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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Marcelo Tosatti Cc: Leonardo Bras , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org Subject: [RFC PATCH v1 1/2] kvm: Implement guest_exit_last_time() Date: Thu, 28 Mar 2024 14:19:46 -0300 Message-ID: <20240328171949.743211-2-leobras@redhat.com> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20240328171949.743211-1-leobras@redhat.com> References: <20240328171949.743211-1-leobras@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Keep track of the last time a cpu ran guest_exit(), and provide a helper to make this information available to other files. Signed-off-by: Leonardo Bras Reviewed-by: Leonardo Bras --- include/linux/kvm_host.h | 13 +++++++++++++ virt/kvm/kvm_main.c | 3 +++ 2 files changed, 16 insertions(+) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 48f31dcd318a..be90d83d631a 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -470,31 +470,44 @@ static __always_inline void guest_state_enter_irqoff(= void) { instrumentation_begin(); trace_hardirqs_on_prepare(); lockdep_hardirqs_on_prepare(); instrumentation_end(); =20 guest_context_enter_irqoff(); lockdep_hardirqs_on(CALLER_ADDR0); } =20 +DECLARE_PER_CPU(unsigned long, kvm_last_guest_exit); + +/* + * Returns time (jiffies) for the last guest exit in current cpu + */ +static inline unsigned long guest_exit_last_time(void) +{ + return this_cpu_read(kvm_last_guest_exit); +} + /* * Exit guest context and exit an RCU extended quiescent state. * * Between guest_context_enter_irqoff() and guest_context_exit_irqoff() it= is * unsafe to use any code which may directly or indirectly use RCU, tracing * (including IRQ flag tracing), or lockdep. All code in this period must = be * non-instrumentable. */ static __always_inline void guest_context_exit_irqoff(void) { context_tracking_guest_exit(); + + /* Keeps track of last guest exit */ + this_cpu_write(kvm_last_guest_exit, jiffies); } =20 /* * Stop accounting time towards a guest. * Must be called after exiting guest context. */ static __always_inline void guest_timing_exit_irqoff(void) { instrumentation_begin(); /* Flush the guest cputime we spent on the guest */ diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index fb49c2a60200..732b1ab43ac9 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -103,20 +103,23 @@ EXPORT_SYMBOL_GPL(halt_poll_ns_shrink); */ =20 DEFINE_MUTEX(kvm_lock); LIST_HEAD(vm_list); =20 static struct kmem_cache *kvm_vcpu_cache; =20 static __read_mostly struct preempt_ops kvm_preempt_ops; static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_running_vcpu); =20 +DEFINE_PER_CPU(unsigned long, kvm_last_guest_exit); +EXPORT_SYMBOL_GPL(kvm_last_guest_exit); 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Thu, 28 Mar 2024 10:20:06 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHu853ijuqNBx1nyDj4vEt9gDt5cv976tFHHSJ9WoGLzNibEXGLlQqgvCaSZT7OfU274ydjLg== X-Received: by 2002:a05:6808:1242:b0:3c3:dfec:d9ee with SMTP id o2-20020a056808124200b003c3dfecd9eemr3965545oiv.32.1711646406669; Thu, 28 Mar 2024 10:20:06 -0700 (PDT) Received: from LeoBras.redhat.com ([2804:1b3:a801:d7ed:4b57:3fcd:d5e6:a613]) by smtp.gmail.com with ESMTPSA id m13-20020ad45dcd000000b00696944e3ce6sm809078qvh.74.2024.03.28.10.20.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Mar 2024 10:20:06 -0700 (PDT) From: Leonardo Bras To: Paolo Bonzini , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Marcelo Tosatti Cc: Leonardo Bras , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org Subject: [RFC PATCH v1 2/2] rcu: Ignore RCU in nohz_full cpus if it was running a guest recently Date: Thu, 28 Mar 2024 14:19:47 -0300 Message-ID: <20240328171949.743211-3-leobras@redhat.com> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20240328171949.743211-1-leobras@redhat.com> References: <20240328171949.743211-1-leobras@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" In current code, we can ignore the RCU request on a nohz_full cpu for up to a second if it has interrupted idle or userspace tasks, since those are quiescent states, and will probably return to it soon thus not requiring to run a softirq or a rcuc thread. Running a guest is also considered to be a quiescent state, and will follow the same logic, so it makes sense to also ignore the RCU request in this case. This solves a latency issue of a latency-sensitive workload running on a guest pinned in nohz_full cpu: if the guest goes out for any reason, and a synchronize_rcu() is requested between guest exit and a timer interrupt, then invoke_rcu_core() is called, and introduce latency due to either a softirq, or a reschedule to run rcuc, if the host is a PREEMPT_RT kernel. Suggested-by: Marcelo Tosatti Signed-off-by: Leonardo Bras Acked-by: Paul E. McKenney Reviewed-by: Leonardo Bras --- kernel/rcu/tree_plugin.h | 14 ++++++++++++++ kernel/rcu/tree.c | 4 +++- 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 36a8b5dbf5b5..16f3cf2e15df 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -5,20 +5,21 @@ * or preemptible semantics. * * Copyright Red Hat, 2009 * Copyright IBM Corporation, 2009 * * Author: Ingo Molnar * Paul E. McKenney */ =20 #include "../locking/rtmutex_common.h" +#include "linux/kvm_host.h" =20 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp) { /* * In order to read the offloaded state of an rdp in a safe * and stable way and prevent from its value to be changed * under us, we must either hold the barrier mutex, the cpu * hotplug lock (read or write) or the nocb lock. Local * non-preemptible reads are also safe. NOCB kthreads and * timers have their own means of synchronization against the @@ -1260,10 +1261,23 @@ static bool rcu_nohz_full_cpu(void) =20 /* * Bind the RCU grace-period kthreads to the housekeeping CPU. */ static void rcu_bind_gp_kthread(void) { if (!tick_nohz_full_enabled()) return; housekeeping_affine(current, HK_TYPE_RCU); } + +/* + * true if for this cpu guest exit is at most over a second ago, + * false otherwise + */ +static bool rcu_recent_guest_exit(void) +{ +#ifdef CONFIG_KVM + return time_before(jiffies, guest_exit_last_time() + HZ); +#else + return false; +#endif +} diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index d9642dd06c25..e5ce00bf1898 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -148,20 +148,21 @@ static void rcu_report_qs_rnp(unsigned long mask, str= uct rcu_node *rnp, static struct task_struct *rcu_boost_task(struct rcu_node *rnp); static void invoke_rcu_core(void); static void rcu_report_exp_rdp(struct rcu_data *rdp); static void sync_sched_exp_online_cleanup(int cpu); static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rn= p); static bool rcu_rdp_is_offloaded(struct rcu_data *rdp); static bool rcu_rdp_cpu_online(struct rcu_data *rdp); static bool rcu_init_invoked(void); static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf); static void rcu_init_new_rnp(struct rcu_node *rnp_leaf); +static bool rcu_recent_guest_exit(void); =20 /* * rcuc/rcub/rcuop kthread realtime priority. The "rcuop" * real-time priority(enabling/disabling) is controlled by * the extra CONFIG_RCU_NOCB_CPU_CB_BOOST configuration. */ static int kthread_prio =3D IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0; module_param(kthread_prio, int, 0444); =20 /* Delay in jiffies for grace-period initialization delays, debug only. */ @@ -3931,21 +3932,22 @@ static int rcu_pending(int user) lockdep_assert_irqs_disabled(); =20 /* Check for CPU stalls, if enabled. */ check_cpu_stall(rdp); =20 /* Does this CPU need a deferred NOCB wakeup? */ if (rcu_nocb_need_deferred_wakeup(rdp, RCU_NOCB_WAKE)) return 1; =20 /* Is this a nohz_full CPU in userspace or idle? 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