Subject: Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12
To: paulmck@linux.vnet.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>
 <20170419132213.GA17362@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, 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
From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Wed, 19 Apr 2017 15:25:47 +0200
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On 04/19/2017 03:22 PM, Paul E. McKenney 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?
> 
> It was an 8x slowdown in boot time of a guest OS running UEFI, from
> five seconds to forty seconds.  The fix restored the original boot time.
> 
> Why didn't I report the slowdown in my cover letter?  Because I didn't
> realize that I had created such a stupid bug!  ;-)
> 
> Why didn't my testing reveal the bug?  Because in my rcutorture testing,
> the buggy code runs about as fast as the original, and the fixed new code
> runs about an order of magnitude faster.  This is because rcutorture's
> performance statistics are mostly sensitive to throughput, while Marc's
> boot-time run is mostly sensitive to latency.
> 
>> 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)
> 
> And here is the patch that restored Marc's boot speed.  It simply changes
> the original (buggy) fixed delay for no delay in the expedited case and
> the same fixed delay in the non-expedited case.
> 
> 							Thanx, Paul

Ok, so it was not a fundamental rework, it was just a bug.
Then nevermind :-)