Date:   Thu, 10 Sep 2020 18:34:33 +0200
From:   SeongJae Park <sjpark@...zon.com>
To:     unlisted-recipients:; (no To-header on input)
CC:     SeongJae Park <sjpark@...zon.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...nel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Greg Thelen <gthelen@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Michal Koutný <mkoutny@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: introduce per-memcg reclaim interface

> On Wed, Sep 9, 2020 at 11:37 PM SeongJae Park <sjpark@...zon.com> wrote:
> >
> > On 2020-09-09T14:57:52-07:00 Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > > Introduce an memcg interface to trigger memory reclaim on a memory cgroup.
> > >
> > > Use cases:
> > > ----------
> > >
> > > 1) Per-memcg uswapd:
> > >
> > > Usually applications consists of combination of latency sensitive and
> > > latency tolerant tasks. For example, tasks serving user requests vs
> > > tasks doing data backup for a database application. At the moment the
> > > kernel does not differentiate between such tasks when the application
> > > hits the memcg limits. So, potentially a latency sensitive user facing
> > > task can get stuck in high reclaim and be throttled by the kernel.
> > >
> > > Similarly there are cases of single process applications having two set
> > > of thread pools where threads from one pool have high scheduling
> > > priority and low latency requirement. One concrete example from our
> > > production is the VMM which have high priority low latency thread pool
> > > for the VCPUs while separate thread pool for stats reporting, I/O
> > > emulation, health checks and other managerial operations. The kernel
> > > memory reclaim does not differentiate between VCPU thread or a
> > > non-latency sensitive thread and a VCPU thread can get stuck in high
> > > reclaim.
> > >
> > > One way to resolve this issue is to preemptively trigger the memory
> > > reclaim from a latency tolerant task (uswapd) when the application is
> > > near the limits. Finding 'near the limits' situation is an orthogonal
> > > problem.
> > >
> > > 2) Proactive reclaim:
> > >
> > > This is a similar to the previous use-case, the difference is instead of
> > > waiting for the application to be near its limit to trigger memory
> > > reclaim, continuously pressuring the memcg to reclaim a small amount of
> > > memory. This gives more accurate and uptodate workingset estimation as
> > > the LRUs are continuously sorted and can potentially provide more
> > > deterministic memory overcommit behavior. The memory overcommit
> > > controller can provide more proactive response to the changing behavior
> > > of the running applications instead of being reactive.
> > >
> > > Benefit of user space solution:
> > > -------------------------------
> > >
> > > 1) More flexible on who should be charged for the cpu of the memory
> > > reclaim. For proactive reclaim, it makes more sense to centralized the
> > > overhead while for uswapd, it makes more sense for the application to
> > > pay for the cpu of the memory reclaim.
> > >
> > > 2) More flexible on dedicating the resources (like cpu). The memory
> > > overcommit controller can balance the cost between the cpu usage and
> > > the memory reclaimed.
> > >
> > > 3) Provides a way to the applications to keep their LRUs sorted, so,
> > > under memory pressure better reclaim candidates are selected. This also
> > > gives more accurate and uptodate notion of working set for an
> > > application.
> > >
> > > Questions:
> > > ----------
> > >
> > > 1) Why memory.high is not enough?
> > >
> > > memory.high can be used to trigger reclaim in a memcg and can
> > > potentially be used for proactive reclaim as well as uswapd use cases.
> > > However there is a big negative in using memory.high. It can potentially
> > > introduce high reclaim stalls in the target application as the
> > > allocations from the processes or the threads of the application can hit
> > > the temporary memory.high limit.
> > >
> > > Another issue with memory.high is that it is not delegatable. To
> > > actually use this interface for uswapd, the application has to introduce
> > > another layer of cgroup on whose memory.high it has write access.
> > >
> > > 2) Why uswapd safe from self induced reclaim?
> > >
> > > This is very similar to the scenario of oomd under global memory
> > > pressure. We can use the similar mechanisms to protect uswapd from self
> > > induced reclaim i.e. memory.min and mlock.
> > >
> > > Interface options:
> > > ------------------
> > >
> > > Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
> > > trigger reclaim in the target memory cgroup.
> > >
> > > In future we might want to reclaim specific type of memory from a memcg,
> > > so, this interface can be extended to allow that. e.g.
> > >
> > > $ echo 10M [all|anon|file|kmem] > memory.reclaim
> > >
> > > However that should be when we have concrete use-cases for such
> > > functionality. Keep things simple for now.
> > >
> > > Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> > > ---
> > >  Documentation/admin-guide/cgroup-v2.rst |  9 ++++++
> > >  mm/memcontrol.c                         | 37 +++++++++++++++++++++++++
> > >  2 files changed, 46 insertions(+)
> > >
> > > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > > index 6be43781ec7f..58d70b5989d7 100644
> > > --- a/Documentation/admin-guide/cgroup-v2.rst
> > > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > > @@ -1181,6 +1181,15 @@ PAGE_SIZE multiple when read back.
> > >       high limit is used and monitored properly, this limit's
> > >       utility is limited to providing the final safety net.
> > >
> > > +  memory.reclaim
> > > +     A write-only file which exists on non-root cgroups.
> > > +
> > > +     This is a simple interface to trigger memory reclaim in the
> > > +     target cgroup. Write the number of bytes to reclaim to this
> > > +     file and the kernel will try to reclaim that much memory.
> > > +     Please note that the kernel can over or under reclaim from
> > > +     the target cgroup.
> > > +
> > >    memory.oom.group
> > >       A read-write single value file which exists on non-root
> > >       cgroups.  The default value is "0".
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 75cd1a1e66c8..2d006c36d7f3 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -6456,6 +6456,38 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
> > >       return nbytes;
> > >  }
> > >
> > > +static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
> > > +                           size_t nbytes, loff_t off)
> > > +{
> > > +     struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> > > +     unsigned int nr_retries = MAX_RECLAIM_RETRIES;
> > > +     unsigned long nr_to_reclaim, nr_reclaimed = 0;
> > > +     int err;
> > > +
> > > +     buf = strstrip(buf);
> > > +     err = page_counter_memparse(buf, "", &nr_to_reclaim);
> > > +     if (err)
> > > +             return err;
> > > +
> > > +     while (nr_reclaimed < nr_to_reclaim) {
> > > +             unsigned long reclaimed;
> > > +
> > > +             if (signal_pending(current))
> > > +                     break;
> > > +
> > > +             reclaimed = try_to_free_mem_cgroup_pages(memcg,
> > > +                                             nr_to_reclaim - nr_reclaimed,
> > > +                                             GFP_KERNEL, true);
> > > +
> > > +             if (!reclaimed && !nr_retries--)
> > > +                     break;
> >
> > Shouldn't the if condition use '||' instead of '&&'?
> 
> I copied the pattern from memory_high_write().
> 
> > I think it could be
> > easier to read if we put the 'nr_retires' condition in the while condition as
> > below (just my personal preference, though).
> >
> >     while (nr_reclaimed < nr_to_reclaim && nr_retires--)
> >
> 
> The semantics will be different. In my version, it means tolerate
> MAX_RECLAIM_RETRIES reclaim failures and your suggestion means total
> MAX_RECLAIM_RETRIES tries.
> 
> Please note that try_to_free_mem_cgroup_pages() internally does
> 'nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX)', so, we might need
> more than MAX_RECLAIM_RETRIES successful tries to actually reclaim the
> amount of memory the user has requested.

Thanks, understood your intention and agreed on the point.

Reviewed-by: SeongJae Park <sjpark@...zon.de>


Thanks,
SeongJae Park

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