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SubjectRe: [PATCH 2/5] x86,smp: proportional backoff for ticket spinlocks
On 01/08/2013 05:50 PM, Eric Dumazet wrote:
> On Tue, 2013-01-08 at 17:32 -0500, Rik van Riel wrote:
>> Subject: x86,smp: proportional backoff for ticket spinlocks
>>
>> Simple fixed value proportional backoff for ticket spinlocks.
>> By pounding on the cacheline with the spin lock less often,
>> bus traffic is reduced. In cases of a data structure with
>> embedded spinlock, the lock holder has a better chance of
>> making progress.
>>
>> If we are next in line behind the current holder of the
>> lock, we do a fast spin, so as not to waste any time when
>> the lock is released.
>>
>> The number 50 is likely to be wrong for many setups, and
>> this patch is mostly to illustrate the concept of proportional
>> backup. The next patch automatically tunes the delay value.
>>
>> Signed-off-by: Rik van Riel <riel@redhat.com>
>> Signed-off-by: Michel Lespinasse <walken@google.com>
>> ---
>> arch/x86/kernel/smp.c | 23 ++++++++++++++++++++---
>> 1 files changed, 20 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
>> index 20da354..aa743e9 100644
>> --- a/arch/x86/kernel/smp.c
>> +++ b/arch/x86/kernel/smp.c
>> @@ -117,11 +117,28 @@ static bool smp_no_nmi_ipi = false;
>> */
>> void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
>> {
>> + __ticket_t head = inc.head, ticket = inc.tail;
>> + __ticket_t waiters_ahead;
>> + unsigned loops;
>> +
>> for (;;) {
>> - cpu_relax();
>> - inc.head = ACCESS_ONCE(lock->tickets.head);
>> + waiters_ahead = ticket - head - 1;
>> + /*
>> + * We are next after the current lock holder. Check often
>> + * to avoid wasting time when the lock is released.
>> + */
>> + if (!waiters_ahead) {
>> + do {
>> + cpu_relax();
>> + } while (ACCESS_ONCE(lock->tickets.head) != ticket);
>> + break;
>> + }
>> + loops = 50 * waiters_ahead;
>> + while (loops--)
>> + cpu_relax();
>>
>> - if (inc.head == inc.tail)
>> + head = ACCESS_ONCE(lock->tickets.head);
>> + if (head == ticket)
>> break;
>> }
>> }
>>
>
> Reviewed-by: Eric Dumazet <edumazet@google.com>
>
> In my tests, I used the following formula :
>
> loops = 50 * ((ticket - head) - 1/2);
>
> or :
>
> delta = ticket - head;
> loops = delay * delta - (delay >> 1);

I suppose that rounding down the delta might result
in more stable results, due to undersleeping less
often.

> (And I didnt use the special :
>
> if (!waiters_ahead) {
> do {
> cpu_relax();
> } while (ACCESS_ONCE(lock->tickets.head) != ticket);
> break;
> }
>
> Because it means this wont help machines with 2 cpus.
>
> (or more generally if there _is_ contention, but with
> one lock holder and one lock waiter)

Machines with 2 CPUs should not need help, because the
cpu_relax() alone gives enough of a pause that the lock
holder can make progress.

It may be interesting to try out your rounding-down of
delta, to see if that makes things better.

--
All rights reversed


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