Jason Low <jason.l...@hp.com> writes: > When running workloads on 2+ socket systems, based on perf profiles, the > update_cfs_rq_blocked_load function constantly shows up as taking up a > noticeable % of run time. This is especially apparent on an 8 socket > machine. For example, when running the AIM7 custom workload, we see: > > 4.18% reaim [kernel.kallsyms] [k] update_cfs_rq_blocked_load > > Much of the contention is in __update_cfs_rq_tg_load_contrib when we > update the tg load contribution stats. However, it turns out that in many > cases, they don't need to be updated and "tg_contrib" is 0. > > This patch adds a check in __update_cfs_rq_tg_load_contrib to skip updating > tg load contribution stats when nothing needs to be updated. This reduces the > cacheline contention that would be unnecessary. In the above case, with the > patch, perf reports the total time spent in this function went down by more > than a factor of 3x: > > 1.18% reaim [kernel.kallsyms] [k] update_cfs_rq_blocked_load > > Signed-off-by: Jason Low <jason.l...@hp.com> Reviewed-by: Ben Segall <bseg...@google.com>
That said, it might be better to remove force_update for this function, or make it just reduce the minimum to /64 or something. If the test is easy to run it would be good to see what it's like just removing the force_update param for this function to see if it's worth worrying about or if the zero case catches ~all the perf gain. Paul, your thoughts? > --- > kernel/sched/fair.c | 3 +++ > 1 files changed, 3 insertions(+), 0 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index bfa3c86..8d4cc72 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -2377,6 +2377,9 @@ static inline void > __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq, > tg_contrib = cfs_rq->runnable_load_avg + cfs_rq->blocked_load_avg; > tg_contrib -= cfs_rq->tg_load_contrib; > > + if (!tg_contrib) > + return; > + > if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) { > atomic_long_add(tg_contrib, &tg->load_avg); > cfs_rq->tg_load_contrib += tg_contrib; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/