| Message ID | 20160406172309.25877.66499.stgit@Solace.fritz.box (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 06/04/16 18:23, Dario Faggioli wrote: > In short, the point is making sure that the actual switch > of scheduler and the remapping of the scheduler's runqueue > lock occur in the same critical section, protected by the > "old" scheduler's lock (and not, e.g., in the free_pdata > hook, as it is now for Credit2 and RTDS). > > Not doing so, is (at least) racy. In fact, for instance, > if we switch cpu X from, Credit2 to Credit, we do: > > schedule_cpu_switch(x, csched2 --> csched): > //scheduler[x] is csched2 > //schedule_lock[x] is csched2_lock > csched_alloc_pdata(x) > csched_init_pdata(x) > pcpu_schedule_lock(x) ----> takes csched2_lock > scheduler[X] = csched > pcpu_schedule_unlock(x) --> unlocks csched2_lock > [1] > csched2_free_pdata(x) > pcpu_schedule_lock(x) --> takes csched2_lock > schedule_lock[x] = csched_lock > spin_unlock(csched2_lock) > > While, if we switch cpu X from, Credit to Credit2, we do: > > schedule_cpu_switch(X, csched --> csched2): > //scheduler[x] is csched > //schedule_lock[x] is csched_lock > csched2_alloc_pdata(x) > csched2_init_pdata(x) > pcpu_schedule_lock(x) --> takes csched_lock > schedule_lock[x] = csched2_lock > spin_unlock(csched_lock) > [2] > pcpu_schedule_lock(x) ----> takes csched2_lock > scheduler[X] = csched2 > pcpu_schedule_unlock(x) --> unlocks csched2_lock > csched_free_pdata(x) > > And if we switch cpu X from RTDS to Credit2, we do: > > schedule_cpu_switch(X, RTDS --> csched2): > //scheduler[x] is rtds > //schedule_lock[x] is rtds_lock > csched2_alloc_pdata(x) > csched2_init_pdata(x) > pcpu_schedule_lock(x) --> takes rtds_lock > schedule_lock[x] = csched2_lock > spin_unlock(rtds_lock) > pcpu_schedule_lock(x) ----> takes csched2_lock > scheduler[x] = csched2 > pcpu_schedule_unlock(x) --> unlocks csched2_lock > rtds_free_pdata(x) > spin_lock(rtds_lock) > ASSERT(schedule_lock[x] == rtds_lock) [3] > schedule_lock[x] = DEFAULT_SCHEDULE_LOCK [4] > spin_unlock(rtds_lock) > > So, the first problem is that, if anything related to > scheduling, and involving CPU, happens at [1] or [2], we: > - take csched2_lock, > - operate on Credit1 functions and data structures, > which is no good! > > The second problem is that the ASSERT at [3] triggers, and > the third that at [4], we screw up the lock remapping we've > done for ourself in csched2_init_pdata()! > > The first problem arises because there is a window during > which the lock is already the new one, but the scheduler is > still the old one. The other two, becase we let schedulers > mess with the lock (re)mapping done by others. > > This patch, therefore, introduces a new hook in the scheduler > interface, called switch_sched, meant at being used when > switching scheduler on a CPU, and implements it for the > various schedulers (that needs it: i.e., all except ARINC653), > so that things are done in the proper order and under the > protection of the best suited (set of) lock(s). It is > necessary to add the hook (as compared to keep doing things > in generic code), because different schedulers may have > different locking schemes. > > Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com> Hey Dario! Everything here looks good, except for one thing: the scheduler lock for arinc653 scheduler. :-) What happens now if you assign a cpu to credit2, and then assign it to arinc653? Since arinc doesn't implement the switch_sched() functionality, the per-cpu scheduler lock will still point to the credit2 lock, won't it? Which will *work*, although it will add unnecessary contention to the credit2 lock; until the lock goes away, at which point vcpu_schedule_lock*() will essentially be using a wild pointer. -George > --- > Cc: George Dunlap <george.dunlap@eu.citrix.com> > Cc: Meng Xu <mengxu@cis.upenn.edu> > Cc: Tianyang Chen <tiche@seas.upenn.edu> > --- > Changes from v1: > > new patch, basically, coming from squashing what were > 4 patches in v1. In any case, with respect to those 4 > patches: > - runqueue lock is back being taken in schedule_cpu_switch(), > as