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SubjectRe: [Xen-devel] [PATCH RFC V7 0/12] Paravirtualized ticketlocks
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On Thu, 2012-04-19 at 21:12 +0100, Raghavendra K T wrote:
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
>
> 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(&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
> 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@lists.xen.org
> http://lists.xen.org/xen-devel




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