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

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Message-Id: <20161107200315.GA3438@linux.vnet.ibm.com>
Date:   Mon, 7 Nov 2016 12:03:15 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        rostedt <rostedt@...dmis.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled

On Mon, Nov 07, 2016 at 06:10:14PM +0000, Mathieu Desnoyers wrote:
> 
> ----- On Nov 7, 2016, at 1:06 PM, Paul E. McKenney paulmck@...ux.vnet.ibm.com wrote:
> 
> > On Mon, Nov 07, 2016 at 05:08:59PM +0000, Mathieu Desnoyers wrote:
> >> ----- On Nov 3, 2016, at 1:49 PM, Paul E. McKenney paulmck@...ux.vnet.ibm.com
> >> wrote:
> >> 
> >> > On Thu, Nov 03, 2016 at 10:29:28AM -0600, Mathieu Desnoyers wrote:
> >> >> Userspace applications should be allowed to expect the membarrier system
> >> >> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on
> >> >> nohz_full CPUs, but synchronize_sched() does not take those into
> >> >> account.
> >> >> 
> >> >> Given that we do not want unrelated processes to be able to affect
> >> >> real-time sensitive nohz_full CPUs, simply return ENOSYS when membarrier
> >> >> is invoked on a kernel with enabled nohz_full CPUs.
> >> >> 
> >> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> >> >> CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> >> > 
> >> > Acked-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> >> 
> >> Hi Paul,
> >> 
> >> Do you plan to pick it up through your tree, or I should sent
> >> it directly to Linus ?
> > 
> > Your choice.  I believe that the original went some other way, but I
> > would be fine carrying this one.
> 
> Not sure what you mean by "the original" ? And which other way ?
> I have not been notified about this.

If I remember correctly, you sent the original sys_membarrier()
patch through akpm or similar.

> If you can carry this patch it would be very much appreciated,

Will do!

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> >							Thanx, Paul
> > 
> >> Thanks,
> >> 
> >> Mathieu
> >> 
> >> > 
> >> >> CC: Josh Triplett <josh@...htriplett.org>
> >> >> CC: Steven Rostedt <rostedt@...dmis.org>
> >> >> CC: Lai Jiangshan <jiangshanlai@...il.com>
> >> >> CC: <stable@...r.kernel.org>	[3.10+]
> >> >> ---
> >> >>  kernel/membarrier.c | 4 ++++
> >> >>  1 file changed, 4 insertions(+)
> >> >> 
> >> >> diff --git a/kernel/membarrier.c b/kernel/membarrier.c
> >> >> index 536c727..9f9284f 100644
> >> >> --- a/kernel/membarrier.c
> >> >> +++ b/kernel/membarrier.c
> >> >> @@ -16,6 +16,7 @@
> >> >> 
> >> >>  #include <linux/syscalls.h>
> >> >>  #include <linux/membarrier.h>
> >> >> +#include <linux/tick.h>
> >> >> 
> >> >>  /*
> >> >>   * Bitmask made from a "or" of all commands within enum membarrier_cmd,
> >> >> @@ -51,6 +52,9 @@
> >> >>   */
> >> >>  SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> >> >>  {
> >> >> +	/* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
> >> >> +	if (tick_nohz_full_enabled())
> >> >> +		return -ENOSYS;
> >> >>  	if (unlikely(flags))
> >> >>  		return -EINVAL;
> >> >>  	switch (cmd) {
> >> >> --
> >> >> 2.1.4
> >> 
> >> --
> >> Mathieu Desnoyers
> >> EfficiOS Inc.
> >> http://www.efficios.com
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>