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Subject[PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

This series replaces the existing paravirtualized spinlock mechanism
with a paravirtualized ticketlock mechanism. The series provides
implementation for both Xen and KVM.

Changes in V9:
- Changed spin_threshold to 32k to avoid excess halt exits that are
causing undercommit degradation (after PLE handler improvement).
- Added kvm_irq_delivery_to_apic (suggested by Gleb)
- Optimized halt exit path to use PLE handler

V8 of PVspinlock was posted last year. After Avi's suggestions to look
at PLE handler's improvements, various optimizations in PLE handling
have been tried.

With this series we see that we could get little more improvements on top
of that.

Ticket locks have an inherent problem in a virtualized case, because
the vCPUs are scheduled rather than running concurrently (ignoring
gang scheduled vCPUs). This can result in catastrophic performance
collapses when the vCPU scheduler doesn't schedule the correct "next"
vCPU, and ends up scheduling a vCPU which burns its entire timeslice
spinning. (Note that this is not the same problem as lock-holder
preemption, which this series also addresses; that's also a problem,
but not catastrophic).

(See Thomas Friebel's talk "Prevent Guests from Spinning Around" for more details.)

Currently we deal with this by having PV spinlocks, which adds a layer
of indirection in front of all the spinlock functions, and defining a
completely new implementation for Xen (and for other pvops users, but
there are none at present).

PV ticketlocks keeps the existing ticketlock implemenentation
(fastpath) as-is, but adds a couple of pvops for the slow paths:

- If a CPU has been waiting for a spinlock for SPIN_THRESHOLD
iterations, then call out to the __ticket_lock_spinning() pvop,
which allows a backend to block the vCPU rather than spinning. This
pvop can set the lock into "slowpath state".

- When releasing a lock, if it is in "slowpath state", the call
__ticket_unlock_kick() to kick the next vCPU in line awake. If the
lock is no longer in contention, it also clears the slowpath flag.

The "slowpath state" is stored in the LSB of the within the lock tail
ticket. This has the effect of reducing the max number of CPUs by
half (so, a "small ticket" can deal with 128 CPUs, and "large ticket"

For KVM, one hypercall is introduced in hypervisor,that allows a vcpu to kick
another vcpu out of halt state.
The blocking of vcpu is done using halt() in (lock_spinning) slowpath.

Overall, it results in a large reduction in code, it makes the native
and virtualized cases closer, and it removes a layer of indirection
around all the spinlock functions.

The fast path (taking an uncontended lock which isn't in "slowpath"
state) is optimal, identical to the non-paravirtualized case.

The inner part of ticket lock code becomes:
inc = xadd(&lock->tickets, inc);

if (likely(inc.head == inc.tail))
goto out;
for (;;) {
unsigned count = SPIN_THRESHOLD;
do {
if (ACCESS_ONCE(lock->tickets.head) == inc.tail)
goto out;
} while (--count);
__ticket_lock_spinning(lock, inc.tail);
out: barrier();
which results in:
push %rbp
mov %rsp,%rbp

mov $0x200,%eax
lock xadd %ax,(%rdi)
movzbl %ah,%edx
cmp %al,%dl
jne 1f # Slowpath if lock in contention

pop %rbp

1: and $-2,%edx
movzbl %dl,%esi

2: mov $0x800,%eax
jmp 4f

3: pause
sub $0x1,%eax
je 5f

4: movzbl (%rdi),%ecx
cmp %cl,%dl
jne 3b

pop %rbp

5: callq *__ticket_lock_spinning
jmp 2b

with CONFIG_PARAVIRT_SPINLOCKS=n, the code has changed slightly, where
the fastpath case is straight through (taking the lock without
contention), and the spin loop is out of line:

push %rbp
mov %rsp,%rbp

mov $0x100,%eax
lock xadd %ax,(%rdi)
movzbl %ah,%edx
cmp %al,%dl
jne 1f

pop %rbp

1: pause
movzbl (%rdi),%eax
cmp %dl,%al
jne 1b

pop %rbp

The unlock code is complicated by the need to both add to the lock's
"head" and fetch the slowpath flag from "tail". This version of the
patch uses a locked add to do this, followed by a test to see if the
slowflag is set. The lock prefix acts as a full memory barrier, so we
can be sure that other CPUs will have seen the unlock before we read
the flag (without the barrier the read could be fetched from the
store queue before it hits memory, which could result in a deadlock).

This is is all unnecessary complication if you're not using PV ticket
locks, it also uses the jump-label machinery to use the standard
"add"-based unlock in the non-PV case.

static_key_false(&paravirt_ticketlocks_enabled))) {
arch_spinlock_t prev;
prev = *lock;
add_smp(&lock->tickets.head, TICKET_LOCK_INC);

/* add_smp() is a full mb() */
if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
__ticket_unlock_slowpath(lock, prev);
} else
__add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
which generates:
push %rbp
mov %rsp,%rbp

nop5 # replaced by 5-byte jmp 2f when PV enabled

# non-PV unlock
addb $0x2,(%rdi)

1: pop %rbp

### PV unlock ###
2: movzwl (%rdi),%esi # Fetch prev

lock addb $0x2,(%rdi) # Do unlock

testb $0x1,0x1(%rdi) # Test flag
je 1b # Finished if not set

### Slow path ###
add $2,%sil # Add "head" in old lock state
mov %esi,%edx
and $0xfe,%dh # clear slowflag for comparison
movzbl %dh,%eax
cmp %dl,%al # If head == tail (uncontended)
je 4f # clear slowpath flag

