Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | [PATCH v2 8/9] sched/fair: Split select_task_rq_fair want_affine logic | Date | Wed, 11 Mar 2020 18:16:00 +0000 |
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The domain loop within select_task_rq_fair() depends on a few bits of input, namely the SD flag we're looking for and whether we want_affine.
For !want_affine, the domain loop will walk up the hierarchy to reach the highest domain with the requested sd_flag (SD_BALANCE_{WAKE, FORK, EXEC}) set. In other words, that's a call to highest_flag_domain(). Note that this is a static information wrt a given SD hierarchy, so we can cache that - but that comes in a later patch to ease reviewing.
For want_affine, we'll walk up the hierarchy to reach the first domain with SD_LOAD_BALANCE, SD_WAKE_AFFINE, and that spans the tasks's prev_cpu. We still save a pointer to the last visited domain that had the requested sd_flag set, which means that if we fail to go through the affine condition (e.g. no domain had SD_WAKE_AFFINE) we'll use the same SD as we would have found if we had !want_affine.
Split the domain loop in !want_affine and want_affine paths. As it is, this leads to two domain walks instead of a single one, but stay tuned for the next patch.
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> --- kernel/sched/fair.c | 29 ++++++++++++++++++----------- 1 file changed, 18 insertions(+), 11 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index f98dac0c7f82..a6fca6817e92 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6620,26 +6620,33 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) } rcu_read_lock(); + + sd = highest_flag_domain(cpu, sd_flag); + + /* + * If !want_affine, we just look for the highest domain where + * sd_flag is set. + */ + if (!want_affine) + goto scan; + + /* + * Otherwise we look for the lowest domain with SD_WAKE_AFFINE and that + * spans both 'cpu' and 'prev_cpu'. + */ for_each_domain(cpu, tmp) { - /* - * If both 'cpu' and 'prev_cpu' are part of this domain, - * cpu is a valid SD_WAKE_AFFINE target. - */ - if (want_affine && (tmp->flags & SD_WAKE_AFFINE) && + if ((tmp->flags & SD_WAKE_AFFINE) && cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) { if (cpu != prev_cpu) new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync); - sd = NULL; /* Prefer wake_affine over balance flags */ + /* Prefer wake_affine over SD lookup */ + sd = NULL; break; } - - if (tmp->flags & sd_flag) - sd = tmp; - else if (!want_affine) - break; } +scan: if (unlikely(sd)) { /* Slow path */ new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag); -- 2.24.0
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