Re: [PATCH 2/5] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

[Ingo Molnar]

* Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:

> 
> 
> On 10/12/2015 00:12, Andy Lutomirski wrote:
> > From: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > 
> > 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 45ns on my machine.
> > With this change, it takes 29ns, which is almost as fast as the pure TSC
> > implementation.
> > 
> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > ---
> >  arch/x86/entry/vdso/vclock_gettime.c | 81 ++++++++++++++++++++----------------
> >  1 file changed, 46 insertions(+), 35 deletions(-)
> > 
> > diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
> > index ca94fa649251..c325ba1bdddf 100644
> > --- a/arch/x86/entry/vdso/vclock_gettime.c
> > +++ b/arch/x86/entry/vdso/vclock_gettime.c
> > @@ -78,47 +78,58 @@ 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.
> >  	 */
> > -	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. */
> > +		tsc = rdtsc_ordered();
> > +
> > +		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;
> > 
> 
> Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Thanks. I've added your Reviewed-by to the 1/5 patch as well - to be able to put 
the whole series into the tip:x86/entry tree. Let me know if you'd like it to be 
done differently.

Thanks,

	Ingo
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