Message-ID: <1335885292.13683.150.camel@twins>
Subject: Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb_others
From: Peter Zijlstra <peterz@infradead.org>
To: Avi Kivity <avi@redhat.com>
Cc: "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>, mingo@elte.hu,
        jeremy@goop.org, mtosatti@redhat.com, kvm@vger.kernel.org,
        x86@kernel.org, vatsa@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        hpa@zytor.com
Date: Tue, 01 May 2012 17:14:52 +0200
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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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