linux-kernel - Re: Low TCP throughput due to vmpressure with swap enabled

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Date:   Thu, 8 Dec 2022 00:31:36 +0000
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Ivan Babrou <ivan@...udflare.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, cgroups@...r.kernel.org,
        kernel-team <kernel-team@...udflare.com>
Subject: Re: Low TCP throughput due to vmpressure with swap enabled

On Wed, Dec 07, 2022 at 01:53:00PM +0100, Johannes Weiner wrote:
[...]
> 
> I don't mind doing that if necessary, but looking at the code I don't
> see why it would be.
> 
> The socket code sets protocol memory pressure on allocations that run
> into limits, and clears pressure on allocations that succeed and
> frees. Why shouldn't we do the same thing for memcg?
> 

I think you are right. Let's go with whatever you have for now as this
will reduce vmpressure dependency.

However I think there are still open issues that needs to be addressed
in the future:

1. Unlike TCP memory accounting, memcg has to account/charge user
memory, kernel memory and tcp/netmem. So, it might make more sense to
enter the pressure state in try_charge_memcg() function. This means
charging of user memory or kernel memory can also put the memcg under
socket pressure.

2. On RX path, the memcg charge can succeed due to GFP_ATOMIC flag.
Should we reset the pressure state in that case?

3. On uncharge path, unlike network stack, should we unconditionally
reset the socket pressure state?

Shakeel