netdev - Re: Re: [PATCH net-next v5 3/3] sock: Fix misuse of sk_under_memory

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Date: Mon, 5 Jun 2023 16:34:21 +0800
From: Abel Wu <wuyun.abel@...edance.com>
To: Shakeel Butt <shakeelb@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Hocko <mhocko@...nel.org>, Vladimir Davydov <vdavydov.dev@...il.com>,
 Muchun Song <muchun.song@...ux.dev>, Simon Horman
 <simon.horman@...igine.com>, netdev@...r.kernel.org, linux-mm@...ck.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH net-next v5 3/3] sock: Fix misuse of
 sk_under_memory_pressure()

On 6/3/23 4:53 AM, Shakeel Butt wrote:
> On Fri, Jun 02, 2023 at 04:11:35PM +0800, Abel Wu wrote:
>> The status of global socket memory pressure is updated when:
>>
>>    a) __sk_mem_raise_allocated():
>>
>> 	enter: sk_memory_allocated(sk) >  sysctl_mem[1]
>> 	leave: sk_memory_allocated(sk) <= sysctl_mem[0]
>>
>>    b) __sk_mem_reduce_allocated():
>>
>> 	leave: sk_under_memory_pressure(sk) &&
>> 		sk_memory_allocated(sk) < sysctl_mem[0]
> 
> There is also sk_page_frag_refill() where we can enter the global
> protocol memory pressure on actual global memory pressure i.e. page
> allocation failed. However this might be irrelevant from this patch's
> perspective as the focus is on the leaving part.

Leaving prot pressure or not under actual global vm pressure is
something similar to what you concerned last time (prot & memcg
is now intermingled), as this will mix prot & global together.

To decouple global info from prot level pressure, a new variable
might be needed. But I doubt the necessity as this seems to be a
rare case but a constant overhead on net core path to check the
global status (although can be relieved by static key).

And after a second thought, failing in skb_page_frag_refill()
doesn't necessarily mean there is global memory pressure since
it can due to the mpol/memset of the current task.

> 
>>
>> So the conditions of leaving global pressure are inconstant, which
> 
> *inconsistent

Will fix in next version.

> 
>> may lead to the situation that one pressured net-memcg prevents the
>> global pressure from being cleared when there is indeed no global
>> pressure, thus the global constrains are still in effect unexpectedly
>> on the other sockets.
>>
>> This patch fixes this by ignoring the net-memcg's pressure when
>> deciding whether should leave global memory pressure.
>>
>> Fixes: e1aab161e013 ("socket: initial cgroup code.")
>> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
> 
> This patch looks good.

Thanks!
	Abel