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    SubjectRe: [PATCH 3/3] vmevent: Implement cross event type
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    On Mon, Apr 9, 2012 at 2:38 AM, Anton Vorontsov
    <anton.vorontsov@linaro.org> wrote:
    > This patch implements a new event type, it will trigger whenever a
    > value crosses a user-specified threshold. It works two-way, i.e. when
    > a value crosses the threshold from a lesser values side to a greater
    > values side, and vice versa.
    >
    > We use the event type in an userspace low-memory killer: we get a
    > notification when memory becomes low, so we start freeing memory by
    > killing unneeded processes, and we get notification when memory hits
    > the threshold from another side, so we know that we freed enough of
    > memory.
    >
    > Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
    > ---
    >  include/linux/vmevent.h              |    9 +++++++++
    >  mm/vmevent.c                         |   21 +++++++++++++++++++++
    >  tools/testing/vmevent/vmevent-test.c |   15 ++++++++++-----
    >  3 files changed, 40 insertions(+), 5 deletions(-)
    >
    > diff --git a/include/linux/vmevent.h b/include/linux/vmevent.h
    > index 64357e4..00cc04f 100644
    > --- a/include/linux/vmevent.h
    > +++ b/include/linux/vmevent.h
    > @@ -22,6 +22,15 @@ enum {
    >         * Sample value is less than user-specified value
    >         */
    >        VMEVENT_ATTR_STATE_VALUE_LT     = (1UL << 0),
    > +       /*
    > +        * Sample value crossed user-specified value
    > +        */
    > +       VMEVENT_ATTR_STATE_VALUE_CROSS  = (1UL << 2),
    > +
    > +       /* Last saved state, used internally by the kernel. */
    > +       __VMEVENT_ATTR_STATE_LAST       = (1UL << 30),
    > +       /* Not first sample, used internally by the kernel. */
    > +       __VMEVENT_ATTR_STATE_NFIRST     = (1UL << 31),
    >  };
    >
    >  struct vmevent_attr {
    > diff --git a/mm/vmevent.c b/mm/vmevent.c
    > index a56174f..f8fd2d6 100644
    > --- a/mm/vmevent.c
    > +++ b/mm/vmevent.c
    > @@ -1,5 +1,6 @@
    >  #include <linux/anon_inodes.h>
    >  #include <linux/atomic.h>
    > +#include <linux/compiler.h>
    >  #include <linux/vmevent.h>
    >  #include <linux/syscalls.h>
    >  #include <linux/timer.h>
    > @@ -94,6 +95,26 @@ static bool vmevent_match(struct vmevent_watch *watch)
    >                if (attr->state & VMEVENT_ATTR_STATE_VALUE_LT) {
    >                        if (value < attr->value)
    >                                return true;
    > +               } else if (attr->state & VMEVENT_ATTR_STATE_VALUE_CROSS) {
    > +                       bool fst = !(attr->state & __VMEVENT_ATTR_STATE_NFIRST);
    > +                       bool old = attr->state & __VMEVENT_ATTR_STATE_LAST;
    > +                       bool new = value < attr->value;
    > +                       bool chg = old ^ new;
    > +                       bool ret = chg;
    > +
    > +                       /*
    > +                        * This is not 'lt' or 'gt' match, so on the first
    > +                        * sample assume we crossed the threshold.
    > +                        */
    > +                       if (unlikely(fst)) {
    > +                               attr->state |= __VMEVENT_ATTR_STATE_NFIRST;
    > +                               ret = true;
    > +                       }
    > +
    > +                       attr->state &= ~__VMEVENT_ATTR_STATE_LAST;
    > +                       attr->state |= new ? __VMEVENT_ATTR_STATE_LAST : 0;
    > +
    > +                       return ret;
    >                }
    >        }
    >

    Can't we implement this by specifying both VMEVENT_ATTR_STATE_VALUE_LT
    and VMEVENT_ATTR_STATE_VALUE_GT in userspace? I assume the problem
    with current approach is that you get more than one notifications,
    right? We can implement a "single-shot" flag to deal with that.
    --
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