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

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Date:   Mon, 7 Nov 2016 18:10:14 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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 you can carry this patch it would be very much appreciated,

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