linux-kernel - Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty

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Message-ID: <20190103200111.GD31793@dhcp22.suse.cz>
Date:   Thu, 3 Jan 2019 21:01:11 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     hannes@...xchg.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty

On Thu 03-01-19 11:49:32, Yang Shi wrote:
> 
> 
> On 1/3/19 11:23 AM, Michal Hocko wrote:
> > On Thu 03-01-19 11:10:00, Yang Shi wrote:
> > > 
> > > On 1/3/19 10:53 AM, Michal Hocko wrote:
> > > > On Thu 03-01-19 10:40:54, Yang Shi wrote:
> > > > > On 1/3/19 10:13 AM, Michal Hocko wrote:
> > [...]
> > > > > > Is there any reason for your scripts to be strictly sequential here? In
> > > > > > other words why cannot you offload those expensive operations to a
> > > > > > detached context in _userspace_?
> > > > > I would say it has not to be strictly sequential. The above script is just
> > > > > an example to illustrate the pattern. But, sometimes it may hit such pattern
> > > > > due to the complicated cluster scheduling and container scheduling in the
> > > > > production environment, for example the creation process might be scheduled
> > > > > to the same CPU which is doing force_empty. I have to say I don't know too
> > > > > much about the internals of the container scheduling.
> > > > In that case I do not see a strong reason to implement the offloding
> > > > into the kernel. It is an additional code and semantic to maintain.
> > > Yes, it does introduce some additional code and semantic, but IMHO, it is
> > > quite simple and very straight forward, isn't it? Just utilize the existing
> > > css offline worker. And, that a couple of lines of code do improve some
> > > throughput issues for some real usecases.
> > I do not really care it is few LOC. It is more important that it is
> > conflating force_empty into offlining logic. There was a good reason to
> > remove reparenting/emptying the memcg during the offline. Considering
> > that you can offload force_empty from userspace trivially then I do not
> > see any reason to implement it in the kernel.
> 
> Er, I may not articulate in the earlier email, force_empty can not be
> offloaded from userspace *trivially*. IOWs the container scheduler may
> unexpectedly overcommit something due to the stall of synchronous force
> empty, which can't be figured out by userspace before it actually happens.
> The scheduler doesn't know how long force_empty would take. If the
> force_empty could be offloaded by kernel, it would make scheduler's life
> much easier. This is not something userspace could do.

What exactly prevents
(
echo 1 > $memecg/force_empty
rmdir $memcg
) &

so that this sequence doesn't really block anything?
-- 
Michal Hocko
SUSE Labs