On Thu, May 07, 2015 at 02:49:39PM +0200, Ingo Molnar wrote: > > * Ingo Molnar <mi...@kernel.org> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/