linux-kernel - Re: [v3 0/9] parallelized "struct page" zeroing

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Message-ID: <20170510211131.GD1590@bombadil.infradead.org>
Date:   Wed, 10 May 2017 14:11:31 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     David Miller <davem@...emloft.net>
Cc:     mhocko@...nel.org, pasha.tatashin@...cle.com,
        linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com
Subject: Re: [v3 0/9] parallelized "struct page" zeroing

On Wed, May 10, 2017 at 02:00:26PM -0400, David Miller wrote:
> From: Matthew Wilcox <willy@...radead.org>
> Date: Wed, 10 May 2017 10:17:03 -0700
> > On Wed, May 10, 2017 at 11:19:43AM -0400, David Miller wrote:
> >> I guess it might be clearer if you understand what the block
> >> initializing stores do on sparc64.  There are no memory accesses at
> >> all.
> >> 
> >> The cpu just zeros out the cache line, that's it.
> >> 
> >> No L3 cache line is allocated.  So this "wipe everything" behavior
> >> will not happen in the L3.
> > 
> > There's either something wrong with your explanation or my reading
> > skills :-)
> > 
> > "There are no memory accesses"
> > "No L3 cache line is allocated"
> > 
> > You can have one or the other ... either the CPU sends a cacheline-sized
> > write of zeroes to memory without allocating an L3 cache line (maybe
> > using the store buffer?), or the CPU allocates an L3 cache line and sets
> > its contents to zeroes, probably putting it in the last way of the set
> > so it's the first thing to be evicted if not touched.
> 
> There is no conflict in what I said.
> 
> Only an L2 cache line is allocated and cleared.  L3 is left alone.

I thought SPARC had inclusive caches.  So allocating an L2 cacheline
would necessitate allocating an L3 cacheline.  Or is this an exception
to the normal order of things?