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SubjectRe: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor
On Fri, Dec 21, 2012 at 06:56:13PM -0500, Rik van Riel wrote:
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 4e44840..e44c56f 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -113,19 +113,62 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1);
> static bool smp_no_nmi_ipi = false;
>
> /*
> - * Wait on a congested ticket spinlock.
> + * Wait on a congested ticket spinlock. Many spinlocks are embedded in
> + * data structures; having many CPUs pounce on the cache line with the
> + * spinlock simultaneously can slow down the lock holder, and the system
> + * as a whole.
> + *
> + * To prevent total performance collapse in case of bad spinlock contention,
> + * perform proportional backoff. The per-cpu value of delay is automatically
> + * tuned to limit the number of times spinning CPUs poll the lock before
> + * obtaining it. This limits the amount of cross-CPU traffic required to obtain
> + * a spinlock, and keeps system performance from dropping off a cliff.
> + *
> + * There is a tradeoff. If we poll too often, the whole system is slowed
> + * down. If we sleep too long, the lock will go unused for a period of
> + * time. Adjusting "delay" to poll, on average, 2.7 times before the
> + * lock is obtained seems to result in low bus traffic. The combination
> + * of aiming for a non-integer amount of average polls, and scaling the
> + * sleep period proportionally to how many CPUs are ahead of us in the
> + * queue for this ticket lock seems to reduce the amount of time spent
> + * "oversleeping" the release of the lock.
> */
> +#define MIN_SPINLOCK_DELAY 1
> +#define MAX_SPINLOCK_DELAY 1000
> +DEFINE_PER_CPU(int, spinlock_delay) = { MIN_SPINLOCK_DELAY };
> void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> {
> + /*
> + * Use the raw per-cpu pointer; preemption is disabled in the
> + * spinlock code. This avoids put_cpu_var once we have the lock.
> + */
> + int *delay_ptr = &per_cpu(spinlock_delay, smp_processor_id());
> + int delay = *delay_ptr;

I'm confused by the above comment. Why not just:

int delay = this_cpu_read(spinlock_delay);
?

> +
> for (;;) {
> - int loops = 50 * (__ticket_t)(inc.tail - inc.head);
> + int loops = delay * (__ticket_t)(inc.tail - inc.head);
> while (loops--)
> cpu_relax();
>
> inc.head = ACCESS_ONCE(lock->tickets.head);
> - if (inc.head == inc.tail)
> + if (inc.head == inc.tail) {
> + /* Decrease the delay, since we may have overslept. */
> + if (delay > MIN_SPINLOCK_DELAY)
> + delay--;
> break;
> + }
> +
> + /*
> + * The lock is still busy, the delay was not long enough.
> + * Going through here 2.7 times will, on average, cancel
> + * out the decrement above. Using a non-integer number
> + * gets rid of performance artifacts and reduces oversleeping.
> + */
> + if (delay < MAX_SPINLOCK_DELAY &&
> + ((inc.head & 3) == 0 || (inc.head & 7) == 1))
> + delay++;
> }
> + *delay_ptr = delay;

this_cpu_write(spinlock_delay, delay);

Too bad you posted this just before break. I currently have access to a
40 core box, and I would have loved to test this. But right now I have
it testing other things, and hopefully I'll still have access to it
after the break.

-- Steve

> }
>
> /*
>
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