Messages in this thread Patch in this message | | | From | Venkatesh Pallipadi <> | Subject | [PATCH] sched: Buggy comparison in check_preempt_tick | Date | Fri, 24 Dec 2010 16:26:53 -0800 |
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A preempt comparison line in check_preempt_tick has two bugs. * It compares signed and unsigned quantities, which breaks when signed quantity happens to be negative * It compares runtime and vruntime, which breaks when there are niced tasks
The bug was initially found by linsched[1]. Change here fixes both the problems.
On x86-64, the signed unsigned compare results in tasks running _longer_ than their expected time slice as a false resched_task() gets signalled after 4 ticks (on tick after preceding sysctl_sched_min_granularity check) and currently running task gets picked again and runs for another ideal_slice interval.
With 2 busy loops on a single CPU and trace_printks inside this buggy check triggering resched task and in pick_next_task shows this:
[001] 510.524336: pick_next_task_fair: loop (5939) [001] 510.536326: pick_next_task_fair: loop (5883) [001] 510.540319: task_tick_fair: delta -4897059, ideal_runtime 11994146 [001] 510.540321: pick_next_task_fair: loop (5883) [001] 510.544306: task_tick_fair: delta -906540, ideal_runtime 11994146 [001] 510.544309: pick_next_task_fair: loop (5883) [001] 510.556306: pick_next_task_fair: loop (5939) [001] 510.560301: task_tick_fair: delta -7105824, ideal_runtime 11994146 [001] 510.560304: pick_next_task_fair: loop (5939) [001] 510.564298: task_tick_fair: delta -3105461, ideal_runtime 11994146 [001] 510.564300: pick_next_task_fair: loop (5939) [001] 510.576288: pick_next_task_fair: loop (5883) [001] 510.580282: task_tick_fair: delta -4897210, ideal_runtime 11994146 [001] 510.580285: pick_next_task_fair: loop (5883) [001] 510.584278: task_tick_fair: delta -897348, ideal_runtime 11994146 [001] 510.584281: pick_next_task_fair: loop (5883) [001] 510.596269: pick_next_task_fair: loop (5939)
That is 20 ms slice for each task, with some redundant resched_tasks and with the fix it is expected ~12ms slices (on 16 cpu system).
[1] - http://lwn.net/Articles/409680/
Signed-off-by: Venkatesh Pallipadi <venki@google.com> --- kernel/sched_fair.c | 4 +++- 1 files changed, 3 insertions(+), 1 deletions(-)
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 00ebd76..fc5ffbd 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -871,8 +871,10 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr) if (cfs_rq->nr_running > 1) { struct sched_entity *se = __pick_next_entity(cfs_rq); s64 delta = curr->vruntime - se->vruntime; + unsigned long ideal_vruntime; - if (delta > ideal_runtime) + ideal_vruntime = calc_delta_fair(ideal_runtime, curr); + if (delta > (s64)ideal_vruntime) resched_task(rq_of(cfs_rq)->curr); } } -- 1.7.3.1
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