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    SubjectRe: [RFC 1/3] /dev/low_mem_notify
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    On Wed, Jan 18, 2012 at 11:06 AM,  <leonid.moiseichuk@nokia.com> wrote:
    > Would be possible to not use percents for thesholds? Accounting in pages even
    > not so difficult to user-space.

    How does that work with memory hotplug?

    On Wed, Jan 18, 2012 at 11:06 AM, <leonid.moiseichuk@nokia.com> wrote:
    > Also, looking on vmnotify_match I understand that events propagated to
    > user-space only in case threshold trigger change state from 0 to 1 but not
    > back, 1-> 0 is very useful event as well.
    >
    > Would be possible to use for threshold pointed value(s) e.g. according to
    > enum zone_state_item, because kinds of memory to track could be different?
    > E.g. to tracking paging activity NR_ACTIVE_ANON and NR_ACTIVE_FILE could be
    > interesting, not only free.

    I don't think there's anything in the ABI that would prevent that.

    >> +struct vmnotify_event {
    >> + /* Size of the struct for ABI extensibility. */
    >> + __u32 size;
    >> +
    >> + __u64 nr_avail_pages;
    >> +
    >> + __u64 nr_swap_pages;
    >> +
    >> + __u64 nr_free_pages;
    >> +};
    >
    > Two fields here most likely session-constant, (nr_avail_pages and
    > nr_swap_pages), seems not much sense to report them in every event. If we
    > have memory/swap hotplug user-space can use sysinfo() call.

    I actually changed the ABI to look like this:

    struct vmnotify_event {
    /*
    * Size of the struct for ABI extensibility.
    */
    __u32 size;

    __u64 attrs;

    __u64 attr_values[];
    };

    So userspace can decide which fields to include in notifications.

    On Wed, Jan 18, 2012 at 11:06 AM, <leonid.moiseichuk@nokia.com> wrote:
    >> +static void vmnotify_sample(struct vmnotify_watch *watch) {
    > ...
    >> + si_meminfo(&si);
    >> + event.nr_avail_pages = si.totalram;
    >> +
    >> +#ifdef CONFIG_SWAP
    >> + si_swapinfo(&si);
    >> + event.nr_swap_pages = si.totalswap;
    >> +#endif
    >> +
    >
    > Why not to use global_page_state() directly? si_meminfo() and especial
    > si_swapinfo are quite expensive call.

    Sure, we can do that. Feel free to send a patch :-).

    >> +static void vmnotify_start_timer(struct vmnotify_watch *watch) {
    >> + u64 sample_period = watch->config.sample_period_ns;
    >> +
    >> + hrtimer_init(&watch->timer, CLOCK_MONOTONIC,
    >> HRTIMER_MODE_REL);
    >> + watch->timer.function = vmnotify_timer_fn;
    >> +
    >> + hrtimer_start(&watch->timer, ns_to_ktime(sample_period),
    >> +HRTIMER_MODE_REL_PINNED); }
    >
    > Do I understand correct you allocate timer for every user-space client and
    > propagate events every pointed interval? What will happened with system if
    > we have a timer but need to turn CPU off? The timer must not be a reason to
    > wakeup if user-space is sleeping.

    No idea what happens. The sampling code is just a proof of concept thing and I
    expect it to be buggy as hell. :-)

    Pekka


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