Date: Wed, 19 Apr 2017 17:43:43 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com, marc.zyngier@arm.com
Subject: Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12
Message-ID: <20170419154343.osootv4bcwfppnlc@hirez.programming.kicks-ass.net>
References: <20170412165441.GA17149@linux.vnet.ibm.com>
 <20170417232714.GA19013@linux.vnet.ibm.com>
 <20170419112845.3rt3zlyzuzert647@hirez.programming.kicks-ass.net>
 <20170419130245.GJ3956@linux.vnet.ibm.com>
 <20170419131553.cii62nl2si3lwm7w@hirez.programming.kicks-ass.net>
 <20170419153703.GQ3956@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170419153703.GQ3956@linux.vnet.ibm.com>
User-Agent: NeoMutt/20170113 (1.7.2)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1304
Lines: 29

On Wed, Apr 19, 2017 at 08:37:03AM -0700, Paul E. McKenney wrote:
> On Wed, Apr 19, 2017 at 03:15:53PM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 19, 2017 at 06:02:45AM -0700, Paul E. McKenney wrote:
> > > On Wed, Apr 19, 2017 at 01:28:45PM +0200, Peter Zijlstra wrote:
> > > > 
> > > > So the thing Maz complained about is because KVM assumes
> > > > synchronize_srcu() is 'free' when there is no srcu_read_lock() activity.
> > > > This series 'breaks' that.
> > > > 
> > > > I've not looked hard enough at the new SRCU to see if its possible to
> > > > re-instate that feature.
> > > 
> > > And with the fix I gave Maz, the parallelized version is near enough
> > > to being free as well.  It was just a stupid bug on my part: I forgot
> > > to check for expedited when scheduling callbacks.
> > 
> > Right, although for the old SRCU it was true for !expedited as well.
> 
> Which is all good fun until someone does a call_srcu() on each and
> every munmap() syscall.  ;-)

Well, that being a different SRCU domain doesn't affect the KVM memslot
domain thingy ;-)

> But the current code is much better housebroken.  ;-)

It is. But a workload that manages to hit sync_expedited in a loop on
all CPUs is still O(n^2) work. And the more sync_expedited instances we
have, the more likely that becomes.