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Message-ID: <20120507082928.GI16608@gmail.com>
Date: Mon, 7 May 2012 10:29:28 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Avi Kivity <avi@...hat.com>
Cc: Jeremy Fitzhardinge <jeremy@...p.org>,
Greg Kroah-Hartman <gregkh@...e.de>,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
"H. Peter Anvin" <hpa@...or.com>,
Marcelo Tosatti <mtosatti@...hat.com>, X86 <x86@...nel.org>,
Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
Avi Kivity <avi@...hat.com>,
Attilio Rao <attilio.rao@...rix.com>,
Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Virtualization <virtualization@...ts.linux-foundation.org>,
Xen Devel <xen-devel@...ts.xensource.com>,
linux-doc@...r.kernel.org, KVM <kvm@...r.kernel.org>,
Andi Kleen <andi@...stfloor.org>,
Stefano Stabellini <stefano.stabellini@...citrix.com>,
Stephan Diestelhorst <stephan.diestelhorst@....com>,
LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks
* Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> wrote:
> This series replaces the existing paravirtualized spinlock mechanism
> with a paravirtualized ticketlock mechanism. The series provides
> implementation for both Xen and KVM.(targeted for 3.5 window)
>
> Note: This needs debugfs changes patch that should be in Xen / linux-next
> https://lkml.org/lkml/2012/3/30/687
>
> Changes in V8:
> - Reabsed patches to 3.4-rc4
> - Combined the KVM changes with ticketlock + Xen changes (Ingo)
> - Removed CAP_PV_UNHALT since it is redundant (Avi). But note that we
> need newer qemu which uses KVM_GET_SUPPORTED_CPUID ioctl.
> - Rewrite GET_MP_STATE condition (Avi)
> - Make pv_unhalt = bool (Avi)
> - Move out reset pv_unhalt code to vcpu_run from vcpu_block (Gleb)
> - Documentation changes (Rob Landley)
> - Have a printk to recognize that paravirt spinlock is enabled (Nikunj)
> - Move out kick hypercall out of CONFIG_PARAVIRT_SPINLOCK now
> so that it can be used for other optimizations such as
> flush_tlb_ipi_others etc. (Nikunj)
>
> 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).
>
> 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);
> 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)
> 3) Use kvm_irq_delivery_to_apic() in kvm hypercall (suggested by Gleb)
> 4) Any cleanups for e.g. Xen/KVM common code for debugfs.
>
> PS: TODOs are no blockers for the current series merge.
>
> 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
>
> Benchmarking on the current set of patches will be posted soon.
>
> Thoughts? Comments? Suggestions?. It would be nice to see
> Acked-by/Reviewed-by/Tested-by for the patch series.
>
> 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
>
> Srivatsa Vaddagiri (3):
> Add a hypercall to KVM hypervisor to support pv-ticketlocks
> Added configuration support to enable debug information for KVM Guests
> Paravirtual ticketlock support for linux guests running on KVM hypervisor
>
> Raghavendra K T (3):
> x86/ticketlock: Don't inline _spin_unlock when using paravirt
> spinlocks
> Fold pv_unhalt flag into GET_MP_STATE ioctl to aid migration
> Add documentation on Hypercalls and features used for PV spinlock
>
> Andrew Jones (1):
> Split out rate limiting from jump_label.h
>
> Stefano Stabellini (1):
> xen: Enable PV ticketlocks on HVM Xen
> ---
> PS: Had to trim down recipient list because, LKML archive does not support
> list > 20. Though many more people should have been in To/CC list.
>
> Ticketlock links:
> V7 : https://lkml.org/lkml/2012/4/19/335
> V6 : https://lkml.org/lkml/2012/3/21/161
>
> KVM patch links:
> V6: https://lkml.org/lkml/2012/4/23/123
>
> V5 kernel changes:
> https://lkml.org/lkml/2012/3/23/50
> Qemu changes for V5:
> http://lists.gnu.org/archive/html/qemu-devel/2012-03/msg04455.html
>
> V4 kernel changes:
> https://lkml.org/lkml/2012/1/14/66
> Qemu changes for V4:
> http://www.mail-archive.com/kvm@vger.kernel.org/msg66450.html
>
> V3 kernel Changes:
> https://lkml.org/lkml/2011/11/30/62
> Qemu patch for V3:
> http://lists.gnu.org/archive/html/qemu-devel/2011-12/msg00397.html
>
> V2 kernel changes :
> https://lkml.org/lkml/2011/10/23/207
>
> Previous discussions : (posted by Srivatsa V).
