WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

[Xen-devel] [PATCH 0/8] [PATCH RFC V2] Paravirtualized ticketlocks

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: [Xen-devel] [PATCH 0/8] [PATCH RFC V2] Paravirtualized ticketlocks
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 2 Sep 2011 16:54:07 -0700
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>, Nick Piggin <npiggin@xxxxxxxxx>, KVM <kvm@xxxxxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Delivery-date: Fri, 02 Sep 2011 17:06:30 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

[ Changes since last posting:
  - fold all the cleanup/bugfix patches into their base patches
  - change spin_lock to make sure fastpath has no cruft in it
  - make sure it doesn't attempt to inline unlock
]

NOTE: this series is based on tip.git tip/x86/spinlocks

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"
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
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, 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);
        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

        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 very straightforward:
        __ticket_unlock_release(lock);
        if (unlikely(__ticket_in_slowpath(lock)))
                __ticket_unlock_slowpath(lock);

which generates:
        push   %rbp
        mov    %rsp,%rbp

        addb   $0x2,(%rdi)
        testb  $0x1,0x1(%rdi)
        jne    1f

        pop    %rbp
        retq   

        ### SLOWPATH START
1:      movzwl (%rdi),%edx
        movzbl %dh,%ecx
        mov    %edx,%eax
        and    $-2,%ecx # clear TICKET_SLOWPATH_FLAG
        mov    %cl,%dh
        cmp    %dl,%cl  # test to see if lock is uncontended
        je     3f

2:      movzbl %dl,%esi
        callq  *__ticket_unlock_kick    # kick anyone waiting
        pop    %rbp
        retq   

3:      lock cmpxchg %dx,(%rdi) # use cmpxchg to safely write back flag
        jmp    2b
        ### SLOWPATH END

The fastpath is pretty straightforward, but it is definitely more
complex than a simple "incb (%rdi)" - which is still generated (and
inlined) when PARAVIRT_SPINLOCKS is disabled.

Thoughts? Comments? Suggestions?

Thanks,
        J

Jeremy Fitzhardinge (8):
  x86/spinlocks: replace pv spinlocks with pv ticketlocks
  x86/ticketlock: don't inline _spin_unlock when using paravirt
    spinlocks
  x86/ticketlock: collapse a layer of functions
  xen/pvticketlock: Xen implementation for PV ticket locks
  x86/pvticketlock: use callee-save for lock_spinning
  x86/ticketlocks: when paravirtualizing ticket locks, increment by 2
  x86/ticketlock: add slowpath logic
  xen/pvticketlock: allow interrupts to be enabled while blocking

 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       |  134 ++++++++++-----
 arch/x86/include/asm/spinlock_types.h |   16 ++-
 arch/x86/kernel/paravirt-spinlocks.c  |   16 +--
 arch/x86/xen/spinlock.c               |  311 ++++++++-------------------------
 kernel/Kconfig.locks                  |    2 +-
 8 files changed, 193 insertions(+), 329 deletions(-)

-- 
1.7.6


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel