On 22/04/20 06:07, Davidlohr Bueso wrote:
> The use of any sort of waitqueue (simple or regular) for
> wait/waking vcpus has always been an overkill and semantically
> wrong. Because this is per-vcpu (which is blocked) there is
> only ever a single waiting vcpu, thus no need for any sort of
> queue.
>
> As such, make use of the rcuwait primitive, with the following
> considerations:
>
> - rcuwait already provides the proper barriers that serialize
> concurrent waiter and waker.
>
> - Task wakeup is done in rcu read critical region, with a
> stable task pointer.
>
> - Because there is no concurrency among waiters, we need
> not worry about rcuwait_wait_event() calls corrupting
> the wait->task. As a consequence, this saves the locking
> done in swait when modifying the queue. This also applies
> to per-vcore wait for powerpc kvm-hv.
>
> The x86 tscdeadline_latency test mentioned in 8577370fb0cb
> ("KVM: Use simple waitqueue for vcpu->wq") shows that, on avg,
> latency is reduced by around 15-20% with this change.
>
> Cc: Paul Mackerras <paulus@xxxxxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-mips@xxxxxxxxxxxxxxx
> Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> arch/mips/kvm/mips.c | 6 ++----
> arch/powerpc/include/asm/kvm_book3s.h | 2 +-
> arch/powerpc/include/asm/kvm_host.h | 2 +-
> arch/powerpc/kvm/book3s_hv.c | 22 ++++++++--------------
> arch/powerpc/kvm/powerpc.c | 2 +-
> arch/x86/kvm/lapic.c | 2 +-
> include/linux/kvm_host.h | 10 +++++-----
> virt/kvm/arm/arch_timer.c | 2 +-
> virt/kvm/arm/arm.c | 9 +++++----
> virt/kvm/async_pf.c | 3 +--
> virt/kvm/kvm_main.c | 19 +++++++++----------
> 11 files changed, 35 insertions(+), 44 deletions(-)
>
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index 8f05dd0a0f4e..fad6acce46e4 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -284,8 +284,7 @@ static enum hrtimer_restart kvm_mips_comparecount_wakeup(struct hrtimer *timer)
> kvm_mips_callbacks->queue_timer_int(vcpu);
>
> vcpu->arch.wait = 0;
> - if (swq_has_sleeper(&vcpu->wq))
> - swake_up_one(&vcpu->wq);
> + rcuwait_wake_up(&vcpu->wait);
>
> return kvm_mips_count_timeout(vcpu);
> }
> @@ -511,8 +510,7 @@ int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,
>
> dvcpu->arch.wait = 0;
>
> - if (swq_has_sleeper(&dvcpu->wq))
> - swake_up_one(&dvcpu->wq);
> + rcuwait_wake_up(&dvcpu->wait);
>
> return 0;
> }
> diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
> index 506e4df2d730..6e5d85ba588d 100644
> --- a/arch/powerpc/include/asm/kvm_book3s.h
> +++ b/arch/powerpc/include/asm/kvm_book3s.h
> @@ -78,7 +78,7 @@ struct kvmppc_vcore {
> struct kvm_vcpu *runnable_threads[MAX_SMT_THREADS];
> struct list_head preempt_list;
> spinlock_t lock;
> - struct swait_queue_head wq;
> + struct rcuwait wait;
> spinlock_t stoltb_lock; /* protects stolen_tb and preempt_tb */
> u64 stolen_tb;
> u64 preempt_tb;
> diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> index 1dc63101ffe1..337047ba4a56 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -751,7 +751,7 @@ struct kvm_vcpu_arch {
> u8 irq_pending; /* Used by XIVE to signal pending guest irqs */
> u32 last_inst;
>
> - struct swait_queue_head *wqp;
> + struct rcuwait *waitp;
> struct kvmppc_vcore *vcore;
> int ret;
> int trap;
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 93493f0cbfe8..b8d42f523ca7 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -230,13 +230,11 @@ static bool kvmppc_ipi_thread(int cpu)
> static void kvmppc_fast_vcpu_kick_hv(struct kvm_vcpu *vcpu)
> {
> int cpu;
> - struct swait_queue_head *wqp;
> + struct rcuwait *wait;
>
> - wqp = kvm_arch_vcpu_wq(vcpu);
> - if (swq_has_sleeper(wqp)) {
> - swake_up_one(wqp);
> + wait = kvm_arch_vcpu_get_wait(vcpu);
> + if (rcuwait_wake_up(wait))
> ++vcpu->stat.halt_wakeup;
> - }
>
