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Date: Fri, 11 Dec 2015 09:42:03 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
Marcelo Tosatti <mtosatti@...hat.com>,
Radim Krcmar <rkrcmar@...hat.com>,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Alexander Graf <agraf@...e.de>,
Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 2/5] x86, vdso, pvclock: Simplify and speed up the vdso
pvclock reader
On 11/12/2015 08:52, Ingo Molnar wrote:
>
> * Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>>
>>
>> On 10/12/2015 00:12, Andy Lutomirski wrote:
>>> From: Andy Lutomirski <luto@...capital.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@...capital.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@...hat.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
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