linux-kernel - Re: [PATCH] Fix: disable sys_membarrier when nohz

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Date:   Thu, 17 Nov 2016 10:17:25 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled

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