> cpu = READ_ONCE(vcpu->arch.thread_cpu);
> if (cpu >= 0 && kvmppc_ipi_thread(cpu))
> @@ -2125,7 +2123,7 @@ static struct kvmppc_vcore *kvmppc_vcore_create(struct kvm *kvm, int id)
>
> spin_lock_init(&vcore->lock);
> spin_lock_init(&vcore->stoltb_lock);
> - init_swait_queue_head(&vcore->wq);
> + rcuwait_init(&vcore->wait);
> vcore->preempt_tb = TB_NIL;
> vcore->lpcr = kvm->arch.lpcr;
> vcore->first_vcpuid = id;
> @@ -3784,7 +3782,6 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
> ktime_t cur, start_poll, start_wait;
> int do_sleep = 1;
> u64 block_ns;
> - DECLARE_SWAITQUEUE(wait);
>
> /* Poll for pending exceptions and ceded state */
> cur = start_poll = ktime_get();
> @@ -3812,10 +3809,7 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
> }
> }
>
> - prepare_to_swait_exclusive(&vc->wq, &wait, TASK_INTERRUPTIBLE);
> -
> if (kvmppc_vcore_check_block(vc)) {
> - finish_swait(&vc->wq, &wait);
> do_sleep = 0;
> /* If we polled, count this as a successful poll */
> if (vc->halt_poll_ns)
> @@ -3828,8 +3822,8 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
> vc->vcore_state = VCORE_SLEEPING;
> trace_kvmppc_vcore_blocked(vc, 0);
> spin_unlock(&vc->lock);
> - schedule();
> - finish_swait(&vc->wq, &wait);
> + rcuwait_wait_event(&vc->wait,
> + kvmppc_vcore_check_block(vc), TASK_INTERRUPTIBLE);
> spin_lock(&vc->lock);
> vc->vcore_state = VCORE_INACTIVE;
> trace_kvmppc_vcore_blocked(vc, 1);
> @@ -3940,7 +3934,7 @@ static int kvmppc_run_vcpu(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
> kvmppc_start_thread(vcpu, vc);
> trace_kvm_guest_enter(vcpu);
> } else if (vc->vcore_state == VCORE_SLEEPING) {
> - swake_up_one(&vc->wq);
> + rcuwait_wake_up(&vc->wait);
> }
>
> }
> @@ -4279,7 +4273,7 @@ static int kvmppc_vcpu_run_hv(struct kvm_run *run, struct kvm_vcpu *vcpu)
> }
> user_vrsave = mfspr(SPRN_VRSAVE);
>
> - vcpu->arch.wqp = &vcpu->arch.vcore->wq;
> + vcpu->arch.waitp = &vcpu->arch.vcore->wait;
> vcpu->arch.pgdir = kvm->mm->pgd;
> vcpu->arch.state = KVMPPC_VCPU_BUSY_IN_HOST;
>
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index e15166b0a16d..4a074b587520 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -751,7 +751,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> if (err)
> goto out_vcpu_uninit;
>
> - vcpu->arch.wqp = &vcpu->wq;
> + vcpu->arch.waitp = &vcpu->wait;
> kvmppc_create_vcpu_debugfs(vcpu, vcpu->vcpu_id);
> return 0;
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9af25c97612a..fb2f56ba171d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1833,7 +1833,7 @@ void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)
> /* If the preempt notifier has already run, it also called apic_timer_expired */
> if (!apic->lapic_timer.hv_timer_in_use)
> goto out;
> - WARN_ON(swait_active(&vcpu->wq));
> + WARN_ON(rcu_dereference(vcpu->wait.task));
> cancel_hv_timer(apic);
> apic_timer_expired(apic);
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6d58beb65454..fc34021546bd 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -23,7 +23,7 @@
> #include <linux/irqflags.h>
> #include <linux/context_tracking.h>
> #include <linux/irqbypass.h>
> -#include <linux/swait.h>
> +#include <linux/rcuwait.h>
> #include <linux/refcount.h>
> #include <linux/nospec.h>
> #include <asm/signal.h>
> @@ -277,7 +277,7 @@ struct kvm_vcpu {
> struct mutex mutex;
> struct kvm_run *run;
>
> - struct swait_queue_head wq;
> + struct rcuwait wait;
> struct pid __rcu *pid;
> int sigset_active;
> sigset_t sigset;
> @@ -956,12 +956,12 @@ static inline bool kvm_arch_has_assigned_device(struct kvm *kvm)
> }
> #endif
>
> -static inline struct swait_queue_head *kvm_arch_vcpu_wq(struct kvm_vcpu *vcpu)
> +static inline struct rcuwait *kvm_arch_vcpu_get_wait(struct kvm_vcpu *vcpu)
> {
> #ifdef __KVM_HAVE_ARCH_WQP