suggested during review; > - add barriers for making sure all initialization is done > when the new lock is assigned, as sugested during review; > - add comments and ASSERT-s about how and why the adopted > locking scheme is safe, as suggested during review. > --- > xen/common/sched_credit.c | 44 ++++++++++++++++++++++++ > xen/common/sched_credit2.c | 81 +++++++++++++++++++++++++++++++++----------- > xen/common/sched_rt.c | 45 +++++++++++++++++------- > xen/common/schedule.c | 41 +++++++++++++++++----- > xen/include/xen/sched-if.h | 3 ++ > 5 files changed, 172 insertions(+), 42 deletions(-) > > diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c > index 96a245d..540d515 100644 > --- a/xen/common/sched_credit.c > +++ b/xen/common/sched_credit.c > @@ -578,12 +578,55 @@ csched_init_pdata(const struct scheduler *ops, void *pdata, int cpu) > { > unsigned long flags; > struct csched_private *prv = CSCHED_PRIV(ops); > + struct schedule_data *sd = &per_cpu(schedule_data, cpu); > + > + /* > + * This is called either during during boot, resume or hotplug, in > + * case Credit1 is the scheduler chosen at boot. In such cases, the > + * scheduler lock for cpu is already pointing to the default per-cpu > + * spinlock, as Credit1 needs it, so there is no remapping to be done. > + */ > + ASSERT(sd->schedule_lock == &sd->_lock && !spin_is_locked(&sd->_lock)); > > spin_lock_irqsave(&prv->lock, flags); > init_pdata(prv, pdata, cpu); > spin_unlock_irqrestore(&prv->lock, flags); > } > > +/* Change the scheduler of cpu to us (Credit). */ > +static void > +csched_switch_sched(struct scheduler *ops, unsigned int cpu, > + void *pdata, void *vdata) > +{ > + struct schedule_data *sd = &per_cpu(schedule_data, cpu); > + struct csched_private *prv = CSCHED_PRIV(ops); > + struct csched_vcpu *svc = vdata; > + > + ASSERT(svc && is_idle_vcpu(svc->vcpu)); > + > + idle_vcpu[cpu]->sched_priv = vdata; > + > + /* > + * We are holding the runqueue lock already (it's been taken in > + * schedule_cpu_switch()). It actually may or may not be the 'right' > + * one for this cpu, but that is ok for preventing races. > + */ > + spin_lock(&prv->lock); > + init_pdata(prv, pdata, cpu); > + spin_unlock(&prv->lock); > + > + per_cpu(scheduler, cpu) = ops; > + per_cpu(schedule_data, cpu).sched_priv = pdata; > + > + /* > + * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact, > + * if it is free (and it can be) we want that anyone that manages > + * taking it, finds all the initializations we've done above in place. > + */ > + smp_mb(); > + sd->schedule_lock = &sd->_lock; > +} > + > #ifndef NDEBUG > static inline void > __csched_vcpu_check(struct vcpu *vc) > @@ -2067,6 +2110,7 @@ static const struct scheduler sched_credit_def = { > .alloc_pdata = csched_alloc_pdata, > .init_pdata = csched_init_pdata, > .free_pdata = csched_free_pdata, > + .switch_sched = csched_switch_sched, > .alloc_domdata = csched_alloc_domdata, > .free_domdata = csched_free_domdata, > > diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c > index 8989eea..60c6f5b 100644 > --- a/xen/common/sched_credit2.c > +++ b/xen/common/sched_credit2.c > @@ -1971,12 +1971,12 @@ static void deactivate_runqueue(struct csched2_private *prv, int rqi) > cpumask_clear_cpu(rqi, &prv->active_queues); > } > > -static void > +/* Returns the ID of the runqueue the cpu is assigned to. */ > +static unsigned > init_pdata(struct csched2_private *prv, unsigned int cpu) > { > unsigned rqi; > struct csched2_runqueue_data *rqd; > - spinlock_t *old_lock; > > ASSERT(spin_is_locked(&prv->lock)); > ASSERT(!cpumask_test_cpu(cpu, &prv->initialized)); > @@ -2007,44 +2007,89 @@ init_pdata(struct csched2_private *prv, unsigned int cpu) > activate_runqueue(prv, rqi); > } > > - /* IRQs already disabled */ > - old_lock = pcpu_schedule_lock(cpu); > - > - /* Move spinlock to new runq lock. */ > - per_cpu(schedule_data, cpu).schedule_lock = &rqd->lock; > - > /* Set the runqueue map */ > prv->runq_map[cpu] = rqi; > > cpumask_set_cpu(cpu, &rqd->idle); > cpumask_set_cpu(cpu, &rqd->active); > - > - /* _Not_ pcpu_schedule_unlock(): per_cpu().schedule_lock changed! */ > - spin_unlock(old_lock); > - > cpumask_set_cpu(cpu, &prv->initialized); > > - return; > + return rqi; > } > > static void > csched2_init_pdata(const struct scheduler *ops, void *pdata, int cpu) > { > struct csched2_private *prv = CSCHED2_PRIV(ops); > + spinlock_t *old_lock; > unsigned long flags; > + unsigned rqi; > > spin_lock_irqsave(&prv->lock, flags); > - init_pdata(prv, cpu); > + old_lock = pcpu_schedule_lock(cpu); > + > + rqi = init_pdata(prv, cpu); > + /* Move the scheduler lock to the new runq lock. */ > + per_cpu(schedule_data, cpu).schedule_lock = &prv->rqd[rqi].lock; > + > + /* _Not_ pcpu_schedule_unlock(): schedule_lock may have changed! */ > + spin_unlock(old_lock); > spin_unlock_irqrestore(&prv->lock, flags); > } > > +/* Change the scheduler of cpu to us (Credit2). */ > +static void > +csched2_switch_sched(struct scheduler *new_ops, unsigned int cpu, > + void *pdata, void *vdata) > +{ > + struct csched2_private *prv = CSCHED2_PRIV(new_ops); > + struct csched2_vcpu *svc = vdata; > + unsigned rqi; > + > + ASSERT(!pdata && svc && is_idle_vcpu(svc->vcpu)); > + > + /* > + * We own one runqueue lock already (from schedule_cpu_switch()). This > + * looks like it violates this scheduler's locking rules, but it does > + * not, as what we own is the lock of another scheduler, that hence has > + * no particular (ordering) relationship with our private global lock. > + * And owning exactly that one (the lock of the old scheduler of this > + * cpu) is what is necessary to prevent races. > + */ > + spin_lock_irq(&prv->lock); > + > + idle_vcpu[cpu]->sched_priv = vdata; > + > + rqi = init_pdata(prv, cpu); > + > + /* > + * Now that we know what runqueue we'll go in, double check what's said > + * above: the lock we already hold is not the one of this runqueue of > + * this scheduler, and so it's safe to have taken it /before/ our > + * private global lock. > + */ > + ASSERT(per_cpu(schedule_data, cpu).schedule_lock != &prv->rqd[rqi].lock); > + > + per_cpu(scheduler, cpu) = new_ops; > + per_cpu(schedule_data, cpu).sched_priv = NULL; /* no pdata */ > + > + /* > + * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact, > + * if it is free (and it can be) we want that anyone that manages > + * taking it, find all the initializations we've done above in place. > + */ > + smp_mb(); > + per_cpu(schedule_data, cpu).schedule_lock = &prv->rqd[rqi].lock; > + > + spin_unlock_irq(&prv->lock); > +} > + > static void > csched2_free_pdata(const struct scheduler *ops, void *pcpu, int cpu) > { > unsigned long flags; > struct csched2_private *prv = CSCHED2_PRIV(ops); > struct csched2_runqueue_data *rqd; > - struct schedule_data *sd = &per_cpu(schedule_data, cpu); > int rqi; > > spin_lock_irqsave(&prv->lock, flags); > @@ -2072,11 +2117,6 @@ csched2_free_pdata(const struct scheduler *ops, void *pcpu, int cpu) > deactivate_runqueue(prv, rqi); > } > > - /* Move spinlock to the original lock. */ > - ASSERT(sd->schedule_lock == &rqd->lock); > - ASSERT(!spin_is_locked(&sd->_lock)); > - sd->schedule_lock = &sd->_lock; > - > spin_unlock(&rqd->lock); > > cpumask_clear_cpu(cpu, &prv->initialized); > @@ -2170,6 +2210,7 @@ static const struct scheduler sched_credit2_def = { > .free_vdata = csched2_free_vdata, > .init_pdata = csched2_init_pdata, > .free_pdata = csched2_free_pdata, > + .switch_sched = csched2_switch_sched, > .alloc_domdata = csched2_alloc_domdata, > .free_domdata = csched2_free_domdata, > }; > diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c > index b96bd93..3bb8c71 100644 > --- a/xen/common/sched_rt.c > +++ b/xen/common/sched_rt.c > @@ -682,6 +682,37 @@ rt_init_pdata(const struct scheduler *ops, void *pdata, int cpu) > spin_unlock_irqrestore(old_lock, flags); > } > > +/* Change the scheduler of cpu to us (RTDS). */ > +static void > +rt_switch_sched(struct scheduler *new_ops, unsigned int cpu, > + void *pdata, void *vdata) > +{ > + struct rt_private *prv = rt_priv(new_ops); > + struct rt_vcpu *svc = vdata; > + > + ASSERT(!pdata && svc && is_idle_vcpu(svc->vcpu)); > + > + /* > + * We are holding the runqueue lock already (it's been taken in > + * schedule_cpu_switch()). It's actually