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    Subject[PATCH] perf_events: add sampling period randomization support (v2)
    Date
    This patch adds support for randomizing the sampling period.  Randomization
    is very useful to mitigate the bias that exists with sampling. The random
    number generator does not need to be sophisticated. This patch uses the
    builtin random32() generator.

    Randomization is activated by setting perf_event_attr.random_period_width
    to a non-zero value. It represents the width of the mask to apply to the
    random value, i.e, maximum range of variation. The random value is applied
    AROUND the period, i.e., period may be longer or shorter with a maximum
    variation of half the bit width. Thus, on average the sampling period remains
    equal to the initial period passed in perf_event_attr.sample_period.

    Note that randomization is not available when a target interrupt rate
    (freq) is enabled.

    The last used period can be collected using the PERF_SAMPLE_PERIOD flag
    in sample_type.

    Randomization is implemented in generic code thus it applies to all
    PMU models and software events.

    Signed-off-by: Stephane Eranian <eranian@google.com>

    --
    include/linux/perf_event.h | 3 +++
    kernel/perf_event.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
    2 files changed, 47 insertions(+)

    diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    index 04f06b4..9848e08 100644
    --- a/include/linux/perf_event.h
    +++ b/include/linux/perf_event.h
    @@ -214,6 +214,8 @@ struct perf_event_attr {
    __u32 bp_type;
    __u64 bp_addr;
    __u64 bp_len;
    +
    + __u8 random_period_width;
    };

    /*
    @@ -877,6 +879,7 @@ extern int perf_swevent_get_recursion_context(void);
    extern void perf_swevent_put_recursion_context(int rctx);
    extern void perf_event_enable(struct perf_event *event);
    extern void perf_event_disable(struct perf_event *event);
    +extern void perf_randomize_event_period(struct perf_event *event);
    #else
    static inline void
    perf_event_task_sched_in(struct task_struct *task) { }
    diff --git a/kernel/perf_event.c b/kernel/perf_event.c
    index a661e79..c581a99 100644
    --- a/kernel/perf_event.c
    +++ b/kernel/perf_event.c
    @@ -30,6 +30,7 @@
    #include <linux/perf_event.h>
    #include <linux/ftrace_event.h>
    #include <linux/hw_breakpoint.h>
    +#include <linux/random.h>

    #include <asm/irq_regs.h>

    @@ -3866,6 +3867,9 @@ static int __perf_event_overflow(struct perf_event *event, int nmi,
    else
    perf_event_output(event, nmi, data, regs);

    + if (event->attr.random_period_width)
    + perf_randomize_event_period(event);
    +
    return ret;
    }

    @@ -3876,6 +3880,22 @@ int perf_event_overflow(struct perf_event *event, int nmi,
    return __perf_event_overflow(event, nmi, 1, data, regs);
    }

    +void perf_randomize_event_period(struct perf_event *event)
    +{
    + u64 mask = ((1ULL << event->attr.random_period_width) - 1);
    + u64 new_seed;
    +
    + new_seed = random32();
    +
    + if (unlikely(mask >> 32))
    + new_seed |= (u64)random32() << 32;
    +
    + event->hw.sample_period = event->attr.sample_period
    + + (new_seed & mask) - (mask >> 1);
    +
    + event->hw.last_period = event->hw.sample_period;
    +}
    +
    /*
    * Generic software event infrastructure
    */
    @@ -4665,6 +4685,28 @@ done:
    return event;
    }

    +static int perf_check_random_period(struct perf_event_attr *attr)
    +{
    + u8 width = attr->random_period_width;
    + u64 period = attr->sample_period;
    + u64 half;
    +
    + if (width > 63 || attr->freq)
    + return -EINVAL;
    +
    + half = (1ULL << (width - 1)) - 1;
    +
    + /* overflow */
    + if ((period + half) < period)
    + return -EINVAL;
    +
    + /* underflow and zero */
    + if (period <= half)
    + return -EINVAL;
    +
    + return 0;
    +}
    +
    static int perf_copy_attr(struct perf_event_attr __user *uattr,
    struct perf_event_attr *attr)
    {
    @@ -4736,6 +4778,8 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
    if (attr->read_format & ~(PERF_FORMAT_MAX-1))
    return -EINVAL;

    + if (attr->random_period_width)
    + ret = perf_check_random_period(attr);
    out:
    return ret;


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