linux-kernel - Re: [PATCH 2/2] free_pcppages_bulk: prefetch buddy while not holding lock

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Message-ID: <148a42d8-8306-2f2f-7f7c-86bc118f8ccd@intel.com>
Date:   Wed, 24 Jan 2018 08:57:43 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Aaron Lu <aaron.lu@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 2/2] free_pcppages_bulk: prefetch buddy while not holding
 lock

On 01/24/2018 08:43 AM, Mel Gorman wrote:
> I'm less convinced by this for a microbenchmark. Prefetch has not been a
> universal win in the past and we cannot be sure that it's a good idea on
> all architectures or doesn't have other side-effects such as consuming
> memory bandwidth for data we don't need or evicting cache hot data for
> buddy information that is not used.

I had the same reaction.

But, I think this case is special.  We *always* do buddy merging (well,
before the next patch in the series is applied) and check an order-0
page's buddy to try to merge it when it goes into the main allocator.
So, the cacheline will always come in.

IOW, I don't think this has the same downsides normally associated with
prefetch() since the data is always used.