the runqueue lock of > + * another scheduler, but that is how things need to be, for > + * preventing races. > + */ > + ASSERT(per_cpu(schedule_data, cpu).schedule_lock != &prv->lock); > + > + idle_vcpu[cpu]->sched_priv = vdata; > + per_cpu(scheduler, cpu) = new_ops; > + per_cpu(schedule_data, cpu).sched_priv = NULL; /* no pdata */ > + > + /* > + * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact, > + * if it is free (and it can be) we want that anyone that manages > + * taking it, find all the initializations we've done above in place. > + */ > + smp_mb(); > + per_cpu(schedule_data, cpu).schedule_lock = &prv->lock; > +} > + > static void * > rt_alloc_pdata(const struct scheduler *ops, int cpu) > { > @@ -707,19 +738,6 @@ rt_alloc_pdata(const struct scheduler *ops, int cpu) > static void > rt_free_pdata(const struct scheduler *ops, void *pcpu, int cpu) > { > - struct rt_private *prv = rt_priv(ops); > - struct schedule_data *sd = &per_cpu(schedule_data, cpu); > - unsigned long flags; > - > - spin_lock_irqsave(&prv->lock, flags); > - > - /* Move spinlock back to the default lock */ > - ASSERT(sd->schedule_lock == &prv->lock); > - ASSERT(!spin_is_locked(&sd->_lock)); > - sd->schedule_lock = &sd->_lock; > - > - spin_unlock_irqrestore(&prv->lock, flags); > - > free_cpumask_var(_cpumask_scratch[cpu]); > } > > @@ -1468,6 +1486,7 @@ static const struct scheduler sched_rtds_def = { > .alloc_pdata = rt_alloc_pdata, > .free_pdata = rt_free_pdata, > .init_pdata = rt_init_pdata, > + .switch_sched = rt_switch_sched, > .alloc_domdata = rt_alloc_domdata, > .free_domdata = rt_free_domdata, > .init_domain = rt_dom_init, > diff --git a/xen/common/schedule.c b/xen/common/schedule.c > index 1941613..5559aa1 100644 > --- a/xen/common/schedule.c > +++ b/xen/common/schedule.c > @@ -1635,11 +1635,11 @@ void __init scheduler_init(void) > int schedule_cpu_switch(unsigned int cpu, struct cpupool *c) > { > struct vcpu *idle; > - spinlock_t *lock; > void *ppriv, *ppriv_old, *vpriv, *vpriv_old; > struct scheduler *old_ops = per_cpu(scheduler, cpu); > struct scheduler *new_ops = (c == NULL) ? &ops : c->sched; > struct cpupool *old_pool = per_cpu(cpupool, cpu); > + spinlock_t * old_lock; > > /* > * pCPUs only move from a valid cpupool to free (i.e., out of any pool), > @@ -1658,11 +1658,21 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c) > if ( old_ops == new_ops ) > goto out; > > + /* > + * To setup the cpu for the new scheduler we need: > + * - a valid instance of per-CPU scheduler specific data, as it is > + * allocated by SCHED_OP(alloc_pdata). Note that we do not want to > + * initialize it yet (i.e., we are not calling SCHED_OP(init_pdata)). > + * That will be done by the target scheduler, in SCHED_OP(switch_sched), > + * in proper ordering and with locking. > + * - a valid instance of per-vCPU scheduler specific data, for the idle > + * vCPU of cpu. That is what the target scheduler will use for the > + * sched_priv field of the per-vCPU info of the idle domain. > + */ > idle = idle_vcpu[cpu]; > ppriv = SCHED_OP(new_ops, alloc_pdata, cpu); > if ( IS_ERR(ppriv) ) > return PTR_ERR(ppriv); > - SCHED_OP(new_ops, init_pdata, ppriv, cpu); > vpriv = SCHED_OP(new_ops, alloc_vdata, idle, idle->domain->sched_priv); > if ( vpriv == NULL ) > { > @@ -1670,17 +1680,30 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c) > return -ENOMEM; > } > > - lock = pcpu_schedule_lock_irq(cpu); > - > SCHED_OP(old_ops, tick_suspend, cpu); > + > + /* > + * The actual switch, including (if necessary) the rerouting of the > + * scheduler lock to whatever new_ops prefers, needs to happen in one > + * critical section, protected by old_ops' lock, or races are possible. > + * It is, in fact, the lock of another scheduler that we are taking (the > + * scheduler of the cpupool that cpu still belongs to). But that is ok > + * as, anyone trying to schedule on this cpu will spin until when we > + * release that lock (bottom of this function). When he'll get the lock > + * --thanks to the loop inside *_schedule_lock() functions-- he'll notice > + * that the lock itself changed, and retry acquiring the