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SubjectRe: [RFC PATCH 1/3] x86,smp: move waiting on contended lock out of line
On Fri, Dec 21, 2012 at 06:50:38PM -0500, Rik van Riel wrote:
> Subject: x86,smp: move waiting on contended ticket lock out of line
>
> Moving the wait loop for congested loops to its own function allows
> us to add things to that wait loop, without growing the size of the
> kernel text appreciably.
>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---

Reviewed-by: Rafael Aquini <aquini@redhat.com>


> arch/x86/include/asm/spinlock.h | 13 +++++++------
> arch/x86/kernel/smp.c | 14 ++++++++++++++
> 2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index 33692ea..2a45eb0 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -34,6 +34,8 @@
> # define UNLOCK_LOCK_PREFIX
> #endif
>
> +extern void ticket_spin_lock_wait(arch_spinlock_t *, struct __raw_tickets);
> +
> /*
> * Ticket locks are conceptually two parts, one indicating the current head of
> * the queue, and the other indicating the current tail. The lock is acquired
> @@ -53,12 +55,11 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>
> inc = xadd(&lock->tickets, inc);
>
> - for (;;) {
> - if (inc.head == inc.tail)
> - break;
> - cpu_relax();
> - inc.head = ACCESS_ONCE(lock->tickets.head);
> - }
> + if (inc.head == inc.tail)
> + goto out;
> +
> + ticket_spin_lock_wait(lock, inc);
> + out:
> barrier(); /* make sure nothing creeps before the lock is taken */
> }
>
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 48d2b7d..20da354 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -113,6 +113,20 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1);
> static bool smp_no_nmi_ipi = false;
>
> /*
> + * Wait on a congested ticket spinlock.
> + */
> +void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> +{
> + for (;;) {
> + cpu_relax();
> + inc.head = ACCESS_ONCE(lock->tickets.head);
> +
> + if (inc.head == inc.tail)
> + break;
> + }
> +}
> +
> +/*
> * this function sends a 'reschedule' IPI to another CPU.
> * it goes straight through and wastes no time serializing
> * anything. Worst case is that we lose a reschedule ...


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