linux-kernel - Re: [PATCH v6 1/2] memory_hotplug: Free pages as higher order

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Message-ID: <20181106200823.GT27423@dhcp22.suse.cz>
Date:   Tue, 6 Nov 2018 21:08:23 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Arun KS <arunks@...eaurora.org>
Cc:     arunks.linux@...il.com, akpm@...ux-foundation.org, vbabka@...e.cz,
        osalvador@...e.de, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, getarunks@...il.com
Subject: Re: [PATCH v6 1/2] memory_hotplug: Free pages as higher order

On Tue 06-11-18 21:01:29, Arun KS wrote:
> On 2018-11-06 19:36, Michal Hocko wrote:
> > On Tue 06-11-18 11:33:13, Arun KS wrote:
> > > When free pages are done with higher 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%. Modify external
> > > providers of online callback to align with the change.
> > > 
> > > This patch modifies totalram_pages, zone->managed_pages and
> > > totalhigh_pages outside managed_page_count_lock. A follow up
> > > series will be send to convert these variable to atomic to
> > > avoid readers potentially seeing a store tear.
> > 
> > Is there any reason to rush this through rather than wait for counters
> > conversion first?
> 
> Sure Michal.
> 
> Conversion patch, https://patchwork.kernel.org/cover/10657217/ is currently
> incremental to this patch.

The ordering should be other way around. Because as things stand with
this patch first it is possible to introduce a subtle race prone
updates. As I've said I am skeptical the race would matter, really, but
there is no real reason to risk for that. Especially when you have the
other (first) half ready.

-- 
Michal Hocko
SUSE Labs