Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor

[Rik van Riel <riel@redhat.com>]

On 12/27/2012 01:49 PM, Eric Dumazet wrote:
> On Thu, 2012-12-27 at 09:35 -0500, Rik van Riel wrote:
>
>>
>> The lock acquisition time depends on the holder of the lock,
>> and what the CPUs ahead of us in line will do with the lock,
>> not on the caller IP of the spinner.
>
> That would be true only for general cases.
>
> In network land, we do have spinlock acquisition time depending on the
> context.
>
> A garbage collector usually runs for longer time than the regular fast
> path.

Won't the garbage collector running, hold up the lock
acquisition time by OTHER acquirers?

> But even without gc, its pretty often we have consumer/producers that
> don't have the same amount of work to perform per lock/unlock sections.
>
> The socket lock per example, might be held for very small sections for
> process contexts (lock_sock() / release_sock()), but longer sections
> from softirq context. Of course, severe lock contention on a socket
> seems unlikely in real workloads.

If one actor holds the lock for longer than the
others, surely it would be the others that suffer
in lock acquisition time?

>> Therefore, I am not convinced that hashing on the caller IP
>> will add much, if anything, except increasing the chance
>> that we end up not backing off when we should...
>>
>> IMHO it would be good to try keeping this solution as simple
>> as we can get away with.
>>
>
> unsigned long hash = (unsigned long)lock ^
>                       (unsigned long)__builtin_return_address(1);
>
> seems simple enough to me, but I get your point.
>
> I also recorded the max 'delay' value reached on my machine to check how
> good MAX_SPINLOCK_DELAY value was :
>
> [   89.628265] cpu 16 delay 3710
> [   89.631230] cpu 6 delay 2930
> [   89.634120] cpu 15 delay 3186
> [   89.637092] cpu 18 delay 3789
> [   89.640071] cpu 22 delay 4012
> [   89.643080] cpu 11 delay 3389
> [   89.646057] cpu 21 delay 3123
> [   89.649035] cpu 9 delay 3295
> [   89.651931] cpu 3 delay 3063
> [   89.654811] cpu 14 delay 3335
>
> Although it makes no performance difference to use a bigger/smaller one.

I guess we want a larger value.

With your hashed lock approach, we can get away with
larger values - they will not penalize other locks
the same way a single value per cpu might have.

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