linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)

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Message-ID: <20100113053126.GC6781@linux.vnet.ibm.com>
Date:	Tue, 12 Jan 2010 21:31:26 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nicholas Miell <nmiell@...cast.net>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	josh@...htriplett.org, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier (v5)

On Tue, Jan 12, 2010 at 09:00:23PM -0800, Nicholas Miell wrote:
> On Tue, 2010-01-12 at 20:37 -0500, Mathieu Desnoyers wrote:
> > + * sys_membarrier - issue memory barrier on current process running threads
> > + * @expedited: (0) Lowest overhead. Few milliseconds latency.
> > + *             (1) Few microseconds latency.
> > + *
> 
> Alternate ABI proposal, keeping the possibility of future expansion in
> mind:
> 
> /*
> Mandatory flags to the membarrier system call that the kernel must
> understand are in the high 16 bits.
> */
> #define MEMBARRIER_MANDATORY_MASK 0xFFFF0000
> 
> /*
> Optional hints that the kernel can ignore are in the low 16 bits.
> */
> #define MEMBARRIER_OPTIONAL_MASK 0x0000FFFF
> 
> #define MEMBARRIER_EXPEDITED 1
> 
> extern int membarrier(unsigned int flags);
> 
> And then add to the system call itself:
> 
> if ((flags & MEMBARRIER_MANDATORY_MASK) != 0)
> 	return -EINVAL;

Why is it OK to ignore the developer's request for an expedited
membarrer()?  The guy who expected the syscall to complete in a few
microseconds might not be so happy to have it take many milliseconds.
By the same token, the guy who specified non-expedited so as to minimally
disturb other threads in the system might not be so happy to see them
all be IPIed for no good reason.  ;-)

							Thanx, Paul
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