Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH 4/6] perf: Optimize get_recursion_context() | Date | Sat, 31 Oct 2020 12:11:42 +0000 |
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From: Peter Zijlstra > Sent: 30 October 2020 23:02 > > On Fri, Oct 30, 2020 at 04:22:48PM -0400, Steven Rostedt wrote: > > As this is something that ftrace recursion also does, perhaps we should > > move this into interrupt.h so that anyone that needs a counter can get > > it quickly, and not keep re-implementing it. > > Works for me, however: > > > /* > > * Quickly find what context you are in. > > * 0 - normal > > * 1 - softirq > > * 2 - hard interrupt > > * 3 - NMI > > */ > > static inline int irq_context() > > { > > unsigned int pc = preempt_count(); > > int rctx = 0; > > unsigned > > > > > if (pc & (NMI_MASK)) > > rctx++; > > if (pc & (NMI_MASK | HARDIRQ_MASK)) > > rctx++; > > if (pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)) > > rctx++; > > > > return rctx; > > } > > otherwise you'll get an extra instruction to sign extend it, which is > daft (yes, i've been staring at GCC output far too much). > > Also, gcc-9 does worse (like 1 byte iirc) with: > > rctx += !!(pc & (NMI_MASK)); > rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK)); > rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)); > > but gcc-10 doesn't seem to care.
You've made be look at some gcc output (it's raining).
The gcc 7.5.0 I have handy probably generates the best code for:
unsigned char q_2(unsigned int pc) { unsigned char rctx = 0;
rctx += !!(pc & (NMI_MASK)); rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK)); rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));
return rctx; }
0000000000000000 <q_2>: 0: f7 c7 00 00 f0 00 test $0xf00000,%edi # clock 0 6: 0f 95 c0 setne %al # clock 1 9: f7 c7 00 00 ff 00 test $0xff0000,%edi # clock 0 f: 0f 95 c2 setne %dl # clock 1 12: 01 c2 add %eax,%edx # clock 2 14: 81 e7 00 01 ff 00 and $0xff0100,%edi 1a: 0f 95 c0 setne %al 1d: 01 d0 add %edx,%eax # clock 3 1f: c3 retq
I doubt that is beatable.
I've annotated the register dependency chain. Likely to be 3 (or maybe 4) clocks. The other versions are a lot worse (7 or 8) without allowing for 'sbb' taking 2 clocks on a lot of Intel cpus.
David
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