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

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Date:   Tue, 8 Nov 2016 11:15:51 +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 3:03 PM, Paul E. McKenney paulmck@...ux.vnet.ibm.com wrote:

> 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.

Ah right, the original implementation, yes.

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

Especially since the regression is somewhat related to RCU
behavior wrt nohz_full, getting it through your tree seems
relevant.

Thanks!

Mathieu

> 
>							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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com