Re: [RFC] perf: fix building for ARCv1

[Vineet Gupta]

On Monday 19 October 2015 03:22 PM, Peter Zijlstra wrote:
> On Mon, Oct 19, 2015 at 09:46:35AM +0000, Vineet Gupta wrote:
>> On ARC we could use the atomic EXchange to implement a user space only binary
>> semaphore - these atomic ops will be small duration so it is OK to spin wait for a
>> little bit. That's how the old pthread library worked for ARC w/o any atomic support.
> That has the obvious problem of lock-holder-preemption and the horrible
> performance issues that result from that.
>
> I think the syscall at least has deterministic behaviour, whereas that
> userspace spin loop has this abysmal worst case thing.

I don't have issue with adding the syscall per-se. But that comes with it's own
headaches of ABI change - more importantly it requires several things to match,
libc, kernel...  It would be easier if change was confined to say perf.

Can we use existing syscall(s) - again this is what our good old pthread library
code did.

static void __pthread_acquire(int * spinlock)
{
  int cnt = 0;
  struct timespec tm;

  READ_MEMORY_BARRIER();

  while (testandset(spinlock)) {   <---- atomic EXchange
    if (cnt < 50) {
      sched_yield();
      cnt++;
    } else {
      tm.tv_sec = 0;
      tm.tv_nsec = 2000001;
      nanosleep(&tm, ((void *)0));
      cnt = 0;
    }
  }

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