> - return vcpu->arch.wqp;
> + return vcpu->arch.waitp;
> #else
> - return &vcpu->wq;
> + return &vcpu->wait;
> #endif
> }
>
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 93bd59b46848..b2805105bbe5 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -593,7 +593,7 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
> if (map.emul_ptimer)
> soft_timer_cancel(&map.emul_ptimer->hrtimer);
>
> - if (swait_active(kvm_arch_vcpu_wq(vcpu)))
> + if (rcu_dereference(kvm_arch_vpu_get_wait(vcpu)) != NULL)
> kvm_timer_blocking(vcpu);
>
> /*
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 48d0ec44ad77..f94a10bb1251 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -579,16 +579,17 @@ void kvm_arm_resume_guest(struct kvm *kvm)
>
> kvm_for_each_vcpu(i, vcpu, kvm) {
> vcpu->arch.pause = false;
> - swake_up_one(kvm_arch_vcpu_wq(vcpu));
> + rcuwait_wake_up(kvm_arch_vcpu_get_wait(vcpu));
> }
> }
>
> static void vcpu_req_sleep(struct kvm_vcpu *vcpu)
> {
> - struct swait_queue_head *wq = kvm_arch_vcpu_wq(vcpu);
> + struct rcuwait *wait = kvm_arch_vcpu_get_wait(vcpu);
>
> - swait_event_interruptible_exclusive(*wq, ((!vcpu->arch.power_off) &&
> - (!vcpu->arch.pause)));
> + rcuwait_wait_event(*wait,
> + (!vcpu->arch.power_off) &&(!vcpu->arch.pause),
> + TASK_INTERRUPTIBLE);
>
> if (vcpu->arch.power_off || vcpu->arch.pause) {
> /* Awaken to handle a signal, request we sleep again later. */
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index 15e5b037f92d..10b533f641a6 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -80,8 +80,7 @@ static void async_pf_execute(struct work_struct *work)
>
> trace_kvm_async_pf_completed(addr, cr2_or_gpa);
>
> - if (swq_has_sleeper(&vcpu->wq))
> - swake_up_one(&vcpu->wq);
> + rcuwait_wake_up(&vcpu->wait);
>
> mmput(mm);
> kvm_put_kvm(vcpu->kvm);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 74bdb7bf3295..835fb109badf 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -341,7 +341,7 @@ static void kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
> vcpu->kvm = kvm;
> vcpu->vcpu_id = id;
> vcpu->pid = NULL;
> - init_swait_queue_head(&vcpu->wq);
> + rcuwait_init(&vcpu->wait);
> kvm_async_pf_vcpu_init(vcpu);
>
> vcpu->pre_pcpu = -1;
> @@ -2671,7 +2671,6 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> {
> ktime_t start, cur;
> - DECLARE_SWAITQUEUE(wait);
> bool waited = false;
> u64 block_ns;
>
> @@ -2697,8 +2696,9 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> } while (single_task_running() && ktime_before(cur, stop));
> }
>
> + prepare_to_rcuwait(&vcpu->wait);
> for (;;) {
> - prepare_to_swait_exclusive(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
> + set_current_state(TASK_INTERRUPTIBLE);
>
> if (kvm_vcpu_check_block(vcpu) < 0)
> break;
> @@ -2706,8 +2706,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> waited = true;
> schedule();
> }
> -
> - finish_swait(&vcpu->wq, &wait);
> + finish_rcuwait(&vcpu->wait);
> cur = ktime_get();
> out:
> kvm_arch_vcpu_unblocking(vcpu);
> @@ -2738,11 +2737,10 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_block);
>
> bool kvm_vcpu_wake_up(struct kvm_vcpu *vcpu)
> {
> - struct swait_queue_head *wqp;
> + struct rcuwait *wait;
>
> - wqp = kvm_arch_vcpu_wq(vcpu);
> - if (swq_has_sleeper(wqp)) {
> - swake_up_one(wqp);
> + wait = kvm_arch_vcpu_get_wait(vcpu);
> + if (rcuwait_wake_up(wait)) {
> WRITE_ONCE(vcpu->ready, true);
> ++vcpu->stat.halt_wakeup;
> return true;
> @@ -2884,7 +2882,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
> continue;
> if (vcpu == me)
> continue;
> - if (swait_active(&vcpu->wq) && !vcpu_dy_runnable(vcpu))
> + if (rcu_dereference(vcpu->wait.task) &&
> + !vcpu_dy_runnable(vcpu))
> continue;
> if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
> !kvm_arch_vcpu_in_kernel(vcpu))
>
