Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 17 Nov 2016 15:02:18 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:


> That's an interesting approach. I would be tempted to give it a
> per-thread (rather than per-process) scope.

Sure, per thread, but have it inherit to child processes.

> 
> 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)

Sure why not ;-)

> 
> 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 ?)

No, it shouldn't mention NOHZ_FULL. Perhaps have all tasks do this
regardless, even though it will only affect nohz full ones. But in the
future it may be other tasks as well.

> 
> 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".

Yeah, something like that. That way it wont interrupt tasks that are
running in no-hz-full and don't care about this syscall.

-- Steve