sched/idle: Optimize the generic idle loop
Currently, smp_processor_id() is used to fetch the current CPU in cpu_idle_loop(). Every time the idle thread runs, it fetches the current CPU using smp_processor_id(). Since the idle thread is per CPU, the current CPU is constant, so we can lift the load out of the loop, saving execution cycles/time in the loop. x86-64: Before patch (execution in loop): 148: 0f ae e8 lfence 14b: 65 8b 04 25 00 00 00 00 mov %gs:0x0,%eax 152: 00 153: 89 c0 mov %eax,%eax 155: 49 0f a3 04 24 bt %rax,(%r12) After patch (execution in loop): 150: 0f ae e8 lfence 153: 4d 0f a3 34 24 bt %r14,(%r12) ARM64: Before patch (execution in loop): 168: d5033d9f dsb ld 16c: b9405661 ldr w1,[x19,#84] 170: 1100fc20 add w0,w1,#0x3f 174: 6b1f003f cmp w1,wzr 178: 1a81b000 csel w0,w0,w1,lt 17c: 130c7000 asr w0,w0,#6 180: 937d7c00 sbfiz x0,x0,#3,#32 184: f8606aa0 ldr x0,[x21,x0] 188: 9ac12401 lsr x1,x0,x1 18c: 36000e61 tbz w1,#0,358 After patch (execution in loop): 1a8: d50339df dsb ld 1ac: f8776ac0 ldr x0,[x22,x23] ab0: ea18001f tst x0,x24 1b4: 54000ea0 b.eq 388 Further observance on ARM64 for 4 seconds shows that cpu_idle_loop is called 8672 times. Shifting the code will save instructions executed in loop and eventually time as well. Signed-off-by:Gaurav Jindal <gaurav.jindal@spreadtrum.com> Signed-off-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by:
Sanjeev Yadav <sanjeev.yadav@spreadtrum.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20160512101330.GA488@gauravjindalubtnb.del.spreadtrum.com Signed-off-by:
Ingo Molnar <mingo@kernel.org>
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