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

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/12/2015 08:52, Ingo Molnar wrote:
> 
> * Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
>>
>>
>> On 10/12/2015 00:12, Andy Lutomirski wrote:
>>> From: Andy Lutomirski <luto@amacapital.net>
>>>
>>> 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@amacapital.net>
>>> ---
>>>  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@redhat.com>
> 
> 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.

The 1/5 patch is entirely in KVM and is not necessary for the rest of
the series to work.  I would like it to be separate, because Marcelo has
not yet chimed in to say why it was necessary.

Can you just apply patches 2-5?

Paolo