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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id q68sm5779056pjq.45.2021.06.04.18.42.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Jun 2021 18:42:38 -0700 (PDT) From: Nicholas Piggin To: Andrew Morton Cc: Nicholas Piggin , Randy Dunlap , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, Anton Blanchard , Andy Lutomirski Subject: [PATCH v4 3/4] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Date: Sat, 5 Jun 2021 11:42:15 +1000 Message-Id: <20210605014216.446867-4-npiggin@gmail.com> X-Mailer: git-send-email 2.23.0 In-Reply-To: <20210605014216.446867-1-npiggin@gmail.com> References: <20210605014216.446867-1-npiggin@gmail.com> MIME-Version: 1.0 Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=EfkqsrLH; spf=pass (imf04.hostedemail.com: domain of npiggin@gmail.com designates 209.85.166.50 as permitted sender) smtp.mailfrom=npiggin@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: EBF1599F X-Stat-Signature: sfrhyy4kz7qm7inqooc1ix6y9mipn7wj X-HE-Tag: 1622867648-798749 Content-Transfer-Encoding: quoted-printable 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 big systems, the mm refcount can become highly contented when doing a lot of context switching with threaded applications (particularly switching between the idle thread and an application thread). Abandoning lazy tlb slows switching down quite a bit in the important user->idle->user cases, so instead implement a non-refcounted scheme that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down any remaining lazy ones. Shootdown IPIs are some concern, but they have not been observed to be a big problem with this scheme (the powerpc implementation generated 314 additional interrupts on a 144 CPU system during a kernel compile). There are a number of strategies that could be employed to reduce IPIs if they turn out to be a problem for some workload. Signed-off-by: Nicholas Piggin --- arch/Kconfig | 13 +++++++++++++ kernel/fork.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 64 insertions(+) diff --git a/arch/Kconfig b/arch/Kconfig index 1cff045cdde6..f8136c893991 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -421,6 +421,19 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM # Use normal mm refcounting for MMU_LAZY_TLB kernel thread references. config MMU_LAZY_TLB_REFCOUNT def_bool y + depends on !MMU_LAZY_TLB_SHOOTDOWN + +# Instead of refcounting the lazy mm struct for kernel thread references +# (which can cause contention with multi-threaded apps on large multipro= cessor +# systems), this option causes __mmdrop to IPI all CPUs in the mm_cpumas= k and +# switch to init_mm if they were using the to-be-freed mm as the lazy tl= b. To +# implement this, architectures must use _lazy_tlb variants of mm refcou= nting +# when releasing kernel thread mm references, and mm_cpumask must includ= e at +# least all possible CPUs in which the mm might be lazy, at the time of = the +# final mmdrop. mmgrab/mmdrop in arch/ code must be switched to _lazy_tl= b +# postfix as necessary. +config MMU_LAZY_TLB_SHOOTDOWN + bool =20 config ARCH_HAVE_NMI_SAFE_CMPXCHG bool diff --git a/kernel/fork.c b/kernel/fork.c index dc06afd725cb..8085ff33c7f6 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -674,6 +674,53 @@ static void check_mm(struct mm_struct *mm) #define allocate_mm() (kmem_cache_alloc(mm_cachep, GFP_KERNEL)) #define free_mm(mm) (kmem_cache_free(mm_cachep, (mm))) =20 +static void do_shoot_lazy_tlb(void *arg) +{ + struct mm_struct *mm =3D arg; + + if (current->active_mm =3D=3D mm) { + WARN_ON_ONCE(current->mm); + current->active_mm =3D &init_mm; + switch_mm(mm, &init_mm, current); + } +} + +static void do_check_lazy_tlb(void *arg) +{ + struct mm_struct *mm =3D arg; + + WARN_ON_ONCE(current->active_mm =3D=3D mm); +} + +static void shoot_lazy_tlbs(struct mm_struct *mm) +{ + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) { + /* + * IPI overheads have not found to be expensive, but they could + * be reduced in a number of possible ways, for example (in + * roughly increasing order of complexity): + * - A batch of mms requiring IPIs could be gathered and freed + * at once. + * - CPUs could store their active mm somewhere that can be + * remotely checked without a lock, to filter out + * false-positives in the cpumask. + * - After mm_users or mm_count reaches zero, switching away + * from the mm could clear mm_cpumask to reduce some IPIs + * (some batching or delaying would help). + * - A delayed freeing and RCU-like quiescing sequence based on + * mm switching to avoid IPIs completely. + */ + on_each_cpu_mask(mm_cpumask(mm), do_shoot_lazy_tlb, (void *)mm, 1); + if (IS_ENABLED(CONFIG_DEBUG_VM)) + on_each_cpu(do_check_lazy_tlb, (void *)mm, 1); + } else { + /* + * In this case, lazy tlb mms are refounted and would not reach + * __mmdrop until all CPUs have switched away and mmdrop()ed. + */ + } +} + /* * Called when the last reference to the mm * is dropped: either by a lazy thread or by @@ -683,6 +730,10 @@ void __mmdrop(struct mm_struct *mm) { BUG_ON(mm =3D=3D &init_mm); WARN_ON_ONCE(mm =3D=3D current->mm); + + /* Ensure no CPUs are using this as their lazy tlb mm */ + shoot_lazy_tlbs(mm); + WARN_ON_ONCE(mm =3D=3D current->active_mm); mm_free_pgd(mm); destroy_context(mm); --=20 2.23.0