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    SubjectRe: [PATCH] writeback: sum memcg dirty counters as needed
    On Fri, Mar 22, 2019 at 11:15 AM Roman Gushchin <guro@fb.com> wrote:
    >
    > On Thu, Mar 07, 2019 at 08:56:32AM -0800, Greg Thelen wrote:
    > > Since commit a983b5ebee57 ("mm: memcontrol: fix excessive complexity in
    > > memory.stat reporting") memcg dirty and writeback counters are managed
    > > as:
    > > 1) per-memcg per-cpu values in range of [-32..32]
    > > 2) per-memcg atomic counter
    > > When a per-cpu counter cannot fit in [-32..32] it's flushed to the
    > > atomic. Stat readers only check the atomic.
    > > Thus readers such as balance_dirty_pages() may see a nontrivial error
    > > margin: 32 pages per cpu.
    > > Assuming 100 cpus:
    > > 4k x86 page_size: 13 MiB error per memcg
    > > 64k ppc page_size: 200 MiB error per memcg
    > > Considering that dirty+writeback are used together for some decisions
    > > the errors double.
    > >
    > > This inaccuracy can lead to undeserved oom kills. One nasty case is
    > > when all per-cpu counters hold positive values offsetting an atomic
    > > negative value (i.e. per_cpu[*]=32, atomic=n_cpu*-32).
    > > balance_dirty_pages() only consults the atomic and does not consider
    > > throttling the next n_cpu*32 dirty pages. If the file_lru is in the
    > > 13..200 MiB range then there's absolutely no dirty throttling, which
    > > burdens vmscan with only dirty+writeback pages thus resorting to oom
    > > kill.
    > >
    > > It could be argued that tiny containers are not supported, but it's more
    > > subtle. It's the amount the space available for file lru that matters.
    > > If a container has memory.max-200MiB of non reclaimable memory, then it
    > > will also suffer such oom kills on a 100 cpu machine.
    > >
    > > The following test reliably ooms without this patch. This patch avoids
    > > oom kills.
    > >
    > > ...
    > >
    > > Make balance_dirty_pages() and wb_over_bg_thresh() work harder to
    > > collect exact per memcg counters when a memcg is close to the
    > > throttling/writeback threshold. This avoids the aforementioned oom
    > > kills.
    > >
    > > This does not affect the overhead of memory.stat, which still reads the
    > > single atomic counter.
    > >
    > > Why not use percpu_counter? memcg already handles cpus going offline,
    > > so no need for that overhead from percpu_counter. And the
    > > percpu_counter spinlocks are more heavyweight than is required.
    > >
    > > It probably also makes sense to include exact dirty and writeback
    > > counters in memcg oom reports. But that is saved for later.
    > >
    > > Signed-off-by: Greg Thelen <gthelen@google.com>
    > > ---
    > > include/linux/memcontrol.h | 33 +++++++++++++++++++++++++--------
    > > mm/memcontrol.c | 26 ++++++++++++++++++++------
    > > mm/page-writeback.c | 27 +++++++++++++++++++++------
    > > 3 files changed, 66 insertions(+), 20 deletions(-)
    > >
    > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
    > > index 83ae11cbd12c..6a133c90138c 100644
    > > --- a/include/linux/memcontrol.h
    > > +++ b/include/linux/memcontrol.h
    > > @@ -573,6 +573,22 @@ static inline unsigned long memcg_page_state(struct mem_cgroup *memcg,
    > > return x;
    > > }
    >
    > Hi Greg!
    >
    > Thank you for the patch, definitely a good problem to be fixed!
    >
    > >
    > > +/* idx can be of type enum memcg_stat_item or node_stat_item */
    > > +static inline unsigned long
    > > +memcg_exact_page_state(struct mem_cgroup *memcg, int idx)
    > > +{
    > > + long x = atomic_long_read(&memcg->stat[idx]);
    > > +#ifdef CONFIG_SMP
    >
    > I doubt that this #ifdef is correct without corresponding changes
    > in __mod_memcg_state(). As now, we do use per-cpu buffer which spills
    > to an atomic value event if !CONFIG_SMP. It's probably something
    > that we want to change, but as now, #ifdef CONFIG_SMP should protect
    > only "if (x < 0)" part.

    Ack. I'll fix it.

    > > + int cpu;
    > > +
    > > + for_each_online_cpu(cpu)
    > > + x += per_cpu_ptr(memcg->stat_cpu, cpu)->count[idx];
    > > + if (x < 0)
    > > + x = 0;
    > > +#endif
    > > + return x;
    > > +}
    >
    > Also, isn't it worth it to generalize memcg_page_state() instead?
    > By adding an bool exact argument? I believe dirty balance is not
    > the only place, where we need a better accuracy.

    Nod. I'll provide a more general version of memcg_page_state(). I'm
    testing updated (forthcoming v2) patch set now with feedback from
    Andrew and Roman.

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