linux-kernel - Re: [RFC] memory_hotplug: Free pages as pageblock

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Message-ID: <20180914091053.GJ20287@dhcp22.suse.cz>
Date:   Fri, 14 Sep 2018 11:10:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Arun KS <arunks@...eaurora.org>
Cc:     Balbir Singh <bsingharora@...il.com>, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, vbabka@...e.cz,
        pasha.tatashin@...cle.com, iamjoonsoo.kim@....com,
        osalvador@...e.de, malat@...ian.org, gregkh@...uxfoundation.org,
        yasu.isimatu@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, arunks.linux@...il.com,
        vinmenon@...eaurora.org
Subject: Re: [RFC] memory_hotplug: Free pages as pageblock_order

On Wed 12-09-18 20:12:30, Arun KS wrote:
> On 2018-09-12 18:47, Michal Hocko wrote:
> > On Wed 12-09-18 22:57:43, Balbir Singh wrote:
> > > On Wed, Sep 12, 2018 at 12:38:53PM +0200, Michal Hocko wrote:
> > > > On Wed 12-09-18 14:56:45, Arun KS wrote:
> > > > > When free pages are done with pageblock_order, time spend on
> > > > > coalescing pages by buddy allocator can be reduced. With
> > > > > section size of 256MB, hot add latency of a single section
> > > > > shows improvement from 50-60 ms to less than 1 ms, hence
> > > > > improving the hot add latency by 60%.
> > > >
> > > > Where does the improvement come from? You are still doing the same
> > > > amount of work except that the number of callbacks is lower. Is this the
> > > > real source of 60% improvement?
> > > >
> > > 
> > > It looks like only the first page of the pageblock is initialized, is
> > > some of the cost amortized in terms of doing one initialization for
> > > the page with order (order) and then relying on split_page and helpers
> > > to do the rest? Of course the number of callbacks reduce by a
> > > significant
> > > number as well.
> > 
> > Ohh, I have missed that part. Now when re-reading I can see the reason
> > for the perf improvement. It is most likely the higher order free which
> > ends up being much cheaper. This part makes some sense.
> > 
> > How much is this feasible is another question. Do not forget we have
> > those external providers of the online callback and those would need to
> > be updated as well.
> Sure Michal, I ll look into this.
> 
> > 
> > Btw. the normal memmap init code path does the same per-page free as
> > well. If we really want to speed the hotplug path then I guess the init
> > one would see a bigger improvement and those two should be in sync.
> Thanks for pointers, Will look further.

I haven't looked closer and I will be travelling next week so just hint.
Have a look at the nobootmem and how it frees pages to the page
allocator in __free_pages_boot_core. Seems exactly what you want and it
also answers your question about reference counting.
-- 
Michal Hocko
SUSE Labs