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Date: Mon, 14 Dec 2015 00:16:56 -0800
From: tip-bot for Andy Lutomirski <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: luto@...capital.net, tglx@...utronix.de, hpa@...or.com,
bp@...en8.de, pbonzini@...hat.com, peterz@...radead.org,
mingo@...nel.org, brgerst@...il.com, dvlasenk@...hat.com,
linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: [tip:x86/asm] x86, vdso, pvclock:
Simplify and speed up the vdso pvclock reader
Commit-ID: 6b078f5de7fc0851af4102493c7b5bb07e49c4cb
Gitweb: http://git.kernel.org/tip/6b078f5de7fc0851af4102493c7b5bb07e49c4cb
Author: Andy Lutomirski <luto@...capital.net>
AuthorDate: Thu, 10 Dec 2015 19:20:19 -0800
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 11 Dec 2015 08:56:02 +0100
x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader
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>
Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-mm@...ck.org
Link: http://lkml.kernel.org/r/6b51dcc41f1b101f963945c5ec7093d72bdac429.1449702533.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
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 ca94fa6..c325ba1 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;
--
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