linux-kernel - Re: LPC Traffic Shaping w/ BPF Talk

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Date:   Wed, 21 Nov 2018 20:01:04 -0800
From:   Vlad Dumitrescu <vladum@...gle.com>
To:     dennis@...nel.org
Cc:     Eddie Hao <eddieh@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>, ast@...nel.org,
        tj@...nel.org, cl@...ux.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: LPC Traffic Shaping w/ BPF Talk - percpu followup

On Tue, Nov 20, 2018 at 2:17 PM Dennis Zhou <dennis@...nel.org> wrote:
>
> Hi Eddie, Vlad, and Willem,
>
> A few people mentioned to me that you guys were experiencing issues with
> the percpu memory allocator. I saw the talk slides mention the
> following two bullets:
>
> 1) allocation pattern makes the per cpu allocator reach a highly
>    fragmented state
> 2) sometimes takes a long time (up to 12s) to create the PERCPU_HASH
>    maps at startup
>
> Could you guys elaborate a little more about the above? Some things
> that would help: kernel version, cpu info, and a reproducer if possible?
>
> Also, I did some work last summer to make percpu allocation more
> efficient, which went into the 4.14 kernel. Just to be sure, is that a
> part of the kernel you guys are running?
>
> Thanks,
> Dennis

Hi, Dennis,

Thanks a lot for reaching out and sorry for the delay in answering. I
was trying to build something which shows the problem on a recent
upstream kernel, but I was unable to do so until now.

It seems like I can still reliably reproduce on one of our kernels,
which has a lot of 'percpu: *' patches by you from 4.14, and with an
internal application. Unfortunately, we haven't spent too much time on
this issue, and I will have 'page in' state from a few months back.
Hopefully, I'll be able to reproduce on a vanilla kernel and a simpler
application, that we can publish, in the following days (after TG
weekend).

Thanks,
Vlad