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Date: Tue, 8 Oct 2019 17:43:10 +0000 (UTC)
From: Christopher Lameter <cl@linux.com>
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To: Leonardo Bras <leonardo@linux.ibm.com>
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Subject: Re: [PATCH v5 02/11] powerpc/mm: Adds counting method to monitor
 lockless pgtable walks
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References: <20191003013325.2614-1-leonardo@linux.ibm.com>  <20191003013325.2614-3-leonardo@linux.ibm.com>  <alpine.DEB.2.21.1910081509070.1734@www.lameter.com> <6e8877bff034603e75b35599797a39d9bc4840f1.camel@linux.ibm.com>
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On Tue, 8 Oct 2019, Leonardo Bras wrote:

> > You are creating contention on a single exclusive cacheline. Doesnt this
> > defeat the whole purpose of the lockless page table walk? Use mmap_sem or
> > so should cause the same performance regression?
>
> Sorry, I did not understand that question.
> I mean, this is just a refcount and never causes a lock.

Locks also use atomic operations like a refcount increment. Both require
the cacheline to be in exclusive state. So the impact is very similar.