Messages in this thread | | | Date | Mon, 21 Jan 2019 12:27:10 +0000 | From | Patrick Bellasi <> | Subject | Re: [PATCH v6 03/16] sched/core: uclamp: Map TASK's clamp values into CPU's clamp buckets |
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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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