From: Nicholas Piggin <npiggin@gmail.com>
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>
Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable
Date: Tue, 08 Jun 2021 13:11:29 +1000 [thread overview]
Message-ID: <1623121605.j47gdpccep.astroid@bobo.none> (raw)
In-Reply-To: <20210605014216.446867-3-npiggin@gmail.com>
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.
>
> Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is.
>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Can I give you a couple of incremental patches for 2/4 and 3/4 to
improve the implementation requirement comments a bit for benefit of
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.
# Use normal mm refcounting for MMU_LAZY_TLB kernel thread references.
+# MMU_LAZY_TLB_REFCOUNT=n can improve the scalability of context switching
+# to/from kernel threads when the same mm is running on a lot of CPUs (a large
+# 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 the 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 reference
+# to a kthread ->active_mm (non-arch code has been converted already).
config MMU_LAZY_TLB_REFCOUNT
def_bool y
--
2.23.0
next prev parent reply other threads:[~2021-06-08 3:12 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-05 1:42 [PATCH v4 0/4] shoot lazy tlbs Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 1/4] lazy tlb: introduce lazy mm refcount helper functions Nicholas Piggin
2021-06-07 23:49 ` Andrew Morton
2021-06-08 1:39 ` Nicholas Piggin
2021-06-08 1:48 ` Andrew Morton
2021-06-08 4:11 ` Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable Nicholas Piggin
2021-06-08 3:11 ` Nicholas Piggin [this message]
2021-06-08 16:20 ` Andy Lutomirski
2021-06-14 0:45 ` Nicholas Piggin
2021-06-14 3:52 ` Andy Lutomirski
2021-06-14 4:14 ` Nicholas Piggin
2021-06-14 4:47 ` Nicholas Piggin
2021-06-14 5:21 ` Nicholas Piggin
2021-06-14 16:20 ` Andy Lutomirski
2021-06-15 0:55 ` Nicholas Piggin
2021-06-16 0:14 ` Andy Lutomirski
2021-06-16 1:02 ` Nicholas Piggin
2021-06-17 0:32 ` Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 3/4] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Nicholas Piggin
2021-06-08 3:15 ` Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 4/4] powerpc/64s: enable MMU_LAZY_TLB_SHOOTDOWN Nicholas Piggin
2021-06-07 23:52 ` Andrew Morton
2021-06-08 2:13 ` Nicholas Piggin
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