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Wed, 05 Apr 2023 15:45:59 -0400 X-MC-Unique: rTa71ey2MCCQay6tZWFKhw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 29B30101A550; Wed, 5 Apr 2023 19:45:57 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B72E940C6EC4; Wed, 5 Apr 2023 19:45:55 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 8DE6240134449; Wed, 5 Apr 2023 16:45:32 -0300 (-03) Date: Wed, 5 Apr 2023 16:45:32 -0300 From: Marcelo Tosatti 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, peterz@infradead.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, 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: 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-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 X-Rspamd-Queue-Id: EAD3680024 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: nhpunzae5x6ui5mfp1q97ukupk49bnq8 X-HE-Tag: 1680723964-458330 X-HE-Meta: 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 rLKk5Dww ak544v0pKSDjRcEfeRGR6yA6IEYIHolQBOXLYhDBDgc8IwJ539j6BZnUxBha0l1vS2+7SbPEG67RWvOPi2rbS+qUt9yr/fVdWGdP6ZoONBgyLNjPtdfJ4QRemouLBuULl0LZAfpdJJSYb4JOu1QPtli8M5yd0dPjP417RH8Bc3ZKJZ83957c5q3Uy/rrUhw3+qWgfQp0rISye6F6YSpuhpyms1vSf1ShoWg1qUZCZvbDEmoAnQVbgMKeAMmqnfYqCsh3MatYM0EWOE5SMZoc8QeDOpaeiE+PmsDySzEo0Uo0tCUoWldVPF+9M7A== 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... Use a different mechanism other than an IPI to ensure in progress __get_free_pages_fast() has finished execution. Isnt this codepath slow path enough that it can use synchronize_rcu_expedited?