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

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Message-ID: <20181106140638.GN27423@dhcp22.suse.cz>
Date:   Tue, 6 Nov 2018 15:06:38 +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 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?

The patch as is looks good to me - modulo atomic counters of course. I
cannot really judge whether existing updaters do really race in practice
to take this riskless.

The improvement is nice of course but this is a rare operation and 50ms
vs 1ms is hardly noticeable. So I would rather wait for the preparatory
work to settle. Btw. is there anything blocking that? It seems to be
mostly automated.

-- 
Michal Hocko
SUSE Labs