On Wed, Apr 12, 2017 at 10:39:46AM -0700, Paul E. McKenney wrote: > Currently, IPIs are used to force other CPUs to invalidate their TLBs > in response to a kernel virtual-memory mapping change. This works, but > degrades both battery lifetime (for idle CPUs) and real-time response > (for nohz_full CPUs), and in addition results in unnecessary IPIs due to > the fact that CPUs executing in usermode are unaffected by stale kernel > mappings. It would be better to cause a CPU executing in usermode to > wait until it is entering kernel mode to do the flush, first to avoid > interrupting usemode tasks and second to handle multiple flush requests > with a single flush in the case of a long-running user task. > > This commit therefore reserves a bit at the bottom of the ->dynticks > counter, which is checked upon exit from extended quiescent states. > If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is > invoked, which, if not supplied, is an empty single-pass do-while loop. > If this bottom bit is set on -entry- to an extended quiescent state, > then a WARN_ON_ONCE() triggers. > > This bottom bit may be set using a new rcu_eqs_special_set() function, > which returns true if the bit was set, or false if the CPU turned > out to not be in an extended quiescent state. Please note that this > function refuses to set the bit for a non-nohz_full CPU when that CPU > is executing in usermode because usermode execution is tracked by RCU > as a dyntick-idle extended quiescent state only for nohz_full CPUs. > > Reported-by: Andy Lutomirski <l...@amacapital.net>
Isn't that more a: Requested-by ?
