linux-kernel - Re: [PATCH v4] kvm/irqchip: Speed up KVM_SET_GSI

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Date:	Tue, 29 Apr 2014 09:55:42 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org
CC:	kvm@...r.kernel.org, Marcelo Tosatti <mtosatti@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v4] kvm/irqchip: Speed up KVM_SET_GSI_ROUTING

On 28/04/14 18:39, Paolo Bonzini wrote:
> From: Christian Borntraeger <borntraeger@...ibm.com>
> 
> When starting lots of dataplane devices the bootup takes very long on
> Christian's s390 with irqfd patches. With larger setups he is even
> able to trigger some timeouts in some components.  Turns out that the
> KVM_SET_GSI_ROUTING ioctl takes very long (strace claims up to 0.1 sec)
> when having multiple CPUs.  This is caused by the  synchronize_rcu and
> the HZ=100 of s390.  By changing the code to use a private srcu we can
> speed things up.  This patch reduces the boot time till mounting root
> from 8 to 2 seconds on my s390 guest with 100 disks.
> 
> Uses of hlist_for_each_entry_rcu, hlist_add_head_rcu, hlist_del_init_rcu
> are fine because they do not have lockdep checks (hlist_for_each_entry_rcu
> uses rcu_dereference_raw rather than rcu_dereference, and write-sides
> do not do rcu lockdep at all).
> 
> Note that we're hardly relying on the "sleepable" part of srcu.  We just
> want SRCU's faster detection of grace periods.
> 
> Testing was done by Andrew Theurer using NETPERF.  The difference between
> results "before" and "after" the patch has mean -0.2% and standard deviation
> 0.6%.  Using a paired t-test on the data points says that there is a 2.5%
> probability that the patch is the cause of the performance difference
> (rather than a random fluctuation).
> 
> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Thanks for moving that patch forward to the latest kernel version 
(plus your fixes)

I can confirm that it still fixes the performance issue on s390.

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