linux-kernel - Re: [RFC PATCH] mm: have kswapd only reclaiming use min protection on memcg

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Date:   Wed, 27 Oct 2021 13:52:11 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm: have kswapd only reclaiming use min protection
 on memcg

On Wed 27-10-21 17:19:56, Zhaoyang Huang wrote:
> On Wed, Oct 27, 2021 at 4:26 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Wed 27-10-21 15:46:19, Zhaoyang Huang wrote:
> > > On Wed, Oct 27, 2021 at 3:20 PM Michal Hocko <mhocko@...e.com> wrote:
> > > >
> > > > On Wed 27-10-21 15:01:50, Huangzhaoyang wrote:
> > > > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > > >
> > > > > For the kswapd only reclaiming, there is no chance to try again on
> > > > > this group while direct reclaim has. fix it by judging gfp flag.
> > > >
> > > > There is no problem description (same as in your last submissions. Have
> > > > you looked at the patch submission documentation as recommended
> > > > previously?).
> > > >
> > > > Also this patch doesn't make any sense. Both direct reclaim and kswapd
> > > > use a gfp mask which contains __GFP_DIRECT_RECLAIM (see balance_pgdat
> > > > for the kswapd part)..
> > > ok, but how does the reclaiming try with memcg's min protection on the
> > > alloc without __GFP_DIRECT_RECLAIM?
> >
> > I do not follow. There is no need to protect memcg if the allocation
> > request doesn't have __GFP_DIRECT_RECLAIM because that would fail the
> > charge if a hard limit is reached, see try_charge_memcg and
> > gfpflags_allow_blocking check.
> >
> > Background reclaim, on the other hand never breaches reclaim protection.
> >
> > What is the actual problem you want to solve?
> Imagine there is an allocation with gfp_mask & ~GFP_DIRECT_RECLAIM and
> all processes are under cgroups. Kswapd is the only hope here which
> however has a low efficiency of get_scan_count. I would like to have
> kswapd work as direct reclaim in 2nd round which will have
> protection=memory.min.

Do you have an example where this would be a practical problem? Atomic
allocations should be rather rare.

-- 
Michal Hocko
SUSE Labs