linux-kernel - Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb

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Date:	Tue, 01 May 2012 17:14:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>, mingo@...e.hu,
	jeremy@...p.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	x86@...nel.org, vatsa@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb_others

On Tue, 2012-05-01 at 15:12 +0300, Avi Kivity wrote:
> 
> What's changed is not gup_fast() but the performance of munmap(),
> exit(), and exec(), no?

If it is indeed cache related like you suggested earlier, it would be
the allocation side of things, like fork()/mmap() that suffer since
there's fewer hot pages about, but yes, anything creating/destroying
page-tables.

> What bounds the amount of memory waiting to be freed during an rcu grace
> period?

Most RCU implementations don't have limits, so that could be quite a
lot. I think preemptible RCU has a batch limit at which point it tries
rather hard to force a grace period, but I'm not sure if even that
provides a hard limit.

Practically though, I haven't had reports of PPC/Sparc going funny
because of this.
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