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    SubjectRe: [PATCH] mm:memcg: add __GFP_NOWARN in __memcg_schedule_kmem_cache_create
    On Thu, Apr 19, 2018 at 08:40:05AM +0200, Michal Hocko wrote:
    > On Wed 18-04-18 11:58:00, David Rientjes wrote:
    > > On Wed, 18 Apr 2018, Michal Hocko wrote:
    > >
    > > > > Okay, no problem. However, I don't feel we need ratelimit at this moment.
    > > > > We can do when we got real report. Let's add just one line warning.
    > > > > However, I have no talent to write a poem to express with one line.
    > > > > Could you help me?
    > > >
    > > > What about
    > > > pr_info("Failed to create memcg slab cache. Report if you see floods of these\n");
    > > >

    Thanks you, Michal. However, hmm, floods is very vague to me. 100 time per sec?
    10 time per hour? I guess we need more guide line to trigger user's reporting
    if we really want to do.


    > >
    > > Um, there's nothing actionable here for the user. Even if the message
    > > directed them to a specific email address, what would you ask the user for
    > > in response if they show a kernel log with 100 of these?
    >
    > We would have to think of a better way to create shaddow memcg caches.
    >
    > > Probably ask
    > > them to use sysrq at the time it happens to get meminfo. But any user
    > > initiated sysrq is going to reveal very different state of memory compared
    > > to when the kmalloc() actually failed.
    >
    > Not really.
    >
    > > If this really needs a warning, I think it only needs to be done once and
    > > reveal the state of memory similar to how slub emits oom warnings. But as
    > > the changelog indicates, the system is oom and we couldn't reclaim. We
    > > can expect this happens a lot on systems with memory pressure. What is
    > > the warning revealing that would be actionable?
    >
    > That it actually happens in real workloads and we want to know what
    > those workloads are. This code is quite old and yet this is the first
    > some somebody complains. So it is most probably rare. Maybe because most
    > workloads doesn't create many memcgs dynamically while low on memory.
    > And maybe that will change in future. In any case, having a large splat
    > of meminfo for GFP_NOWAIT is not really helpful. It will tell us what we
    > know already - the memory is low and the reclaim was prohibited. We just
    > need to know that this happens out there.

    The workload was experimenting creating memcg per app on embedded device
    but at this moment, I don't consider kmemcg at this moment so I can live
    with disabling kmemcg, even. Based on it, I cannot say whether it's real
    workload or not.

    When I see replies of this thread, it's arguble to add such one-line
    warn so if you want it strongly, could you handle by yourself?
    Sorry but I don't have any interest on the arguing.

    Thanks.

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