linux-kernel - Re: [PATCH 3/8] net: consolidate memcg socket buffer tracking and accounting

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Message-ID: <20151022184612.GN18351@esperanza>
Date:	Thu, 22 Oct 2015 21:46:12 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, Tejun Heo <tj@...nel.org>,
	<netdev@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/8] net: consolidate memcg socket buffer tracking and
 accounting

On Thu, Oct 22, 2015 at 12:21:31AM -0400, Johannes Weiner wrote:
> The tcp memory controller has extensive provisions for future memory
> accounting interfaces that won't materialize after all. Cut the code
> base down to what's actually used, now and in the likely future.
> 
> - There won't be any different protocol counters in the future, so a
>   direct sock->sk_memcg linkage is enough. This eliminates a lot of
>   callback maze and boilerplate code, and restores most of the socket
>   allocation code to pre-tcp_memcontrol state.
> 
> - There won't be a tcp control soft limit, so integrating the memcg

In fact, the code is ready for the "soft" limit (I mean min, pressure,
max tuple), it just lacks a knob.

>   code into the global skmem limiting scheme complicates things
>   unnecessarily. Replace all that with simple and clear charge and
>   uncharge calls--hidden behind a jump label--to account skb memory.
> 
> - The previous jump label code was an elaborate state machine that
>   tracked the number of cgroups with an active socket limit in order
>   to enable the skmem tracking and accounting code only when actively
>   necessary. But this is overengineered: it was meant to protect the
>   people who never use this feature in the first place. Simply enable
>   the branches once when the first limit is set until the next reboot.
> 
...
> @@ -1136,9 +1090,6 @@ static inline bool sk_under_memory_pressure(const struct sock *sk)
>  	if (!sk->sk_prot->memory_pressure)
>  		return false;
>  
> -	if (mem_cgroup_sockets_enabled && sk->sk_cgrp)
> -		return !!sk->sk_cgrp->memory_pressure;
> -

AFAIU, now we won't shrink the window on hitting the limit, i.e. this
patch subtly changes the behavior of the existing knobs, potentially
breaking them.

Thanks,
Vladimir
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