linux-kernel - Re: __GFP_REPEAT usage in fq_alloc

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Date:   Fri, 6 Jan 2017 17:31:40 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Eric Dumazet <edumazet@...gle.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: __GFP_REPEAT usage in fq_alloc_node

On 01/06/2017 04:39 PM, Eric Dumazet wrote:
> On Fri, Jan 6, 2017 at 7:20 AM, Michal Hocko <mhocko@...nel.org> wrote:
>>
>> Hi Eric,
>> I am currently checking kmalloc with vmalloc fallback users and convert
>> them to a new kvmalloc helper [1]. While I am adding a support for
>> __GFP_REPEAT to kvmalloc [2] I was wondering what is the reason to use
>> __GFP_REPEAT in fq_alloc_node in the first place. c3bd85495aef
>> ("pkt_sched: fq: more robust memory allocation") doesn't mention
>> anything. Could you clarify this please?
>>
>> Thanks!
> 
> I guess this question applies to all __GFP_REPEAT usages in net/ ?
> 
> At the time, tests on the hardware I had in my labs showed that
> vmalloc() could deliver pages spread
> all over the memory and that was a small penalty (once memory is
> fragmented enough, not at boot time)

I wonder what's that cause of the penalty (when accessing the vmapped
area I suppose?) Is it higher risk of collisions cache misses within the
area, compared to consecutive physical adresses?

> I guess this wont be anymore a concern if I can finish my pending work
> about vmalloc() trying to get adjacent pages
> https://lkml.org/lkml/2016/12/21/285
> 
> Thanks.
> 
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