----- On Nov 17, 2016, at 9:50 AM, rostedt rost...@goodmis.org wrote: > On Thu, 17 Nov 2016 13:54:27 +0000 (UTC) > Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote: > > >> >> > >> >> > Acked-by: Lai Jiangshan <jiangshan...@gmail.com> >> >> > >> >> > But I'm afraid, in the future, tick_nohz_full will become a default y >> >> > feature. thus it makes sys_membarrier() always disabled. we might >> >> > need a new MEMBARRIER_CMD_XXX to handle it? >> >> >> >> This may require that we send an IPI to nohz_full CPUs, which will >> >> disturb them real-time wise. Any better ideas ? >> > >> > Restrict the IPIs to CPUs running the process executing the >> > sys_membarrier() system call. This would mean that CPUs only >> > are interrupted by their own application's request. >> >> This would break use-cases of cross-process shared memory. :-( > > Perhaps make this an opt in. That is, all processes that want to be > affected by this can call this function with some flag that sets a flag > in tasks struct. And have that process get an IPI even in no-hz-full > mode if it asked to do it.
That's an interesting approach. I would be tempted to give it a per-thread (rather than per-process) scope. E.g., a thread could do the following to ask to be interrupted by IPIs: membarrier(MEMBARRIER_CMD_REGISTER_EXPEDITED, 0) and could unregister with: membarrier(MEMBARRIER_CMD_UNREGISTER_EXPEDITED, 0) We can then keep a per-thread refcount internally. (not sure the "EXPEDITED" is the right word there... do we want it to be "NOHZ_FULL" instead ?) Then in membarrier(MEMBARRIER_CMD_SHARED, 0), for each nohz_full cpu, we grab the rq lock, and only send an IPI if the running thread is registered as "expedited". Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