> https://lkml.org/lkml/2010/7/26/24
> https://lkml.org/lkml/2011/1/19/212
>
> Ticketlock change history:
> 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)
>
> 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 ]
>
> KVM patch Change history:
> Changes in V6:
> - Rebased to 3.4-rc3
> - Removed debugfs changes patch which should now be in Xen/linux-next.
> (https://lkml.org/lkml/2012/3/30/687)
> - Removed PV_UNHALT_MSR since currently we don't need guest communication,
> and made pv_unhalt folded to GET_MP_STATE (Marcello, Avi[long back])
> - Take jumplabel changes from Ingo/Jason into use (static_key_slow_inc usage)
> - Added inline to spinlock_init in non PARAVIRT case
> - Move arch specific code to arch/x86 and add stubs to other archs (Marcello)
> - Added more comments on pv_unhalt usage etc (Marcello)
>
> Changes in V5:
> - rebased to 3.3-rc6
> - added PV_UNHALT_MSR that would help in live migration (Avi)
> - removed PV_LOCK_KICK vcpu request and pv_unhalt flag (re)added.
> - Changed hypercall documentaion (Alex).
> - mode_t changed to umode_t in debugfs.
> - MSR related documentation added.
> - rename PV_LOCK_KICK to PV_UNHALT.
> - host and guest patches not mixed. (Marcelo, Alex)
> - kvm_kick_cpu now takes cpu so it can be used by flush_tlb_ipi_other
> paravirtualization (Nikunj)
> - coding style changes in variable declarion etc (Srikar)
>
> Changes in V4:
> - reabsed to 3.2.0 pre.
> - use APIC ID for kicking the vcpu and use kvm_apic_match_dest for matching (Avi)
> - fold vcpu->kicked flag into vcpu->requests (KVM_REQ_PVLOCK_KICK) and related
> changes for UNHALT path to make pv ticket spinlock migration friendly(Avi, Marcello)
> - Added Documentation for CPUID, Hypercall (KVM_HC_KICK_CPU)
> and capabilty (KVM_CAP_PVLOCK_KICK) (Avi)
> - Remove unneeded kvm_arch_vcpu_ioctl_set_mpstate call. (Marcello)
> - cumulative variable type changed (int ==> u32) in add_stat (Konrad)
> - remove unneeded kvm_guest_init for !CONFIG_KVM_GUEST case
>
> Changes in V3:
> - rebased to 3.2-rc1
> - use halt() instead of wait for kick hypercall.
> - modify kick hyper call to do wakeup halted vcpu.
> - hook kvm_spinlock_init to smp_prepare_cpus call (moved the call out of head##.c).
> - fix the potential race when zero_stat is read.
> - export debugfs_create_32 and add documentation to API.
> - use static inline and enum instead of ADDSTAT macro.
> - add barrier() in after setting kick_vcpu.
> - empty static inline function for kvm_spinlock_init.
> - combine the patches one and two readuce overhead.
> - make KVM_DEBUGFS depends on DEBUGFS.
> - include debugfs header unconditionally.
>
> Changes in V2:
> - rebased patchesto -rc9
> - synchronization related changes based on Jeremy's changes
> (Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>) pointed by
> Stephan Diestelhorst <stephan.diestelhorst@....com>
> - enabling 32 bit guests
> - splitted patches into two more chunks
>
> Documentation/virtual/kvm/cpuid.txt | 4 +
> Documentation/virtual/kvm/hypercalls.txt | 60 +++++
> arch/x86/Kconfig | 10 +
> arch/x86/include/asm/kvm_host.h | 4 +
> arch/x86/include/asm/kvm_para.h | 16 +-
> 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/kvm.c | 256 ++++++++++++++++++++
> arch/x86/kernel/paravirt-spinlocks.c | 18 +-
> arch/x86/kvm/cpuid.c | 3 +-
> arch/x86/kvm/x86.c | 44 ++++-
> arch/x86/xen/smp.c | 3 +-
> arch/x86/xen/spinlock.c | 387 ++++++++++--------------------
> include/linux/jump_label.h | 26 +--
> include/linux/jump_label_ratelimit.h | 34 +++
> include/linux/kvm_para.h | 1 +
> include/linux/perf_event.h | 1 +
> kernel/jump_label.c | 1 +
> 20 files changed, 673 insertions(+), 381 deletions(-)
This is looking pretty good and complete now - any objections
from anyone to trying this out in a separate x86 topic tree?
Thanks,
Ingo
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