 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 15mVFFUN ffnuiEzjBXplP0YvuXKkAkVDYxcku9UC2rQ2OjF8KsXaQk5VF28uBblLghMbbkJkAZtBekgbpxYpOQoB4rbCnvK7dpQ9+A3qtxtVcceArdQSbmpUrKDqfIEkwf5JOHxOUXDF60EtyHCYZm9r+0vtcRKo/TopKtr5dQsgD5xNfG9RLR/Y8RC4sPWI9HaT6+XJPTf9EH4vt6YuQiJKzZL6Hkk1V8+ahT8hjRhRN6rTTiI8dHxOpVKWjHPb8BIDeI9m7iJ3yWY+EqfUXKN1T0cynh5u+7yYSunu3QzmG X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.