Subject: Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12
To: Peter Zijlstra <peterz@infradead.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>
References: <20170412165441.GA17149@linux.vnet.ibm.com>
 <20170417232714.GA19013@linux.vnet.ibm.com>
 <20170419112845.3rt3zlyzuzert647@hirez.programming.kicks-ass.net>
 <bc6e4342-cb97-9108-174d-e7ab8f6d01e5@de.ibm.com>
 <20170419120847.GB3029@worktop.programming.kicks-ass.net>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        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
From: Marc Zyngier <marc.zyngier@arm.com>
Organization: ARM Ltd
Message-ID: <a322a051-72a8-5beb-8ce5-f0b17c088be6@arm.com>
Date: Wed, 19 Apr 2017 13:51:22 +0100
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On 19/04/17 13:08, Peter Zijlstra wrote:
> 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.

The issue I noticed yesterday has been addressed here:

https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/commit/?h=dev.2017.04.17a&id=6eec94fe40e294b04d32c8ef552e28fa6159bdad

and was triggered by the constant mapping/unmapping of memslots that
QEMU triggers when emulating a NOR flash that UEFI uses for storing its
variables.

So far, I'm not seeing any other spectacular regression introduced by
this series.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...