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Date:   Tue, 08 Jun 2021 13:11:29 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be
 configurable
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Anton Blanchard <anton@ozlabs.org>, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linuxppc-dev@lists.ozlabs.org, Andy Lutomirski <luto@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>
References: <20210605014216.446867-1-npiggin@gmail.com>
        <20210605014216.446867-3-npiggin@gmail.com>
In-Reply-To: <20210605014216.446867-3-npiggin@gmail.com>
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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 <npiggin@gmail.com>

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 <npiggin@gmail.com>
---
 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