linux-kernel - Re: [RFC] perf: fix building for ARCv1

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Date:	Tue, 20 Oct 2015 08:00:46 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Noam Camus <noamc@...hip.com>
CC:	Andi Kleen <andi@...stfloor.org>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
	"dvhart@...ux.intel.com" <dvhart@...ux.intel.com>,
	"dsahern@...il.com" <dsahern@...il.com>,
	"acme@...hat.com" <acme@...hat.com>
Subject: Re: [RFC] perf: fix building for ARCv1

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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