linux-kernel - Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12

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Date:   Wed, 19 Apr 2017 14:08:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        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, dvhart@...ux.intel.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
        marc.zyngier@....com
Subject: Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12

On Wed, Apr 19, 2017 at 01:48:08PM +0200, Christian Borntraeger wrote:
> On 04/19/2017 01:28 PM, 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.
> 
> Why is such a behaviour change not mentioned in the cover letter?
> I could not find anything in the patch descriptions that would
> indicate a slowdown. How much slower did it get?
> 
> But indeed, there are several places at KVM startup which have been
> reworked to srcu since normal rcu was too slow for several usecases.
> (Mostly registering devices and related data structures at startup,
> basically the qemu/kvm coldplug interaction)

I suspect Paul is not considering this a 'normal' RCU feature, and
therefore didn't think about changing this.

I know I was fairly surprised by this requirement when I ran into it;
and only accidentally remembered it now that maz complained.