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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Mon, Jan 5, 2015 at 11:17 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> On Mon, Jan 05, 2015 at 10:56:07AM -0800, Andy Lutomirski wrote:
>> On Mon, Jan 5, 2015 at 7:25 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
>> > On Mon, Dec 22, 2014 at 04:39:57PM -0800, Andy Lutomirski 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.
>> >> +
>> >> +      * 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.
>> >>        */
>> >
>> > Can Xen guarantee that ?
>>
>> I think so, vacuously.  Xen doesn't seem to set PVCLOCK_TSC_STABLE_BIT
>> at all.  I have no idea going forward, though.
>>
>> Xen people?
>>
>> >
>> >> -     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;
>> >> +     }
>> >
>> > This check must be performed after reading a stable pvti.
>> >
>>
>> We can even read it in the middle, guarded by the version checks.
>> I'll do that for v2.
>>
>> >> +
>> >> +     do {
>> >> +             version = pvti->version;
>> >> +
>> >> +             /* This is also a read barrier, so we'll read version 
>> >> first. */
>> >> +             rdtsc_barrier();
>> >> +             tsc = __native_read_tsc();
>> >> +
>> >> +             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);
>> >
>> > The following is possible:
>> >
>> > 1) State: all pvtis marked as PVCLOCK_TSC_STABLE_BIT.
>> > 1) Update request for all vcpus, for a TSC_STABLE_BIT -> ~TSC_STABLE_BIT
>> > transition.
>> > 2) vCPU-1 updates its pvti with new values.
>> > 3) vCPU-0 still has not updated its pvti with new values.
>> > 4) vCPU-1 VM-enters, uses vCPU-0 values, even though it has been
>> > notified of a TSC_STABLE_BIT -> ~TSC_STABLE_BIT transition.
>> >
>> > The update is not actually atomic across all vCPUs, its atomic in
>> > the sense of not allowing visibility of distinct
>> > system_timestamp/tsc_timestamp values.
>> >
>>
>> Hmm.  In step 4, is there a guarantee that vCPU-0 won't VM-enter until
>> it gets marked unstable?
>
> Yes. It will VM-enter after pvti is updated.
>
>> Otherwise the vdso could could just as
>> easily be called from vCPU-1, migrated to vCPU-0, read the data
>> complete with stale stable bit, and get migrated back to vCPU-1.
>
> Right.
>
>> But I thought that KVM currently froze all vCPUs when updating pvti
>> for any of them.  How can this happen?  I admit I don't really
>> understand the update request code.
>
> The update is performed as follows:
>
>         - Stop guest instruction execution on every vCPU, parking them in the 
> host.
>         - Request KVMCLOCK update for every vCPU.
>         - Resume guest instruction execution.
>
> The KVMCLOCK update (==pvti update) is guaranteed to be performed before
> guest instructions are executed again.
>
> But there is no guarantee that vCPU-N has updated its pvti when
> vCPU-M resumes guest instruction execution.

Still confused.  So we can freeze all vCPUs in the host, then update
pvti 1, then resume vCPU 1, then update pvti 0?  In that case, we have
a problem, because vCPU 1 can observe pvti 0 mid-update, and KVM
doesn't increment the version pre-update, and we can return completely
bogus results.

>
> So the cost this patch removes is mainly from __getcpu (==RDTSCP?) ?

It removes a whole bunch of code, an extra barrier, and two __getcpus.

> Perhaps you can use Gleb's idea to stick vcpu id into version field ?

I don't understand how that's useful at all.  If you're reading pvti,
you clearly know the vcpu id.

--Andy

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