sched: prefer wakers
Peter Zijlstra [Wed, 14 Jan 2009 11:39:19 +0000 (12:39 +0100)]
Prefer tasks that wake other tasks to preempt quickly. This improves
performance because more work is available sooner.

The workload that prompted this patch was a kernel build over NFS4 (for some
curious and not understood reason we had to revert commit:
18de9735300756e3ca9c361ef58409d8561dfe0d to make any progress at all)

Without this patch a make -j8 bzImage (of x86-64 defconfig) would take
3m30-ish, with this patch we're down to 2m50-ish.

psql-sysbench/mysql-sysbench show a slight improvement in peak performance as
well, tbench and vmark seemed to not care.

It is possible to improve upon the build time (to 2m20-ish) but that seriously
destroys other benchmarks (just shows that there's more room for tinkering).

Much thanks to Mike who put in a lot of effort to benchmark things and proved
a worthy opponent with a competing patch.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c
kernel/sched_features.h

index 8e1352c..bdf6434 100644 (file)
@@ -1295,16 +1295,63 @@ out:
 }
 #endif /* CONFIG_SMP */
 
-static unsigned long wakeup_gran(struct sched_entity *se)
+/*
+ * Adaptive granularity
+ *
+ * se->avg_wakeup gives the average time a task runs until it does a wakeup,
+ * with the limit of wakeup_gran -- when it never does a wakeup.
+ *
+ * So the smaller avg_wakeup is the faster we want this task to preempt,
+ * but we don't want to treat the preemptee unfairly and therefore allow it
+ * to run for at least the amount of time we'd like to run.
+ *
+ * NOTE: we use 2*avg_wakeup to increase the probability of actually doing one
+ *
+ * NOTE: we use *nr_running to scale with load, this nicely matches the
+ *       degrading latency on load.
+ */
+static unsigned long
+adaptive_gran(struct sched_entity *curr, struct sched_entity *se)
+{
+       u64 this_run = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
+       u64 expected_wakeup = 2*se->avg_wakeup * cfs_rq_of(se)->nr_running;
+       u64 gran = 0;
+
+       if (this_run < expected_wakeup)
+               gran = expected_wakeup - this_run;
+
+       return min_t(s64, gran, sysctl_sched_wakeup_granularity);
+}
+
+static unsigned long
+wakeup_gran(struct sched_entity *curr, struct sched_entity *se)
 {
        unsigned long gran = sysctl_sched_wakeup_granularity;
 
+       if (cfs_rq_of(curr)->curr && sched_feat(ADAPTIVE_GRAN))
+               gran = adaptive_gran(curr, se);
+
        /*
-        * More easily preempt - nice tasks, while not making it harder for
-        * + nice tasks.
+        * Since its curr running now, convert the gran from real-time
+        * to virtual-time in his units.
         */
-       if (!sched_feat(ASYM_GRAN) || se->load.weight > NICE_0_LOAD)
-               gran = calc_delta_fair(sysctl_sched_wakeup_granularity, se);
+       if (sched_feat(ASYM_GRAN)) {
+               /*
+                * By using 'se' instead of 'curr' we penalize light tasks, so
+                * they get preempted easier. That is, if 'se' < 'curr' then
+                * the resulting gran will be larger, therefore penalizing the
+                * lighter, if otoh 'se' > 'curr' then the resulting gran will
+                * be smaller, again penalizing the lighter task.
+                *
+                * This is especially important for buddies when the leftmost
+                * task is higher priority than the buddy.
+                */
+               if (unlikely(se->load.weight != NICE_0_LOAD))
+                       gran = calc_delta_fair(gran, se);
+       } else {
+               if (unlikely(curr->load.weight != NICE_0_LOAD))
+                       gran = calc_delta_fair(gran, curr);
+       }
 
        return gran;
 }
@@ -1331,7 +1378,7 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
        if (vdiff <= 0)
                return -1;
 
-       gran = wakeup_gran(curr);
+       gran = wakeup_gran(curr, se);
        if (vdiff > gran)
                return 1;
 
index da5d93b..76f6175 100644 (file)
@@ -1,5 +1,6 @@
 SCHED_FEAT(NEW_FAIR_SLEEPERS, 1)
-SCHED_FEAT(NORMALIZED_SLEEPER, 1)
+SCHED_FEAT(NORMALIZED_SLEEPER, 0)
+SCHED_FEAT(ADAPTIVE_GRAN, 1)
 SCHED_FEAT(WAKEUP_PREEMPT, 1)
 SCHED_FEAT(START_DEBIT, 1)
 SCHED_FEAT(AFFINE_WAKEUPS, 1)