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    SubjectRe: [PATCH v6 03/16] sched/core: uclamp: Map TASK's clamp values into CPU's clamp buckets
    On 21-Jan 11:15, Peter Zijlstra wrote:
    > On Tue, Jan 15, 2019 at 10:15:00AM +0000, Patrick Bellasi wrote:
    > > +/*
    > > + * Number of utilization clamp buckets.
    > > + *
    > > + * The first clamp bucket (bucket_id=0) is used to track non clamped tasks, i.e.
    > > + * util_{min,max} (0,SCHED_CAPACITY_SCALE). Thus we allocate one more bucket in
    > > + * addition to the compile time configured number.
    > > + */
    > > +#define UCLAMP_BUCKETS (CONFIG_UCLAMP_BUCKETS_COUNT + 1)
    > > +
    > > +/*
    > > + * Utilization clamp bucket
    > > + * @value: clamp value tracked by a clamp bucket
    > > + * @bucket_id: the bucket index used by the fast-path
    > > + * @mapped: the bucket index is valid
    > > + *
    > > + * A utilization clamp bucket maps a:
    > > + * clamp value (value), i.e.
    > > + * util_{min,max} value requested from userspace
    > > + * to a:
    > > + * clamp bucket index (bucket_id), i.e.
    > > + * index of the per-cpu RUNNABLE tasks refcounting array
    > > + *
    > > + * The mapped bit is set whenever a task has been mapped on a clamp bucket for
    > > + * the first time. When this bit is set, any:
    > > + * uclamp_bucket_inc() - for a new clamp value
    > > + * is matched by a:
    > > + * uclamp_bucket_dec() - for the old clamp value
    > > + */
    > > +struct uclamp_se {
    > > + unsigned int value : bits_per(SCHED_CAPACITY_SCALE);
    > > + unsigned int bucket_id : bits_per(UCLAMP_BUCKETS);
    > > + unsigned int mapped : 1;
    > > +};
    >
    > Do we want something like:
    >
    > BUILD_BUG_ON(sizeof(struct uclamp_se) == sizeof(unsigned int));

    Mmm... isn't "!=" what you mean ?

    We cannot use less then an unsigned int for the fields above... am I
    missing something?

    > And/or put a limit on CONFIG_UCLAMP_BUCKETS_COUNT that guarantees that ?

    The number of buckets is currently KConfig limited to a max of 20, which gives:

    UCLAMP_BUCKETS: 21 => 5bits

    Thus, even on 32 bit targets and assuming 21bits for an "extended"
    SCHED_CAPACITY_SCALE range we should always fit into an unsigned int
    and have at least 6 bits for flags.

    Are you afraid of some compiler magic related to bitfields packing ?

    --
    #include <best/regards.h>

    Patrick Bellasi

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