David Laight
2021-Jul-21 20:53 UTC
[PATCH v2 3/4] tools headers UAPI: add cpu_relax() implementation for x86 and arm64
From: Yunsheng Lin> Sent: 20 July 2021 03:22 > > As x86 and arm64 is the two available systems that I can build > and test the cpu_relax() implementation, so only add cpu_relax() > implementation for x86 and arm64, other arches can be added easily > when needed. >...> +#if defined(__i386__) || defined(__x86_64__) > +/* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */ > +static __always_inline void rep_nop(void) > +{ > + asm volatile("rep; nop" ::: "memory"); > +}Beware, Intel increased the stall for 'rep nop' in some recent cpu to IIRC about 200 cycles. They even document that this might have a detrimental effect. It is basically far too long for the sort of thing it makes sense to busy-wait for. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)