Messages in this thread | | | Date | Fri, 26 Nov 2010 11:02:45 -0600 (CST) | From | Christoph Lameter <> | Subject | Re: [thiscpuops upgrade 05/10] x86: Use this_cpu_inc_return for nmi counter |
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On Fri, 26 Nov 2010, Tejun Heo wrote:
> > - __this_cpu_inc(alert_counter); > > - if (__this_cpu_read(alert_counter) == 5 * nmi_hz) > > + if (__this_cpu_inc_return(alert_counter) == 5 * nmi_hz) > > Hmmm... one worry I have is that xadd, being not a very popular > operation, might be slower than add and read. Using it for atomicity > would probably be beneficial in most cases but have you checked this > actually is cheaper?
XADD takes 3 uops. INC 1 and MOV 1 uop. So there is an additiona uop.
However, a memory fetch from l1 takes a mininum 4 cycles. Doing that twice already ends up with at least 8 cycles.
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