linux-kernel - Re: [PATCH v2 00/10] try to reduce fragmenting fallbacks

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Message-ID: <71f5642e-7efa-3d05-b012-defe9d339e0a@suse.cz>
Date:   Wed, 15 Feb 2017 21:11:57 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 00/10] try to reduce fragmenting fallbacks

On 15.2.2017 17:11, Vlastimil Babka wrote:
> On 02/15/2017 03:29 PM, Vlastimil Babka wrote:
>> Results for patch 4 ("count movable pages when stealing from pageblock")
>> are really puzzling me, as it increases the number of fragmenting events
>> for reclaimable allocations, implicating "reclaimable placed with (i.e.
>> falling back to) unmovable" (which is not listed separately above, but
>> follows logically from "reclaimable placed with movable" not changing
>> that much). I really wonder why is that. The patch effectively only
>> changes the decision to change migratetype of a pageblock, it doesn't
>> affect the actual stealing decision (which is always true for
>> RECLAIMABLE anyway, see can_steal_fallback()). Moreover, since we can't
>> distinguish UNMOVABLE from RECLAIMABLE when counting, good_pages is 0
>> and thus even the decision to change pageblock migratetype shouldn't be
>> changed by the patch for this case. I must recheck the implementation...
> 
> Ah, there it is... not enough LISP
> 
> -       if (pages >= (1 << (pageblock_order-1)) ||
> +       /* Claim the whole block if over half of it is free or good type */
> +       if (free_pages + good_pages >= (1 << (pageblock_order-1)) ||

Nope, I was blind and thought that this needs "(free_pages + good_pages)"
because of operator priority wrt shifting, but >= is not shift... bah.