linux-kernel - Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

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Message-Id: <20170727131418.GR3730@linux.vnet.ibm.com>
Date:   Thu, 27 Jul 2017 06:14:18 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

On Thu, Jul 27, 2017 at 10:53:12AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 26, 2017 at 06:01:15PM +0000, Mathieu Desnoyers wrote:
> 
> > Another alternative for a MEMBARRIER_CMD_SHARED_EXPEDITED would be rate-limiting
> > per thread. For instance, we could add a new "ulimit" that would bound the
> > number of expedited membarrier per thread that can be done per millisecond,
> > and switch to synchronize_sched() whenever a thread goes beyond that limit
> > for the rest of the time-slot.
> 
> You forgot to ask yourself how you could abuse this.. just spawn more
> threads.
> 
> Per-thread limits are nearly useless, because spawning new threads is
> cheap.

Agreed -- any per-thread limit has to be some portion/fraction of a
global limit.

							Thanx, Paul