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Date:   Fri, 4 Jan 2019 14:09:06 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, Pavel.Tatashin@microsoft.com,
        mingo@kernel.org, mgorman@techsingularity.net,
        iamjoonsoo.kim@lge.com, tglx@linutronix.de, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm/page_owner: fix for deferred struct page init
Message-ID: <20190104130906.GO31793@dhcp22.suse.cz>
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On Thu 03-01-19 17:22:29, Qian Cai wrote:
> On 1/3/19 3:22 PM, Michal Hocko wrote:
> > On Thu 03-01-19 14:53:47, Qian Cai wrote:
> >> On 1/3/19 2:07 PM, Michal Hocko wrote> So can we make the revert with an
> >> explanation that the patch was wrong?
> >>> If we want to make hacks to catch more objects to be tracked then it
> >>> would be great to have some numbers in hands.
> >>
> >> Well, those numbers are subject to change depends on future start_kernel()
> >> order. Right now, there are many functions could be caught earlier by page owner.
> >>
> >> 	kmemleak_init();
> > [...]
> >> 	sched_init_smp();
> > 
> > The kernel source dump will not tell us much of course. A ball park
> > number whether we are talking about dozen, hundreds or thousands of
> > allocations would tell us something at least, doesn't it.
> > 
> > Handwaving that it might help us some is not particurarly useful. We are
> > already losing some allocations already. Does it matter? Well, that
> > depends, sometimes we do want to catch an owner of particular page and
> > it is sad to find nothing. But how many times have you or somebody else
> > encountered that in practice. That is exactly a useful information to
> > judge an ugly ifdefery in the code. See my point?
> 
> Here is the number without DEFERRED_STRUCT_PAGE_INIT.
> 
> == page_ext_init() after page_alloc_init_late() ==
> Node 0, zone DMA: page owner found early allocated 0 pages
> Node 0, zone DMA32: page owner found early allocated 7009 pages
> Node 0, zone Normal: page owner found early allocated 85827 pages
> Node 4, zone Normal: page owner found early allocated 75063 pages
> 
> == page_ext_init() before kmemleak_init() ==
> Node 0, zone DMA: page owner found early allocated 0 pages
> Node 0, zone DMA32: page owner found early allocated 6654 pages
> Node 0, zone Normal: page owner found early allocated 41907 pages
> Node 4, zone Normal: page owner found early allocated 41356 pages
> 
> So, it told us that it will miss tens of thousands of early page allocation call
> sites.

This is an answer for the first part of the question (how much). The
second is _do_we_care_?

-- 
Michal Hocko
SUSE Labs