Re: [PATCH 3/3] LoongArch: KVM: Set vcpu_is_preempted() macro rather than function

[Huacai Chen]

On Wed, Mar 11, 2026 at 11:51 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
>
>
>
> On 2026/3/11 上午11:35, Huacai Chen wrote:
> > Hi, Bibo,
> >
> > On Wed, Mar 11, 2026 at 11:21 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
> >>
> >> vcpu_is_preempted() is performance sensitive called in function
> >> osq_lock(), here set it as macro. So that parameter is not parsed
> >> at most time, it can avoid cache line thrashing across numa node.
> > I remember that I had suggested you define vcpu_is_preempted() in the
> > header file but you refused.
> yes, I refuse then, that is a tragedy, I am sorry for that:(
>
> By some benchmarks, vcpu_is_preempted() is called in busy-loop method,
> more obvious with more CPUs, it may be a bottleneck.
I also remember that you said the static key and percpu steal_time
structure should be EXPORT_SYMBOL, but you haven't done this patch.

Huacai

>
> Regards
> Bibo Mao
>
> >
> > Huacai
> >
> >>
> >> Here is part of unixbench result on 3C5000 DualWay machine with 32
> >> Cores and 2 Numa node.
> >>            origin    with patch    CONFIG_PARAVIRT disabled
> >> execl     6871.9    7134.2        7190.8
> >> fstime    425.5     959.9         956.1
> >>
> >>  From the test result, with macro method it is almost the same with
> >> CONFIG_PARAVIRT disabled, and there is some improvment compared with
> >> function method.
> >>
> >> Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> >> ---
> >>   arch/loongarch/include/asm/qspinlock.h | 27 +++++++++++++++++++++-----
> >>   arch/loongarch/kernel/paravirt.c       | 15 ++------------
> >>   2 files changed, 24 insertions(+), 18 deletions(-)
> >>
> >> diff --git a/arch/loongarch/include/asm/qspinlock.h b/arch/loongarch/include/asm/qspinlock.h
> >> index 66244801db67..305afd37cdcb 100644
> >> --- a/arch/loongarch/include/asm/qspinlock.h
> >> +++ b/arch/loongarch/include/asm/qspinlock.h
> >> @@ -5,8 +5,10 @@
> >>   #include <linux/jump_label.h>
> >>
> >>   #ifdef CONFIG_PARAVIRT
> >> -
> >> +#include <asm/kvm_para.h>
> >>   DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key);
> >> +DECLARE_STATIC_KEY_FALSE(virt_preempt_key);
> >> +DECLARE_PER_CPU(struct kvm_steal_time, steal_time);
> >>
> >>   #define virt_spin_lock virt_spin_lock
> >>
> >> @@ -34,10 +36,25 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
> >>          return true;
> >>   }
> >>
> >> -#define vcpu_is_preempted vcpu_is_preempted
> >> -
> >> -bool vcpu_is_preempted(int cpu);
> >> -
> >> +/*
> >> + * Macro is better than inline function here
> >> + * With inline function, parameter cpu is parsed even though it is not used.
> >> + * This may cause cache line thrashing across NUMA node.
> >> + * With macro method, parameter cpu is parsed only when it is used.
> >> + */
> >> +#define vcpu_is_preempted(cpu)                                         \
> >> +({                                                                     \
> >> +       bool __val;                                                     \
> >> +                                                                       \
> >> +       if (!static_branch_unlikely(&virt_preempt_key))                 \
> >> +               __val = false;                                          \
> >> +       else {                                                          \
> >> +               struct kvm_steal_time *src;                             \
> >> +               src = &per_cpu(steal_time, cpu);                        \
> >> +               __val = !!(src->preempted & KVM_VCPU_PREEMPTED);        \
> >> +       }                                                               \
> >> +       __val;                                                          \
> >> +})
> >>   #endif /* CONFIG_PARAVIRT */
> >>
> >>   #include <asm-generic/qspinlock.h>
> >> diff --git a/arch/loongarch/kernel/paravirt.c b/arch/loongarch/kernel/paravirt.c
> >> index b74fe6db49ab..2d1206e486e2 100644
> >> --- a/arch/loongarch/kernel/paravirt.c
> >> +++ b/arch/loongarch/kernel/paravirt.c
> >> @@ -10,8 +10,8 @@
> >>   #include <asm/paravirt.h>
> >>
> >>   static int has_steal_clock;
> >> -static DEFINE_PER_CPU(struct kvm_steal_time, steal_time) __aligned(64);
> >> -static DEFINE_STATIC_KEY_FALSE(virt_preempt_key);
> >> +DEFINE_PER_CPU(struct kvm_steal_time, steal_time) __aligned(64);
> >> +DEFINE_STATIC_KEY_FALSE(virt_preempt_key);
> >>   DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key);
> >>
> >>   static bool steal_acc = true;
> >> @@ -261,17 +261,6 @@ static int pv_time_cpu_down_prepare(unsigned int cpu)
> >>          return 0;
> >>   }
> >>
> >> -bool vcpu_is_preempted(int cpu)
> >> -{
> >> -       struct kvm_steal_time *src;
> >> -
> >> -       if (!static_branch_unlikely(&virt_preempt_key))
> >> -               return false;
> >> -
> >> -       src = &per_cpu(steal_time, cpu);
> >> -       return !!(src->preempted & KVM_VCPU_PREEMPTED);
> >> -}
> >> -EXPORT_SYMBOL(vcpu_is_preempted);
> >>   #endif
> >>
> >>   static void pv_cpu_reboot(void *unused)
> >> --
> >> 2.39.3
> >>
> >>
>
>