new one (which > + * will be the correct, remapped one, at that point). > + */ > + old_lock = pcpu_schedule_lock(cpu); > + > vpriv_old = idle->sched_priv; > - idle->sched_priv = vpriv; > - per_cpu(scheduler, cpu) = new_ops; > ppriv_old = per_cpu(schedule_data, cpu).sched_priv; > - per_cpu(schedule_data, cpu).sched_priv = ppriv; > - SCHED_OP(new_ops, tick_resume, cpu); > + SCHED_OP(new_ops, switch_sched, cpu, ppriv, vpriv); > > - pcpu_schedule_unlock_irq(lock, cpu); > + /* _Not_ pcpu_schedule_unlock(): schedule_lock may have changed! */ > + spin_unlock_irq(old_lock); > + > + SCHED_OP(new_ops, tick_resume, cpu); > > SCHED_OP(old_ops, free_vdata, vpriv_old); > SCHED_OP(old_ops, free_pdata, ppriv_old, cpu); > diff --git a/xen/include/xen/sched-if.h b/xen/include/xen/sched-if.h > index 70c08c6..9cebe41 100644 > --- a/xen/include/xen/sched-if.h > +++ b/xen/include/xen/sched-if.h > @@ -137,6 +137,9 @@ struct scheduler { > void (*free_domdata) (const struct scheduler *, void *); > void * (*alloc_domdata) (const struct scheduler *, struct domain *); > > + void (*switch_sched) (struct scheduler *, unsigned int, > + void *, void *); > + > int (*init_domain) (const struct scheduler *, struct domain *); > void (*destroy_domain) (const struct scheduler *, struct domain *); > >
On Thu, 2016-04-07 at 15:54 +0100, George Dunlap wrote: > On 06/04/16 18:23, Dario Faggioli wrote: > > > > This patch, therefore, introduces a new hook in the scheduler > > interface, called switch_sched, meant at being used when > > switching scheduler on a CPU, and implements it for the > > various schedulers (that needs it: i.e., all except ARINC653), > > so that things are done in the proper order and under the > > protection of the best suited (set of) lock(s). It is > > necessary to add the hook (as compared to keep doing things > > in generic code), because different schedulers may have > > different locking schemes. > > > > Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com> > Hey Dario! Everything here looks good, except for one thing: the > scheduler lock for arinc653 scheduler. :-) > Bah! :-/ > What happens now if you > assign a cpu to credit2, and then assign it to arinc653? Since arinc > doesn't implement the switch_sched() functionality, the per-cpu > scheduler lock will still point to the credit2 lock, won't it? > So, to assign a cpu to a pool you have to first remove it from the pool it's currently in. And removing a cpu from its pool, means re-assigning it to the default pool (pool0), which uses as scheduler whatever we chose at boot. (Then, of course, vcpus of domains in the default pool still won't run there, because its corresponding bit in pool0->cpu_valid mask is 0.) So, if you move a cpu from pool0 to an ARINC pool, indeed the lock would keep pointing to pool0's runqueue lock (which will be one of the Credit2's runqueue locks, if the default scheduler is Credit2). And that is the case even if you move a cpu from some non-default pool to an ARINC pool, because you always have to remove the cpu from where it is first, which automatically puts it back in the default pool. This is relevant because... > Which will *work*, although it will add unnecessary contention to the > credit2 lock; until the lock goes away, at which point > vcpu_schedule_lock*() will essentially be using a wild pointer. > ...since we're talking about the lock of the default scheduler, it'll never go away. But indeed I agree that this just adds unnecessary contention which, in case of Credit2 or RTDS, where the lock is shared between pcpus, is something we certainly don't want (good catch!). I'll add a very simple implementation of switch_sched to ARINC, which basically puts back sd->schedule_lock to sd->_lock, and also does the basic operations for actually switching scheduler, which before this patch were done in schedule_cpu_switch(), and hence are also needed in there. Thanks and Regards, Dario
diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c index 96a245d..540d515 100644 --- a/xen/common/sched_credit.c +++ b/xen/common/sched_credit.c @@ -578,12 +578,55 @@ csched_init_pdata(const struct scheduler *ops, void *pdata, int cpu) { unsigned long flags; struct csched_private *prv = CSCHED_PRIV(ops); + struct schedule_data *sd = &per_cpu(schedule_data, cpu); + + /* + * This is called either during during boot, resume or hotplug, in + * case Credit1 is the scheduler chosen at boot. In such cases, the + * scheduler lock for cpu is already pointing to the default per-cpu + * spinlock, as Credit1 needs it, so there is no remapping to be done. + */ + ASSERT(sd->schedule_lock == &sd->_lock && !spin_is_locked(&sd->_lock)); spin_lock_irqsave(&prv->lock, flags); init_pdata(prv, pdata, cpu); spin_unlock_irqrestore(&prv->lock, flags); } +/* Change the scheduler of cpu to us (Credit). */ +static void +csched_switch_sched(struct scheduler *ops, unsigned int cpu, + void *pdata, void *vdata) +{ + struct schedule_data *sd = &per_cpu(schedule_data, cpu); + struct csched_private *prv = CSCHED_PRIV(ops); + struct csched_vcpu *svc = vdata; + + ASSERT(svc && is_idle_vcpu(svc->vcpu)); + + idle_vcpu[cpu]->sched_priv = vdata; + + /* + * We are holding the runqueue lock already (it's been taken in + * schedule_cpu_switch()). It actually may or may not be the 'right' + * one for this cpu, but that is ok for preventing races. + */ + spin_lock(&prv->lock); + init_pdata(prv, pdata, cpu); + spin_unlock(&prv->lock); + + per_cpu(scheduler, cpu) = ops; + per_cpu(schedule_data, cpu).sched_priv = pdata; + + /* + * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact, + * if it is free (and it can be) we want that anyone that manages + * taking it, finds all the initializations we've done above in place. + */ + smp_mb(); + sd->schedule_lock = &sd->_lock; +} + #ifndef NDEBUG static inline void __csched_vcpu_check(struct vcpu *vc) @@ -2067,6 +2110,7 @@ static const struct scheduler sched_credit_def = { .alloc_pdata = csched_alloc_pdata, .init_pdata = csched_init_pdata, .free_pdata = csched_free_pdata, + .switch_sched = csched_switch_sched, .alloc_domdata = csched_alloc_domdata, .free_domdata = csched_free_domdata, diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c index 8989eea..60c6f5b 100644 --- a/xen/common/sched_credit2.c +++ b/xen/common/sched_credit2.c @@ -1971,12 +1971,12 @@ static void deactivate_runqueue(struct csched2_private *prv, int rqi) cpumask_clear_cpu(rqi, &prv->active_queues); } -static void +/* Returns the ID of the runqueue the cpu is assigned to. */ +static unsigned init_pdata(struct csched2_private *prv, unsigned int cpu) { unsigned rqi; struct csched2_runqueue_data *rqd; - spinlock_t *old_lock; ASSERT(spin_is_locked(&prv->lock)); ASSERT(!cpumask_test_cpu(cpu, &prv->initialized)); @@ -2007,44 +2007,89 @@ init_pdata(struct csched2_private *prv, unsigned int cpu) activate_runqueue(prv, rqi); } - /* IRQs already disabled */ - old_lock = pcpu_schedule_lock(cpu); - - /* Move spinlock to new runq lock. */ - per_cpu(schedule_data, cpu).schedule_lock = &rqd->lock; - /* Set the runqueue map */ prv->runq_map[cpu] = rqi; cpumask_set_cpu(cpu, &rqd->idle); cpumask_set_cpu(cpu, &rqd->active); - - /* _Not_ pcpu_schedule_unlock(): per_cpu().schedule_lock changed! */ - spin_unlock(old_lock); - cpumask_set_cpu(cpu, &prv->initialized); - return; + return rqi; } static void csched2_init_pdata(const struct scheduler *ops, void *pdata, int cpu) { struct csched2_private *prv = CSCHED2_PRIV(ops); + spinlock_t *old_lock; unsigned long flags; + unsigned rqi; spin_lock_irqsave(&prv->lock, flags); - init_pdata(prv, cpu); + old_lock = pcpu_schedule_lock(cpu); + + rqi = init_pdata(prv, cpu); + /* Move the scheduler lock to the new runq lock. */ + per_cpu(schedule_data, cpu).schedule_lock = &prv->rqd[rqi].lock; + + /* _Not_ pcpu_schedule_unlock(): schedule_lock may have changed! */ + spin_unlock(old_lock); spin_unlock_irqrestore(&prv->lock, flags); } +/* Change the scheduler of cpu to us (Credit2). */ +static void +csched2_switch_sched(struct scheduler *new_ops, unsigned int cpu, + void *pdata, void *vdata) +{ + struct csched2_private *prv = CSCHED2_PRIV(new_ops); + struct csched2_vcpu *svc = vdata; + unsigned rqi; + + ASSERT(!pdata && svc && is_idle_vcpu(svc->vcpu)); + + /* + * We own one