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

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Date:   Wed, 19 Apr 2017 13:48:08 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 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 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've not looked hard enough at the new SRCU to see if its possible to
> re-instate that feature.
>