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    SubjectRe: [PATCH] avoid race condition in pick_next_task_fair in kernel/sched_fair.c
    From
    On Thu, Dec 23, 2010 at 4:36 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > On Wed, 2010-12-22 at 18:08 +0100, Peter Zijlstra wrote:
    >> On Wed, 2010-12-22 at 16:14 +0100, Miklos Vajna wrote:
    >> > kernel-build is a git build using the config I already sent and after a
    >> > 'git checkout v2.6.36'. I can try to build master as well, so far what I
    >> > saw is that the bug occurs there as well, but less frequently, so maybe
    >> > that's a bit harder to debug.
    >>
    >> Right, so I've got v2.6.36 exploding, current -git and -tip won't
    >> explode for me, tried both about 20 times. So I'll try and figure out
    >> wth makes .36 explode and then see if further kernel still suffer that
    >> problem.
    >
    > OK, so something goes wrong with load accounting,
    >
    > Full log at: http://programming.kicks-ass.net/sekrit/log4
    >
    > I print the entire runqueue before and after each op, the last good one
    > is a post-print:
    >
    > kworker/-9       0d..2. 2013644us : pick_next_task_fair: picked: f6a8de5c, kworker/u:1/1251
    > kworker/-9       0d..2. 2013644us : print_runqueue <-pick_next_task_fair
    > kworker/-9       0d..2. 2013645us : __print_runqueue:  cfs_rq: c2407c34, nr: 3, load: 3072
    > kworker/-9       0d..2. 2013645us : __print_runqueue:  curr: f6a8de5c, comm: kworker/u:1/1251, load: 1024
    > kworker/-9       0d..2. 2013646us : __print_runqueue:  se: f69e6300, load: 1024,
    > kworker/-9       0d..2. 2013646us : __print_runqueue:    cfs_rq: f69e6540, nr: 2, load: 2048
    > kworker/-9       0d..2. 2013646us : __print_runqueue:    curr: (null)
    > kworker/-9       0d..2. 2013647us : __print_runqueue:    se: f69e65a0, load: 1024,
    > kworker/-9       0d..2. 2013647us : __print_runqueue:      cfs_rq: f69e6240, nr: 1, load: 1024
    > kworker/-9       0d..2. 2013648us : __print_runqueue:      curr: (null)
    > kworker/-9       0d..2. 2013648us : __print_runqueue:      se: f725916c, comm: loadkeys/1225, load: 1024
    > kworker/-9       0d..2. 2013649us : __print_runqueue:    se: f69e6660, load: 1024,
    > kworker/-9       0d..2. 2013649us : __print_runqueue:      cfs_rq: f69e6600, nr: 1, load: 1024
    > kworker/-9       0d..2. 2013650us : __print_runqueue:      curr: (null)
    > kworker/-9       0d..2. 2013650us : __print_runqueue:      se: f6a8da0c, comm: systemd-tmpfile/1246, load: 1024
    > kworker/-9       0d..2. 2013651us : __print_runqueue:  se: f6a8cd1c, comm: kworker/u:1/1252, load: 1024
    >
    >
    > And then the next pre-print looks like:
    >
    > systemd--1251    0d..5. 2015398us : enqueue_task_fair <-enqueue_task
    > systemd--1251    0d..5. 2015398us : print_runqueue <-enqueue_task_fair
    > systemd--1251    0d..5. 2015399us : __print_runqueue:  cfs_rq: c2407c34, nr: 3, load: 3072
    > systemd--1251    0d..5. 2015400us : __print_runqueue:  curr: f6a8de5c, comm: systemd-cgroups/1251, load: 1024
    > systemd--1251    0d..5. 2015401us : __print_runqueue:  se: f69e6300, load: 1024,
    > systemd--1251    0d..5. 2015401us : __print_runqueue:    cfs_rq: f69e6540, nr: 2, load: 2048
    > systemd--1251    0d..5. 2015402us : __print_runqueue:    curr: (null)
    > systemd--1251    0d..5. 2015402us : __print_runqueue:    se: f69e65a0, load: 4137574976,

    the load == f69e65a0 == address of se, odd

    Thanks,
    Yong

    > systemd--1251    0d..5. 2015402us : __print_runqueue:      cfs_rq: f69e6240, nr: 1, load: 4137575360
