Re: [RFC PATCH 2/5] x86,smp: proportional backoff for ticket spinlocks

[Michel Lespinasse]

On Thu, Jan 3, 2013 at 3:35 AM, Raghavendra KT
<raghavendra.kt.linux@xxxxxxxxx> wrote:
> [Ccing IBM id]
> On Thu, Jan 3, 2013 at 10:52 AM, Rik van Riel <riel@xxxxxxxxxx> wrote:
>> 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@xxxxxxxxxx>
>> Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
>> ---
>>  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..9c56fe3 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;
>                                              ^^^^^^^^^^^^^^
> Just wondering,
> Does wraparound affects this?

The result gets stored in waiters_ahead, which is unsigned and has
same bit size as ticket and head. So, this takes care of the
wraparound issue.

In other words, you may have to add 1<<8 or 1<<16 if the integer
difference was negative; but you get that for free by just computing
the difference as a 8 or 16 bit unsigned value.


-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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