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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id c21sm9244091pfi.44.2021.06.07.20.11.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 07 Jun 2021 20:11:34 -0700 (PDT) Date: Tue, 08 Jun 2021 13:11:29 +1000 From: Nicholas Piggin Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable To: Andrew Morton References: <20210605014216.446867-1-npiggin@gmail.com> <20210605014216.446867-3-npiggin@gmail.com> In-Reply-To: <20210605014216.446867-3-npiggin@gmail.com> MIME-Version: 1.0 Message-Id: <1623121605.j47gdpccep.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Randy Dunlap , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andy Lutomirski , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Excerpts from Nicholas Piggin's message of June 5, 2021 11:42 am: > Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb mm > when it is context switched. This can be disabled by architectures that > don't require this refcounting if they clean up lazy tlb mms when the > last refcount is dropped. Currently this is always enabled, which is > what existing code does, so the patch is effectively a no-op. >=20 > Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is. >=20 > Signed-off-by: Nicholas Piggin Can I give you a couple of incremental patches for 2/4 and 3/4 to=20 improve the implementation requirement comments a bit for benefit of=20 other archs. Thanks, Nick -- Explain the requirements for lazy tlb mm refcounting in the comment, to help with archs that may want to disable this by some means other than MMU_LAZY_TLB_SHOOTDOWN. Signed-off-by: Nicholas Piggin --- arch/Kconfig | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/arch/Kconfig b/arch/Kconfig index 1cff045cdde6..39d8c7dcf220 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -419,6 +419,16 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM shootdowns should enable this. =20 # Use normal mm refcounting for MMU_LAZY_TLB kernel thread references. +# MMU_LAZY_TLB_REFCOUNT=3Dn can improve the scalability of context switchi= ng +# to/from kernel threads when the same mm is running on a lot of CPUs (a l= arge +# multi-threaded application), by reducing contention on the mm refcount. +# +# This can be disabled if the architecture ensures no CPUs are using an mm= as a +# "lazy tlb" beyond its final refcount (i.e., by the time __mmdrop frees t= he mm +# or its kernel page tables). This could be arranged by arch_exit_mmap(), = or +# final exit(2) TLB flush, for example. arch code must also ensure the +# _lazy_tlb variants of mmgrab/mmdrop are used when dropping the lazy refe= rence +# to a kthread ->active_mm (non-arch code has been converted already). config MMU_LAZY_TLB_REFCOUNT def_bool y =20 --=20 2.23.0