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SubjectRe: [PATCH v2 2/4] sched/fair: add util_est on top of PELT
On Wed, Dec 13, 2017 at 04:36:53PM +0000, Patrick Bellasi wrote:
> On 13-Dec 17:19, Peter Zijlstra wrote:
> > On Tue, Dec 05, 2017 at 05:10:16PM +0000, Patrick Bellasi wrote:
> > > @@ -562,6 +577,12 @@ struct task_struct {
> > >
> > > const struct sched_class *sched_class;
> > > struct sched_entity se;
> > > + /*
> > > + * Since we use se.avg.util_avg to update util_est fields,
> > > + * this last can benefit from being close to se which
> > > + * also defines se.avg as cache aligned.
> > > + */
> > > + struct util_est util_est;

The thing is, since sched_entity has a member with cacheline alignment,
the whole structure must have cacheline alignment, and this util_est
_will_ start on a new line.

See also:

$ pahole -EC task_struct defconfig/kernel/sched/core.o

...
struct sched_avg {
/* typedef u64 */ long long unsigned int last_update_time; /* 576 8 */
/* typedef u64 */ long long unsigned int load_sum; /* 584 8 */
/* typedef u32 */ unsigned int util_sum; /* 592 4 */
/* typedef u32 */ unsigned int period_contrib; /* 596 4 */
long unsigned int load_avg; /* 600 8 */
long unsigned int util_avg; /* 608 8 */
} avg; /* 576 40 */
/* --- cacheline 6 boundary (384 bytes) --- */
} se; /* 192 448 */
/* --- cacheline 8 boundary (512 bytes) was 24 bytes ago --- */
struct util_est {
long unsigned int last; /* 640 8 */
long unsigned int ewma; /* 648 8 */
} util_est; /* 640 16 */
...
The thing is somewhat confused on which cacheline is which, but you'll
see sched_avg landing at 576 (cacheline #9) and util_est at 640 (line
#10).

> > > struct sched_rt_entity rt;
> > > #ifdef CONFIG_CGROUP_SCHED
> > > struct task_group *sched_task_group;

> One goal was to keep util_est variables close to the util_avg used to
> load the filter, for caches affinity sakes.
>
> The other goal was to have util_est data only for Tasks and CPU's
> RQ, thus avoiding unused data for TG's RQ and SE.
>
> Unfortunately the first goal does not allow to achieve completely the
> second and, you right, the solution looks a bit inconsistent.
>
> Do you think we should better disregard cache proximity and move
> util_est_runnable to rq?

proximity is likely important; I'd suggest moving util_est into
sched_entity.

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