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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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