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Date:   Fri, 24 Nov 2023 11:24:11 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Charan Teja Kalla <quic_charante@quicinc.com>
cc:     akpm@linux-foundation.org, mgorman@techsingularity.net,
        mhocko@suse.com, david@redhat.com, vbabka@suse.cz,
        hannes@cmpxchg.org, quic_pkondeti@quicinc.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH V3 1/2] mm: page_alloc: correct high atomic reserve
 calculations
In-Reply-To: <1660034138397b82a0a8b6ae51cbe96bd583d89e.1700821416.git.quic_charante@quicinc.com>
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References: <cover.1700821416.git.quic_charante@quicinc.com> <1660034138397b82a0a8b6ae51cbe96bd583d89e.1700821416.git.quic_charante@quicinc.com>
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On Fri, 24 Nov 2023, Charan Teja Kalla wrote:

> reserve_highatomic_pageblock() aims to reserve the 1% of the managed
> pages of a zone, which is used for the high order atomic allocations.
> 
> It uses the below calculation to reserve:
> static void reserve_highatomic_pageblock(struct page *page, ....) {
> 
>    .......
>    max_managed = (zone_managed_pages(zone) / 100) + pageblock_nr_pages;
> 
>    if (zone->nr_reserved_highatomic >= max_managed)
>        goto out;
> 
>    zone->nr_reserved_highatomic += pageblock_nr_pages;
>    set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
>    move_freepages_block(zone, page, MIGRATE_HIGHATOMIC, NULL);
> 
> out:
>    ....
> }
> 
> Since we are always appending the 1% of zone managed pages count to
> pageblock_nr_pages, the minimum it is turning into 2 pageblocks as the
> nr_reserved_highatomic is incremented/decremented in pageblock sizes.
> 
> Encountered a system(actually a VM running on the Linux kernel) with the
> below zone configuration:
> Normal free:7728kB boost:0kB min:804kB low:1004kB high:1204kB
> reserved_highatomic:8192KB managed:49224kB
> 
> The existing calculations making it to reserve the 8MB(with pageblock
> size of 4MB) i.e. 16% of the zone managed memory.  Reserving such high
> amount of memory can easily exert memory pressure in the system thus may
> lead into unnecessary reclaims till unreserving of high atomic reserves.
> 
> Since high atomic reserves are managed in pageblock size granules, as
> MIGRATE_HIGHATOMIC is set for such pageblock, fix the calculations for
> high atomic reserves as,  minimum is pageblock size , maximum is
> approximately 1% of the zone managed pages.
> 
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>

Acked-by: David Rientjes <rientjes@google.com>