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

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Message-ID: <20170516083601.GB2481@dhcp22.suse.cz>
Date:   Tue, 16 May 2017 10:36:02 +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 16:44:26, Pasha Tatashin wrote:
> On 05/15/2017 03:38 PM, Michal Hocko wrote:
> >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.
> 
> OK, I will think about this.
> 
> I do not really like adding new configs because they tend to clutter the
> code. This is why,

Yes I hate adding new (arch) config options as well. And I still believe
we do not need any here either...

> I wanted to rely on already existing config that I know benefits all
> platforms that use it.

I wouldn't be so sure about this. If any other platform has a similar
issues with small memset as sparc then the overhead is just papered over
by parallel initialization.

> Eventually,
> "CONFIG_DEFERRED_STRUCT_PAGE_INIT" is going to become the default
> everywhere, as there should not be a drawback of using it even on small
> machines.

Maybe and I would highly appreciate that.
-- 
Michal Hocko
SUSE Labs