linux-kernel - Re: [PATCH v2 0/3] Directed kmem charging

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Message-Id: <20180221125426.464f894d29a0b6e525b2e3be@linux-foundation.org>
Date:   Wed, 21 Feb 2018 12:54:26 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Christopher Lameter <cl@...ux.com>, Jan Kara <jack@...e.cz>,
        Amir Goldstein <amir73il@...il.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Greg Thelen <gthelen@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] Directed kmem charging

On Wed, 21 Feb 2018 09:18:35 -0800 Shakeel Butt <shakeelb@...gle.com> wrote:

> On Wed, Feb 21, 2018 at 8:09 AM, Christopher Lameter <cl@...ux.com> wrote:
> > Another way to solve this is to switch the user context right?
> >
> > Isnt it possible to avoid these patches if do the allocation in another
> > task context instead?
> >
> 
> Sorry, can you please explain what you mean by 'switch the user
> context'. Is there any example in kernel which does something similar?
> 
> Another way is by adding a field 'remote_memcg_to_charge' in
> task_struct and set it before the allocation and in memcontrol.c,
> first check if current->remote_memcg_to_charge is set otherwise use
> the memcg of current. Also if we provide a wrapper to do that for the
> user, there will be a lot less plumbing.
> 
> Please let me know if you prefer this approach.

That would be a lot simpler.  Passing function arguments via
task_struct is a bit dirty but is sometimes sooo effective.  You
should've seen how much mess task_struct.journal_info avoided!  And
reclaim_state.

And one always wonders whether we should do a local save/restore before
modifying the task_struct field, so it nests.

What do others think?


Maybe we can rename task_struct.reclaim_state to `struct task_mm_state
*task_mm_state", add remote_memcg_to_charge to struct task_mm_state and
avoid bloating the task_struct?