# Kick next CPU waiting for lock
3: movzbl %sil,%esi
callq *pv_lock_ops.kick

pop %rbp

# Lock no longer contended - clear slowflag
4: mov %esi,%eax
lock cmpxchg %dx,(%rdi) # cmpxchg to clear flag
cmp %si,%ax
jne 3b # If clear failed, then kick

pop %rbp

So when not using PV ticketlocks, the unlock sequence just has a
5-byte nop added to it, and the PV case is reasonable straightforward
aside from requiring a "lock add".

base = 3.10-rc2 kernel
patched = base + this series

The test was on 32 core (model: Intel(R) Xeon(R) CPU X7560) HT disabled
with 32 KVM guest vcpu 8GB RAM.

ebizzy (records/sec) higher is better
base stdev patched stdev %improvement
1x 5574.9000 237.4997 5618.0000 94.0366 0.77311
2x 2741.5000 561.3090 3332.0000 102.4738 21.53930
3x 2146.2500 216.7718 2302.3333 76.3870 7.27237
4x 1663.0000 141.9235 1753.7500 83.5220 5.45701
dbench (Throughput) higher is better
base stdev patched stdev %improvement
1x 14111.5600 754.4525 14645.9900 114.3087 3.78718
2x 2481.6270 71.2665 2667.1280 73.8193 7.47498
3x 1510.2483 31.8634 1503.8792 36.0777 -0.42173
4x 1029.4875 16.9166 1039.7069 43.8840 0.99267

Your suggestions and comments are welcome.

github link:

Please note that we set SPIN_THRESHOLD = 32k with this series,
that would eatup little bit of overcommit performance of PLE machines
and overall performance of non-PLE machines.

The older series was tested by Attilio for Xen implementation [1].

Jeremy Fitzhardinge (9):
x86/spinlock: Replace pv spinlocks with pv ticketlocks
x86/ticketlock: Collapse a layer of functions
xen: Defer spinlock setup until boot CPU setup
xen/pvticketlock: Xen implementation for PV ticket locks
xen/pvticketlocks: Add xen_nopvspin parameter to disable xen pv ticketlocks
x86/pvticketlock: Use callee-save for lock_spinning
x86/pvticketlock: When paravirtualizing ticket locks, increment by 2
x86/ticketlock: Add slowpath logic
xen/pvticketlock: Allow interrupts to be enabled while blocking

Andrew Jones (1):
Split jumplabel ratelimit

Stefano Stabellini (1):
xen: Enable PV ticketlocks on HVM Xen

Srivatsa Vaddagiri (3):
kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks
kvm guest : Add configuration support to enable debug information for KVM Guests
kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

Raghavendra K T (5):
x86/ticketlock: Don't inline _spin_unlock when using paravirt spinlocks
kvm : Fold pv_unhalt flag into GET_MP_STATE ioctl to aid migration
Simplify kvm_for_each_vcpu with kvm_irq_delivery_to_apic
Documentation/kvm : Add documentation on Hypercalls and features used for PV spinlock
Add directed yield in vcpu block path

Link in V8 has links to previous patch series and also whole history.

V8 PV Ticketspinlock for Xen/KVM link:

Documentation/virtual/kvm/cpuid.txt | 4 +
Documentation/virtual/kvm/hypercalls.txt | 13 ++
arch/ia64/include/asm/kvm_host.h | 5 +
arch/powerpc/include/asm/kvm_host.h | 5 +
arch/s390/include/asm/kvm_host.h | 5 +
arch/x86/Kconfig | 10 +
arch/x86/include/asm/kvm_host.h | 7 +-
arch/x86/include/asm/kvm_para.h | 14 +-
arch/x86/include/asm/paravirt.h | 32 +--
arch/x86/include/asm/paravirt_types.h | 10 +-
arch/x86/include/asm/spinlock.h | 128 +++++++----
arch/x86/include/asm/spinlock_types.h | 16 +-
arch/x86/include/uapi/asm/kvm_para.h | 1 +
arch/x86/kernel/kvm.c | 256 +++++++++++++++++++++
arch/x86/kernel/paravirt-spinlocks.c | 18 +-
arch/x86/kvm/cpuid.c | 3 +-
arch/x86/kvm/lapic.c | 5 +-
arch/x86/kvm/x86.c | 39 +++-
arch/x86/xen/smp.c | 3 +-
arch/x86/xen/spinlock.c | 384 ++++++++++---------------------
include/linux/jump_label.h | 26 +--
include/linux/jump_label_ratelimit.h | 34 +++
include/linux/kvm_host.h | 2 +-
include/linux/perf_event.h | 1 +
include/uapi/linux/kvm_para.h | 1 +
kernel/jump_label.c | 1 +
virt/kvm/kvm_main.c | 6 +-
27 files changed, 645 insertions(+), 384 deletions(-)

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