Re: [PATCH] memcg: introduce per-memcg reclaim interface

[Shakeel Butt]

On Wed, Sep 9, 2020 at 11:37 PM SeongJae Park <sjpark@xxxxxxxxxx> wrote:
>
> On 2020-09-09T14:57:52-07:00 Shakeel Butt <shakeelb@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >  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,
> SeongJae Park