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

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Message-ID: <20170515193817.GC7551@dhcp22.suse.cz>
Date:   Mon, 15 May 2017 21:38:18 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pasha Tatashin <pasha.tatashin@...cle.com>
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

On Mon 15-05-17 14:12:10, Pasha Tatashin wrote:
> Hi Michal,
> 
> After looking at your suggested memblock_virt_alloc_core() change again, I
> decided to keep what I have. I do not want to inline
> memblock_virt_alloc_internal(), because it is not a performance critical
> path, and by inlining it we will unnecessarily increase the text size on all
> platforms.

I do not insist but I would really _prefer_ if the bool zero argument
didn't proliferate all over the memblock API.

> Also, because it will be very hard to make sure that no platform regresses
> by making memset() default in _memblock_virt_alloc_core() (as I already
> showed last week at least sun4v SPARC64 will require special changes in
> order for this to work), I decided to make it available only for "deferred
> struct page init" case. As, what is already in the patch.

I do not think this is the right approach. Your measurements just show
that sparc could have a more optimized memset for small sizes. If you
keep the same memset only for the parallel initialization then you
just hide this fact. I wouldn't worry about other architectures. All
sane architectures should simply work reasonably well when touching a
single or only few cache lines at the same time. If some arches really
suffer from small memsets then the initialization should be driven by a
specific ARCH_WANT_LARGE_PAGEBLOCK_INIT rather than making this depend
on DEFERRED_INIT. Or if you are too worried then make it opt-in and make
it depend on ARCH_WANT_PER_PAGE_INIT and make it enabled for x86 and
sparc after memset optimization.

-- 
Michal Hocko
SUSE Labs