Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

[Paul E. McKenney]

On Thu, May 07, 2015 at 02:49:39PM +0200, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> 
> > The TIF_RCU_QS thing is just a fancy way for synchronize_rcu() 
> > (being executed on some other CPU not doing RT work) to 
> > intelligently wait for the remote (RT work doing) CPU to finish 
> > executing kernel code, without polling or so.
> 
> it's basically a cheap IPI being inserted on the remote CPU.
> 
> We need the TIF_RCU_QS callback not just to wait intelligently, but 
> mainly to elapse a grace period, otherwise synchronize_rcu() might not 
> ever make progress: think a SCHED_FIFO task doing some kernel work, 
> synchronize_rcu() stumbling upon it - but the SCHED_FIFO task 
> otherwise never scheduling and never getting any timer irqs either, 
> and thus never entering quiescent state.
> 
> (Cc:-ed Paul too, he might be interested in this as well.)

Hmmm...  So the point is that a NO_HZ_FULL CPU periodically posts
callbacks to indicate that it has passed through a quiescent state,
for example, upon entry to and/or exit from userspace?  These callbacks
would then be offloaded to some other CPU.

But the callback would not be invoked until RCU saw a grace period,
so I must be missing something here...  Probably that the TIF_RCU_QS
callback is not an RCU callback, but something else?

							Thanx, Paul

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