    > systemd--1251    0d..5. 2015403us : __print_runqueue:      curr: (null)
    > systemd--1251    0d..5. 2015404us : __print_runqueue:      se: f725916c, comm: loadkeys/1225, load: 1024
    > systemd--1251    0d..5. 2015404us : __print_runqueue:    se: f69e6660, load: 1024,
    > systemd--1251    0d..5. 2015404us : __print_runqueue:      cfs_rq: f69e6600, nr: 1, load: 1024
    > systemd--1251    0d..5. 2015405us : __print_runqueue:      curr: (null)
    > systemd--1251    0d..5. 2015406us : __print_runqueue:      se: f6a8da0c, comm: systemd-tmpfile/1246, load: 1024
    > systemd--1251    0d..5. 2015406us : __print_runqueue:  se: f6a8cd1c, comm: kworker/u:1/1252, load: 1024
    >
    >
    > The load value being crazy like this leads to dequeue_task_fair() making
    > weird decisions since it checks for load==0 to dequeue groups.
    >
    > Not doing that, then leads to 'dead' groups still being enqueued and
    > eventually referencing freed memory etc..
    >
    > I've no clue why any of this happens, it could be random scribbles,
    > except the se/cfs_rq combo are always off together (although not always
    > by the same amount).
    >
    > Anyway, my brain needs sleep, I'll continue poking tomorrow.
    >
    > ---
    >  kernel/sched_fair.c |   71 +++++++++++++++++++++++++++++++++++++++++++++++++++
    >  1 files changed, 71 insertions(+), 0 deletions(-)
    >
    > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
    > index db3f674..8b8b038 100644
    > --- a/kernel/sched_fair.c
    > +++ b/kernel/sched_fair.c
    > @@ -333,6 +333,8 @@ static void __enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
    >        s64 key = entity_key(cfs_rq, se);
    >        int leftmost = 1;
    >
    > +       trace_printk("%p to %p, %ld tasks\n", se, cfs_rq, cfs_rq->nr_running);
    > +
    >        /*
    >         * Find the right place in the rbtree:
    >         */
    > @@ -364,6 +366,8 @@ static void __enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
    >
    >  static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
    >  {
    > +       trace_printk("%p from %p, %ld left\n", se, cfs_rq, cfs_rq->nr_running);
    > +
    >        if (cfs_rq->rb_leftmost == &se->run_node) {
    >                struct rb_node *next_node;
    >
    > @@ -394,6 +398,53 @@ static struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
    >        return rb_entry(last, struct sched_entity, run_node);
    >  }
    >
    > +#define INDENT(i) ((i)*2), "                                  "
    > +
    > +static void notrace __print_runqueue(struct cfs_rq *cfs_rq, int indent);
    > +
    > +static void __always_inline
    > +__print_entity(struct sched_entity *se, char *name, int indent)
    > +{
    > +       if (!se) {
    > +               trace_printk("%.*s %s: %p\n", INDENT(indent), name, se);
    > +               return;
    > +       }
    > +
    > +       if (entity_is_task(se)) {
    > +               struct task_struct *p = task_of(se);
    > +               trace_printk("%.*s %s: %p, comm: %s/%d, load: %ld\n",
    > +                               INDENT(indent), name,
    > +                               se, p->comm, p->pid, se->load.weight);
    > +       } else {
    > +               trace_printk("%.*s %s: %p, load: %ld, ",
    > +                               INDENT(indent), name,
    > +                               se, se->load.weight);
    > +               __print_runqueue(group_cfs_rq(se), indent+1);
    > +       }
    > +}
    > +
    > +static void notrace __print_runqueue(struct cfs_rq *cfs_rq, int indent)
    > +{
    > +       struct rb_node *n;
    > +
