linux-kernel - Re: [PATCH tip/core/rcu 01/40] rcu: Maintain special bits at bottom of ->dynticks counter

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Message-Id: <20170413161534.GB3956@linux.vnet.ibm.com>
Date:   Thu, 13 Apr 2017 09:15:34 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 01/40] rcu: Maintain special bits at bottom
 of ->dynticks counter

On Thu, Apr 13, 2017 at 11:20:44AM +0200, Peter Zijlstra wrote:
> On Wed, Apr 12, 2017 at 10:39:46AM -0700, Paul E. McKenney wrote:
> > Currently, IPIs are used to force other CPUs to invalidate their TLBs
> > in response to a kernel virtual-memory mapping change.  This works, but
> > degrades both battery lifetime (for idle CPUs) and real-time response
> > (for nohz_full CPUs), and in addition results in unnecessary IPIs due to
> > the fact that CPUs executing in usermode are unaffected by stale kernel
> > mappings.  It would be better to cause a CPU executing in usermode to
> > wait until it is entering kernel mode to do the flush, first to avoid
> > interrupting usemode tasks and second to handle multiple flush requests
> > with a single flush in the case of a long-running user task.
> > 
> > This commit therefore reserves a bit at the bottom of the ->dynticks
> > counter, which is checked upon exit from extended quiescent states.
> > If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
> > invoked, which, if not supplied, is an empty single-pass do-while loop.
> > If this bottom bit is set on -entry- to an extended quiescent state,
> > then a WARN_ON_ONCE() triggers.
> > 
> > This bottom bit may be set using a new rcu_eqs_special_set() function,
> > which returns true if the bit was set, or false if the CPU turned
> > out to not be in an extended quiescent state.  Please note that this
> > function refuses to set the bit for a non-nohz_full CPU when that CPU
> > is executing in usermode because usermode execution is tracked by RCU
> > as a dyntick-idle extended quiescent state only for nohz_full CPUs.
> > 
> > Reported-by: Andy Lutomirski <luto@...capital.net>
> 
> Isn't that more a: Requested-by ?

I am not too worried about the distinction.  Request a feature, report
the lack of a needed feature, or report a bug, but either way I had to
write the code.  ;-)

								Thanx, Paul