On Thu, 2012-04-19 at 21:12 +0100, Raghavendra K T wrote: > From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx> > > This series replaces the existing paravirtualized spinlock mechanism > with a paravirtualized ticketlock mechanism. (targeted for 3.5 window) Which tree is this series going through, tip.git I guess? I don't see it there. Ian. > > Changes in V7: > - Reabsed patches to 3.4-rc3 > - Added jumplabel split patch (originally from Andrew Jones rebased to > 3.4-rc3 > - jumplabel changes from Ingo and Jason taken and now using static_key_* > instead of static_branch. > - using UNINLINE_SPIN_UNLOCK (which was splitted as per suggestion from Linus) > - This patch series is rebased on debugfs patch (that sould be already in > Xen/linux-next https://lkml.org/lkml/2012/3/23/51) > > 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" > http://www.xen.org/files/xensummitboston08/LHP.pdf 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" > 32768). > > This series provides a Xen implementation, KVM implementation will be > posted in next 2-3 days. > > 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); > inc.tail &= ~TICKET_SLOWPATH_FLAG; > > if (likely(inc.head == inc.tail)) > goto out; > for (;;) { > unsigned count = SPIN_THRESHOLD; > do { > if (ACCESS_ONCE(lock->tickets.head) == inc.tail) > goto out; > cpu_relax(); > } 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 > retq > > ### SLOWPATH START > 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 > retq > > 5: callq *__ticket_lock_spinning > jmp 2b > ### SLOWPATH END > > 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 > retq > > ### SLOWPATH START > 1: pause > movzbl (%rdi),%eax > cmp %dl,%al > jne 1b > > pop %rbp > retq > ### SLOWPATH END > > 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. > > if (TICKET_SLOWPATH_FLAG && > static_key_false(¶virt_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 > retq > > ### 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 > retq > > # 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 > retq > > 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". > > TODO: 1) remove CONFIG_PARAVIRT_SPINLOCK ? > 2) experiments on further optimization possibilities. (discussed in V6) > > Results: > ======= > various form of results based on V6 of the patch series are posted in following links > https://lkml.org/lkml/2012/3/21/161 > https://lkml.org/lkml/2012/3/21/198 > > kvm results: > https://lkml.org/lkml/2012/3/23/50 > https://lkml.org/lkml/2012/4/5/73 > > Thoughts? Comments? Suggestions? > > 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 > > Raghavendra K T (1): > x86/ticketlock: don't inline _spin_unlock when using paravirt > spinlocks > > Andrew Jones (1): > split out rate limiting from jump_label.h > > Stefano Stabellini (1): > xen: enable PV ticketlocks on HVM Xen > --- > > V6 : https://lkml.org/lkml/2012/3/21/161 > > Changes in V6 posting: (Raghavendra K T) > - Rebased to linux-3.3-rc6. > - used function+enum in place of macro (better type checking) > - use cmpxchg while resetting zero status for possible race > [suggested by Dave Hansen for KVM patches ] > > arch/x86/Kconfig | 1 + > 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/kernel/paravirt-spinlocks.c | 18 +-- > arch/x86/xen/smp.c | 3 +- > arch/x86/xen/spinlock.c | 383 +++++++++++---------------------- > include/linux/jump_label.h | 26 +--- > include/linux/jump_label_ratelimit.h | 34 +++ > include/linux/perf_event.h | 1 + > kernel/jump_label.c | 1 + > 12 files changed, 279 insertions(+), 374 deletions(-) > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@xxxxxxxxxxxxx > http://lists.xen.org/xen-devel -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
