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Subject[PATCH RFC V5 00/11] Paravirtualized ticketlocks
From: Jeremy Fitzhardinge <>

[ Changes since last posting:
- Use "lock add" for unlock operation rather than "lock xadd"; it is
equivalent to "add; mfence", but more efficient than both "lock
xadd" and "mfence".

I think this version is ready for submission.

NOTE: this series is available in:
git:// upstream/pvticketlock-slowflag
and is based on the previously posted ticketlock cleanup series in
git:// upstream/ticketlock-cleanup

This series replaces the existing paravirtualized spinlock mechanism
with a paravirtualized ticketlock mechanism.

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"

This series provides a Xen implementation, but it should be
straightforward to add a KVM implementation as well.

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.

unlikely(static_branch(&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".

Thoughts? Comments? Suggestions?

Jeremy Fitzhardinge (10):
x86/spinlock: replace pv spinlocks with pv ticketlocks
x86/ticketlock: don't inline _spin_unlock when using paravirt
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
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

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

arch/x86/Kconfig | 3 +
arch/x86/include/asm/paravirt.h | 30 +---
arch/x86/include/asm/paravirt_types.h | 10 +-
arch/x86/include/asm/spinlock.h | 126 +++++++++----
arch/x86/include/asm/spinlock_types.h | 16 +-
arch/x86/kernel/paravirt-spinlocks.c | 18 +--
arch/x86/xen/smp.c | 3 +-
arch/x86/xen/spinlock.c | 331 ++++++++++-----------------------
kernel/Kconfig.locks | 2 +-
9 files changed, 210 insertions(+), 329 deletions(-)


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