Messages in this thread | | | From | Vineet Gupta <> | Subject | Re: [RFC] perf: fix building for ARCv1 | Date | Tue, 20 Oct 2015 08:00:46 +0000 |
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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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