From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Matlack Subject: Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader Date: Wed, 24 Dec 2014 13:30:40 -0800 Message-ID: References: <8d09c16eb39cbe264417cc66c4aca730af10b70b.1419295081.git.luto@amacapital.net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1Y3tWJ-0000Ra-QQ for xen-devel@lists.xenproject.org; Wed, 24 Dec 2014 21:31:04 +0000 Received: by mail-ie0-f181.google.com with SMTP id rl12so6301281iec.40 for ; Wed, 24 Dec 2014 13:31:00 -0800 (PST) In-Reply-To: <8d09c16eb39cbe264417cc66c4aca730af10b70b.1419295081.git.luto@amacapital.net> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andy Lutomirski Cc: kvm list , Gleb Natapov , Marcelo Tosatti , "linux-kernel@vger.kernel.org" , "xen-devel@lists.xenproject.org" , Paolo Bonzini List-Id: xen-devel@lists.xenproject.org On Mon, Dec 22, 2014 at 4:39 PM, 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 > --- > 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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/