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

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Date:   Wed, 17 May 2017 09:50:57 +1000
From:   Benjamin Herrenschmidt <benh@....ibm.com>
To:     David Miller <davem@...emloft.net>, pasha.tatashin@...cle.com
Cc:     linux-s390@...r.kernel.org, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, linux-kernel@...r.kernel.org,
        mhocko@...nel.org, linux-mm@...ck.org, sparclinux@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [v3 0/9] parallelized "struct page" zeroing

On Fri, 2017-05-12 at 13:37 -0400, David Miller wrote:
> > Right now it is larger, but what I suggested is to add a new optimized
> > routine just for this case, which would do STBI for 64-bytes but
> > without membar (do membar at the end of memmap_init_zone() and
> > deferred_init_memmap()
> > 
> > #define struct_page_clear(page)                                 \
> >          __asm__ __volatile__(                                   \
> >          "stxa   %%g0, [%0]%2\n"                                 \
> >          "stxa   %%xg0, [%0 + %1]%2\n"                           \
> >          : /* No output */                                       \
> >          : "r" (page), "r" (0x20), "i"(ASI_BLK_INIT_QUAD_LDD_P))
> > 
> > And insert it into __init_single_page() instead of memset()
> > 
> > The final result is 4.01s/T which is even faster compared to current
> > 4.97s/T
> 
> Ok, indeed, that would work.

On ppc64, that might not. We have a dcbz instruction that clears an
entire cache line at once. That's what we use for memset's and page
clearing. However, 64 bytes is half a cache line on modern processors
so we can't use it with that semantic and would have to fallback to the
slower stores.

Cheers,
Ben.