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    • T
      nohz: prevent tick stop outside of the idle loop · b8f8c3cf
      Thomas Gleixner 提交于
      Jack Ren and Eric Miao tracked down the following long standing
      problem in the NOHZ code:
      
      	scheduler switch to idle task
      	enable interrupts
      
      Window starts here
      
      	----> interrupt happens (does not set NEED_RESCHED)
      	      	irq_exit() stops the tick
      
      	----> interrupt happens (does set NEED_RESCHED)
      
      	return from schedule()
      	
      	cpu_idle(): preempt_disable();
      
      Window ends here
      
      The interrupts can happen at any point inside the race window. The
      first interrupt stops the tick, the second one causes the scheduler to
      rerun and switch away from idle again and we end up with the tick
      disabled.
      
      The fact that it needs two interrupts where the first one does not set
      NEED_RESCHED and the second one does made the bug obscure and extremly
      hard to reproduce and analyse. Kudos to Jack and Eric.
      
      Solution: Limit the NOHZ functionality to the idle loop to make sure
      that we can not run into such a situation ever again.
      
      cpu_idle()
      {
      	preempt_disable();
      
      	while(1) {
      		 tick_nohz_stop_sched_tick(1); <- tell NOHZ code that we
      		 			          are in the idle loop
      
      		 while (!need_resched())
      		       halt();
      
      		 tick_nohz_restart_sched_tick(); <- disables NOHZ mode
      		 preempt_enable_no_resched();
      		 schedule();
      		 preempt_disable();
      	}
      }
      
      In hindsight we should have done this forever, but ... 
      
      /me grabs a large brown paperbag.
      
      Debugged-by: Jack Ren <jack.ren@marvell.com>, 
      Debugged-by: Neric miao <eric.y.miao@gmail.com>
      Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
      b8f8c3cf
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