linux-kernel - Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode

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Date:   Thu, 12 Jul 2018 12:10:28 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        paulmck@...ux.vnet.ibm.com
Cc:     peterz@...radead.org, mhillenb@...zon.de,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when
 entering guest mode

On Thu, 2018-07-12 at 13:00 +0200, Christian Borntraeger wrote:
> 
> On s390 this seems to add about 10ns (~3%) for a guest exit/rentry
> microbenchmark mostly due to  rcu_eqs_enter and rcu_eqs_exit now being
> visible in perf samples. The older interface was cheaper.

Well, the older interface wasn't actually working, which made it
moderately suboptimal :)

But that is fixed by the first patch of the two, so perhaps the second
ought to be conditional on CONFIG_NO_HZ_FULL since it's only really
fixing a real bug there?

Unless we can contrive some way to do the rcu_eqs_enter/exit only on
the paths to/from userspace, and *not* when we loop in the kernel to
handle interrupts (etc.) and immediately re-enter the guest without
returning? That's somewhat non-trivial though...
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