Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

[David Matlack <dmatlack@xxxxxxxxxx>]

On Mon, Dec 22, 2014 at 4:39 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> The pvclock vdso code was too abstracted to understand easily and
> excessively paranoid.  Simplify it for a huge speedup.
>
> This opens the door for additional simplifications, as the vdso no
> longer accesses the pvti for any vcpu other than vcpu 0.
>
> Before, vclock_gettime using kvm-clock took about 64ns on my machine.
> With this change, it takes 19ns, which is almost as fast as the pure TSC
> implementation.
>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>  arch/x86/vdso/vclock_gettime.c | 82 
> ++++++++++++++++++++++++------------------
>  1 file changed, 47 insertions(+), 35 deletions(-)
>
> diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> index 9793322751e0..f2e0396d5629 100644
> --- a/arch/x86/vdso/vclock_gettime.c
> +++ b/arch/x86/vdso/vclock_gettime.c
> @@ -78,47 +78,59 @@ static notrace const struct pvclock_vsyscall_time_info 
> *get_pvti(int cpu)
>
>  static notrace cycle_t vread_pvclock(int *mode)
>  {
> -       const struct pvclock_vsyscall_time_info *pvti;
> +       const struct pvclock_vcpu_time_info *pvti = &get_pvti(0)->pvti;
>         cycle_t ret;
> -       u64 last;
> -       u32 version;
> -       u8 flags;
> -       unsigned cpu, cpu1;
> -
> +       u64 tsc, pvti_tsc;
> +       u64 last, delta, pvti_system_time;
> +       u32 version, pvti_tsc_to_system_mul, pvti_tsc_shift;
>
>         /*
> -        * Note: hypervisor must guarantee that:
> -        * 1. cpu ID number maps 1:1 to per-CPU pvclock time info.
> -        * 2. that per-CPU pvclock time info is updated if the
> -        *    underlying CPU changes.
> -        * 3. that version is increased whenever underlying CPU
> -        *    changes.
> +        * Note: The kernel and hypervisor must guarantee that cpu ID
> +        * number maps 1:1 to per-CPU pvclock time info.
> +        *
> +        * Because the hypervisor is entirely unaware of guest userspace
> +        * preemption, it cannot guarantee that per-CPU pvclock time
> +        * info is updated if the underlying CPU changes or that that
> +        * version is increased whenever underlying CPU changes.
> +        *
> +        * On KVM, we are guaranteed that pvti updates for any vCPU are
> +        * atomic as seen by *all* vCPUs.  This is an even stronger
> +        * guarantee than we get with a normal seqlock.
>          *
> +        * On Xen, we don't appear to have that guarantee, but Xen still
> +        * supplies a valid seqlock using the version field.
> +

Forgotten * here?

> +        * We only do pvclock vdso timing at all if
> +        * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
> +        * mean that all vCPUs have matching pvti and that the TSC is
> +        * synced, so we can just look at vCPU 0's pvti.
>          */
> -       do {
> -               cpu = __getcpu() & VGETCPU_CPU_MASK;
> -               /* TODO: We can put vcpu id into higher bits of pvti.version.
> -                * This will save a couple of cycles by getting rid of
> -                * __getcpu() calls (Gleb).
> -                */
> -
> -               pvti = get_pvti(cpu);
> -
> -               version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
> -
> -               /*
> -                * Test we're still on the cpu as well as the version.
> -                * We could have been migrated just after the first
> -                * vgetcpu but before fetching the version, so we
> -                * wouldn't notice a version change.
> -                */
> -               cpu1 = __getcpu() & VGETCPU_CPU_MASK;
> -       } while (unlikely(cpu != cpu1 ||
> -                         (pvti->pvti.version & 1) ||
> -                         pvti->pvti.version != version));
> -
> -       if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
> +
> +       if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
>                 *mode = VCLOCK_NONE;
> +               return 0;
> +       }
> +
> +       do {
> +               version = pvti->version;
> +
> +               /* This is also a read barrier, so we'll read version first. 
> */
> +               rdtsc_barrier();
> +               tsc = __native_read_tsc();

Is there a reason why you read the tsc inside the loop rather than once
after the loop?

> +
> +               pvti_tsc_to_system_mul = pvti->tsc_to_system_mul;
> +               pvti_tsc_shift = pvti->tsc_shift;
> +               pvti_system_time = pvti->system_time;
> +               pvti_tsc = pvti->tsc_timestamp;
> +
> +               /* Make sure that the version double-check is last. */
> +               smp_rmb();
> +       } while (unlikely((version & 1) || version != pvti->version));
> +
> +       delta = tsc - pvti_tsc;
> +       ret = pvti_system_time +
> +               pvclock_scale_delta(delta, pvti_tsc_to_system_mul,
> +                                   pvti_tsc_shift);
>
>         /* refer to tsc.c read_tsc() comment for rationale */
>         last = gtod->cycle_last;
> --
> 2.1.0
>
> --
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