Messages in this thread Patch in this message |  | | Date | Mon, 24 Feb 2014 13:12:18 +0100 | From | Peter Zijlstra <> | Subject | Re: sched: hang in migrate_swap |
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On Mon, Feb 24, 2014 at 06:14:24PM +0800, Michael wang wrote: > On 02/24/2014 03:10 PM, Peter Zijlstra wrote: > > On Mon, Feb 24, 2014 at 01:19:15PM +0800, Michael wang wrote: > >> Peter, do we accidentally missed this commit? > >> > >> http://git.kernel.org/tip/477af336ba06ef4c32e97892bb0d2027ce30f466 > > > > Ingo dropped it on Saturday because it makes locking_selftest() unhappy. > > > > That is because we call locking_selftest() way before we're ready to > > call schedule() and guess what it does :-/ > > > > I'm not entirely sure what to do.. ideally I'd shoot locking_selftest in > > the head, but clearly that's not entirely desired either. > > ...what about move idle_balance() back to it's old position?
I've always hated that, idle_balance() is very much a fair policy thing and shouldn't live in the core code.
> pull_rt_task() logical could be after idle_balance() if still no FAIR > and DL, then go into the pick loop, that may could make things more > clean & clear, should we have a try?
So the reason pull_{rt,dl}_task() is before idle_balance() is that we don't want to add the execution latency of idle_balance() to the rt/dl task pulling.
Anyway, the below seems to work; it avoids playing tricks with the idle thread and instead uses a magic constant.
The comparison should be faster too; seeing how we avoid dereferencing p->sched_class.
--- Subject: sched: Guarantee task priority in pick_next_task() From: Peter Zijlstra <peterz@infradead.org> Date: Fri Feb 14 12:25:08 CET 2014
Michael spotted that the idle_balance() push down created a task priority problem.
Previously, when we called idle_balance() before pick_next_task() it wasn't a problem when -- because of the rq->lock droppage -- an rt/dl task slipped in.
Similarly for pre_schedule(), rt pre-schedule could have a dl task slip in.
But by pulling it into the pick_next_task() loop, we'll not try a higher task priority again.
Cure this by creating a re-start condition in pick_next_task(); and triggering this from pick_next_task_{rt,fair}().
Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()") Cc: Juri Lelli <juri.lelli@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Steven Rostedt <rostedt@goodmis.org> Reported-by: Michael Wang <wangyun@linux.vnet.ibm.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> --- kernel/sched/core.c | 12 ++++++++---- kernel/sched/fair.c | 13 ++++++++++++- kernel/sched/rt.c | 10 +++++++++- kernel/sched/sched.h | 5 +++++ 4 files changed, 34 insertions(+), 6 deletions(-)
--- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2586,24 +2586,28 @@ static inline void schedule_debug(struct static inline struct task_struct * pick_next_task(struct rq *rq, struct task_struct *prev) { - const struct sched_class *class; + const struct sched_class *class = &fair_sched_class; struct task_struct *p; /* * Optimization: we know that if all tasks are in * the fair class we can call that function directly: */ - if (likely(prev->sched_class == &fair_sched_class && + if (likely(prev->sched_class == class && rq->nr_running == rq->cfs.h_nr_running)) { p = fair_sched_class.pick_next_task(rq, prev); - if (likely(p)) + if (likely(p && p != RETRY_TASK)) return p; } +again: for_each_class(class) { p = class->pick_next_task(rq, prev); - if (p) + if (p) { + if (unlikely(p == RETRY_TASK)) + goto again; return p; + } } BUG(); /* the idle class will always have a runnable task */ --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4687,6 +4687,7 @@ pick_next_task_fair(struct rq *rq, struc struct cfs_rq *cfs_rq = &rq->cfs; struct sched_entity *se; struct task_struct *p; + int new_tasks; again: #ifdef CONFIG_FAIR_GROUP_SCHED @@ -4785,7 +4786,17 @@ pick_next_task_fair(struct rq *rq, struc return p; idle: - if (idle_balance(rq)) /* drops rq->lock */ + /* + * Because idle_balance() releases (and re-acquires) rq->lock, it is + * possible for any higher priority task to appear. In that case we + * must re-start the pick_next_entity() loop. + */ + new_tasks = idle_balance(rq); + + if (rq->nr_running != rq->cfs.h_nr_running) + return RETRY_TASK; + + if (new_tasks) goto again; return NULL; --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct struct task_struct *p; struct rt_rq *rt_rq = &rq->rt; - if (need_pull_rt_task(rq, prev)) + if (need_pull_rt_task(rq, prev)) { pull_rt_task(rq); + /* + * pull_rt_task() can drop (and re-acquire) rq->lock; this + * means a dl task can slip in, in which case we need to + * re-start task selection. + */ + if (unlikely(rq->dl.dl_nr_running)) + return RETRY_TASK; + } if (!rt_rq->rt_nr_running) return NULL; --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1090,6 +1090,8 @@ static const u32 prio_to_wmult[40] = { #define DEQUEUE_SLEEP 1 +#define RETRY_TASK ((void *)-1UL) + struct sched_class { const struct sched_class *next; @@ -1104,6 +1106,9 @@ struct sched_class { * It is the responsibility of the pick_next_task() method that will * return the next task to call put_prev_task() on the @prev task or * something equivalent. + * + * May return RETRY_TASK when it finds a higher prio class has runnable + * tasks. */ struct task_struct * (*pick_next_task) (struct rq *rq, struct task_struct *prev);
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