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

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Date:   Wed, 31 May 2017 23:35:48 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     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, davem@...emloft.net
Subject: Re: [v3 0/9] parallelized "struct page" zeroing

> OK, so why cannot we make zero_struct_page 8x 8B stores, other arches
> would do memset. You said it would be slower but would that be
> measurable? I am sorry to be so persistent here but I would be really
> happier if this didn't depend on the deferred initialization. If this is
> absolutely a no-go then I can live with that of course.

Hi Michal,

This is actually a very good idea. I just did some measurements, and it 
looks like performance is very good.

Here is data from SPARC-M7 with 3312G memory with single thread performance:

Current:
memset() in memblock allocator takes: 8.83s
__init_single_page() take: 8.63s

Option 1:
memset() in __init_single_page() takes: 61.09s (as we discussed because 
of membar overhead, memset should really be optimized to do STBI only 
when size is 1 page or bigger).

Option 2:

8 stores (stx) in __init_single_page(): 8.525s!

So, even for single thread performance we can double the initialization 
speed of "struct page" on SPARC by removing memset() from memblock, and 
using 8 stx in __init_single_page(). It appears we never miss L1 in 
__init_single_page() after the initial 8 stx.

I will update patches with memset() on other platforms, and stx on SPARC.

My experimental code looks like this:

static void __meminit __init_single_page(struct page *page, unsigned 
long pfn, unsigned long zone, int nid)
{
         __asm__ __volatile__(
         "stx    %%g0, [%0 + 0x00]\n"
         "stx    %%g0, [%0 + 0x08]\n"
         "stx    %%g0, [%0 + 0x10]\n"
         "stx    %%g0, [%0 + 0x18]\n"
         "stx    %%g0, [%0 + 0x20]\n"
         "stx    %%g0, [%0 + 0x28]\n"
         "stx    %%g0, [%0 + 0x30]\n"
         "stx    %%g0, [%0 + 0x38]\n"
         :
         :"r"(page));
         set_page_links(page, zone, nid, pfn);
         init_page_count(page);
         page_mapcount_reset(page);
         page_cpupid_reset_last(page);

         INIT_LIST_HEAD(&page->lru);
#ifdef WANT_PAGE_VIRTUAL
         /* The shift won't overflow because ZONE_NORMAL is below 4G. */
         if (!is_highmem_idx(zone))
                 set_page_address(page, __va(pfn << PAGE_SHIFT));
#endif
}

Thank you,
Pasha