    > +       trace_printk("%.*s cfs_rq: %p, nr: %ld, load: %ld\n",
    > +                       INDENT(indent),
    > +                       cfs_rq, cfs_rq->nr_running,
    > +                       cfs_rq->load.weight);
    > +
    > +       __print_entity(cfs_rq->curr, "curr", indent);
    > +
    > +       for (n = cfs_rq->rb_leftmost; n; n = rb_next(n)) {
    > +               struct sched_entity *se = rb_entry(n, struct sched_entity, run_node);
    > +               __print_entity(se, "se", indent);
    > +       }
    > +}
    > +
    > +void print_runqueue(struct cfs_rq *cfs_rq)
    > +{
    > +       __print_runqueue(cfs_rq, 0);
    > +}
    > +
    >  /**************************************************************
    >  * Scheduling class statistics methods:
    >  */
    > @@ -1047,6 +1098,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
    >        struct cfs_rq *cfs_rq;
    >        struct sched_entity *se = &p->se;
    >
    > +       print_runqueue(&rq->cfs);
    >        for_each_sched_entity(se) {
    >                if (se->on_rq)
    >                        break;
    > @@ -1055,6 +1107,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
    >                flags = ENQUEUE_WAKEUP;
    >        }
    >
    > +       trace_printk("add task: %p, %s/%d\n", &p->se, p->comm, p->pid);
    > +       print_runqueue(&rq->cfs);
    > +
    >        hrtick_update(rq);
    >  }
    >
    > @@ -1068,6 +1123,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
    >        struct cfs_rq *cfs_rq;
    >        struct sched_entity *se = &p->se;
    >
    > +       print_runqueue(&rq->cfs);
    >        for_each_sched_entity(se) {
    >                cfs_rq = cfs_rq_of(se);
    >                dequeue_entity(cfs_rq, se, flags);
    > @@ -1077,6 +1133,9 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
    >                flags |= DEQUEUE_SLEEP;
    >        }
    >
    > +       trace_printk("del task: %p, %s/%d\n", &p->se, p->comm, p->pid);
    > +       print_runqueue(&rq->cfs);
    > +
    >        hrtick_update(rq);
    >  }
    >
    > @@ -1719,6 +1778,8 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)
    >        struct cfs_rq *cfs_rq = &rq->cfs;
    >        struct sched_entity *se;
    >
    > +       print_runqueue(&rq->cfs);
    > +
    >        if (!cfs_rq->nr_running)
    >                return NULL;
    >
    > @@ -1731,6 +1792,9 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)
    >        p = task_of(se);
    >        hrtick_start_fair(rq, p);
    >
    > +       trace_printk("picked: %p, %s/%d\n", se, p->comm, p->pid);
    > +       print_runqueue(&rq->cfs);
    > +
    >        return p;
    >  }
    >
    > @@ -1742,10 +1806,15 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
    >        struct sched_entity *se = &prev->se;
    >        struct cfs_rq *cfs_rq;
    >
    > +       print_runqueue(&rq->cfs);
    >        for_each_sched_entity(se) {
    >                cfs_rq = cfs_rq_of(se);
    >                put_prev_entity(cfs_rq, se);
    >        }
    > +
    > +       trace_printk("put: %p, %s/%d\n", se, prev->comm, prev->pid);
    > +       print_runqueue(&rq->cfs);
    > +
    >  }
    >
    >  #ifdef CONFIG_SMP
    > @@ -3819,8 +3888,10 @@ static void set_curr_task_fair(struct rq *rq)
    >  {
    >        struct sched_entity *se = &rq->curr->se;
    >
    > +       print_runqueue(&rq->cfs);
    >        for_each_sched_entity(se)
    >                set_next_entity(cfs_rq_of(se), se);
    > +       print_runqueue(&rq->cfs);
    >  }
    >
    >  #ifdef CONFIG_FAIR_GROUP_SCHED
    >
    > --
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