runqueue lock already (from schedule_cpu_switch()). This + * looks like it violates this scheduler's locking rules, but it does + * not, as what we own is the lock of another scheduler, that hence has + * no particular (ordering) relationship with our private global lock. + * And owning exactly that one (the lock of the old scheduler of this + * cpu) is what is necessary to prevent races. + */ + spin_lock_irq(&prv->lock); + + idle_vcpu[cpu]->sched_priv = vdata; + + rqi = init_pdata(prv, cpu); + + /* + * Now that we know what runqueue we'll go in, double check what's said + * above: the lock we already hold is not the one of this runqueue of + * this scheduler, and so it's safe to have taken it /before/ our + * private global lock. + */ + ASSERT(per_cpu(schedule_data, cpu).schedule_lock != &prv->rqd[rqi].lock); + + per_cpu(scheduler, cpu) = new_ops; + per_cpu(schedule_data, cpu).sched_priv = NULL; /* no pdata */ + + /* + * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact, + * if it is free (and it can be) we want that anyone that manages + * taking it, find all the initializations we've done above in place. + */ + smp_mb(); + per_cpu(schedule_data, cpu).schedule_lock = &prv->rqd[rqi].lock; + + spin_unlock_irq(&prv->lock); +} + static void csched2_free_pdata(const struct scheduler *ops, void *pcpu, int cpu) { unsigned long flags; struct csched2_private *prv = CSCHED2_PRIV(ops); struct csched2_runqueue_data *rqd; - struct schedule_data *sd = &per_cpu(schedule_data, cpu); int rqi; spin_lock_irqsave(&prv->lock, flags); @@ -2072,11 +2117,6 @@ csched2_free_pdata(const struct scheduler *ops, void *pcpu, int cpu) deactivate_runqueue(prv, rqi); } - /* Move spinlock to the original lock. */ - ASSERT(sd->schedule_lock == &rqd->lock); - ASSERT(!spin_is_locked(&sd->_lock)); - sd->schedule_lock = &sd->_lock; - spin_unlock(&rqd->lock); cpumask_clear_cpu(cpu, &prv->initialized); @@ -2170,6 +2210,7 @@ static const struct scheduler sched_credit2_def = { .free_vdata = csched2_free_vdata, .init_pdata = csched2_init_pdata, .free_pdata = csched2_free_pdata, + .switch_sched = csched2_switch_sched, .alloc_domdata = csched2_alloc_domdata, .free_domdata = csched2_free_domdata, }; diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c index b96bd93..3bb8c71 100644 --- a/xen/common/sched_rt.c +++ b/xen/common/sched_rt.c @@ -682,6 +682,37 @@ rt_init_pdata(const struct scheduler *ops, void *pdata, int cpu) spin_unlock_irqrestore(old_lock, flags); } +/* Change the scheduler of cpu to us (RTDS). */ +static void +rt_switch_sched(struct scheduler *new_ops, unsigned int cpu, + void *pdata, void *vdata) +{ + struct rt_private *prv = rt_priv(new_ops); + struct rt_vcpu *svc = vdata; + + ASSERT(!pdata && svc && is_idle_vcpu(svc->vcpu)); + + /* + * We are holding the runqueue lock already (it's been taken in + * schedule_cpu_switch()). It's actually the runqueue lock of + * another scheduler, but that is how things need to be, for + * preventing races. + */ + ASSERT(per_cpu(schedule_data, cpu).schedule_lock != &prv->lock); + + idle_vcpu[cpu]->sched_priv = vdata; + per_cpu(scheduler, cpu) = new_ops; + per_cpu(schedule_data, cpu).sched_priv = NULL; /* no pdata */ + + /* + * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact, + * if it is free (and it can be) we want that anyone that manages + * taking it, find all the initializations we've done above in place. + */ + smp_mb(); + per_cpu(schedule_data, cpu).schedule_lock = &prv->lock; +} + static void * rt_alloc_pdata(const struct scheduler *ops, int cpu) { @@ -707,19 +738,6 @@ rt_alloc_pdata(const struct scheduler *ops, int cpu) static void rt_free_pdata(const struct scheduler *ops, void *pcpu, int cpu) { - struct rt_private *prv = rt_priv(ops); - struct schedule_data *sd = &per_cpu(schedule_data, cpu); - unsigned long flags; - - spin_lock_irqsave(&prv->lock, flags); - - /* Move spinlock back to the default lock */ - ASSERT(sd->schedule_lock == &prv->lock); - ASSERT(!spin_is_locked(&sd->_lock)); - sd->schedule_lock = &sd->_lock; - - spin_unlock_irqrestore(&prv->lock, flags); - free_cpumask_var(_cpumask_scratch[cpu]); } @@ -1468,6 +1486,7 @@ static const struct scheduler sched_rtds_def = { .alloc_pdata = rt_alloc_pdata, .free_pdata = rt_free_pdata, .init_pdata = rt_init_pdata, + .switch_sched = rt_switch_sched, .alloc_domdata = rt_alloc_domdata, .free_domdata = rt_free_domdata, .init_domain = rt_dom_init, diff --git a/xen/common/schedule.c b/xen/common/schedule.c index 1941613..5559aa1 100644 --- a/xen/common/schedule.c +++ b/xen/common/schedule.c @@ -1635,11 +1635,11 @@ void __init scheduler_init(void) int schedule_cpu_switch(unsigned int cpu, struct cpupool *c) { struct vcpu *idle; - spinlock_t *lock; void *ppriv, *ppriv_old, *vpriv, *vpriv_old; struct scheduler *old_ops = per_cpu(scheduler, cpu); struct scheduler *new_ops = (c == NULL) ? &ops : c->sched; struct cpupool *old_pool = per_cpu(cpupool, cpu); + spinlock_t * old_lock; /* * pCPUs only move from a valid cpupool to free (i.e., out of any pool), @@ -1658,11 +1658,21 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c) if ( old_ops == new_ops ) goto out; + /* + * To setup the cpu for the new scheduler we need: + * - a valid instance of per-CPU scheduler specific data, as it is + * allocated by SCHED_OP(alloc_pdata). Note that we do not want to + * initialize it yet (i.e., we are not calling SCHED_OP(init_pdata)). + * That will be done by the target scheduler, in SCHED_OP(switch_sched), + * in proper ordering and with locking. + * - a valid instance of per-vCPU scheduler specific data, for the idle + * vCPU of cpu. That is what the target scheduler will use for the + * sched_priv field of the per-vCPU info of the idle domain. + */ idle = idle_vcpu[cpu]; ppriv = SCHED_OP(new_ops, alloc_pdata, cpu); if ( IS_ERR(ppriv) ) return PTR_ERR(ppriv); - SCHED_OP(new_ops, init_pdata, ppriv, cpu); vpriv = SCHED_OP(new_ops, alloc_vdata, idle, idle->domain->sched_priv); if ( vpriv == NULL ) { @@ -1670,17 +1680,30 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c) return -ENOMEM; } - lock = pcpu_schedule_lock_irq(cpu); - SCHED_OP(old_ops, tick_suspend, cpu); + + /* + * The actual switch, including (if necessary) the rerouting of the + * scheduler lock to whatever new_ops prefers, needs to happen in one + * critical section, protected by old_ops' lock, or races are possible. + * It is, in fact, the lock of another scheduler that we are taking (the + * scheduler of the cpupool that cpu still belongs to). But that is ok + * as, anyone trying to schedule on this cpu will spin until when we + * release that lock (bottom of this function). When he'll get the lock + * --thanks to the loop inside *_schedule_lock() functions-- he'll notice + * that the lock itself changed, and retry acquiring the new one (which + * will be the correct, remapped one, at that point). + */ + old_lock = pcpu_schedule_lock(cpu); + vpriv_old = idle->sched_priv; - idle->sched_priv = vpriv; - per_cpu(scheduler, cpu) = new_ops; ppriv_old = per_cpu(schedule_data, cpu).sched_priv; - per_cpu(schedule_data, cpu).sched_priv = ppriv; - SCHED_OP(new_ops, tick_resume, cpu); + SCHED_OP(new_ops, switch_sched, cpu, ppriv, vpriv); - pcpu_schedule_unlock_irq(lock, cpu); + /* _Not_ pcpu_schedule_unlock(): schedule_lock may have changed! */ + spin_unlock_irq(old_lock); + + SCHED_OP(new_ops, tick_resume, cpu); SCHED_OP(old_ops, free_vdata, vpriv_old); SCHED_OP(old_ops, free_pdata, ppriv_old, cpu); diff --git a/xen/include/xen/sched-if.h b/xen/include/xen/sched-if.h index 70c08c6..9cebe41 100644 --- a/xen/include/xen/sched-if.h +++ b/xen/include/xen/sched-if.h @@ -137,6 +137,9 @@ struct scheduler { void (*free_domdata) (const struct scheduler *, void *); void * (*alloc_domdata) (const struct scheduler *, struct domain *); + void (*switch_sched) (struct scheduler *, unsigned int, + void *, void *); + int (*init_domain) (const struct scheduler *, struct domain *); void (*destroy_domain) (const struct scheduler *, struct domain *);
In short, the point is making sure that the actual switch of scheduler and the remapping of the scheduler's runqueue lock occur in the same critical section, protected by the "old" scheduler's lock (and not, e.g., in the free_pdata hook, as it is now for Credit2 and RTDS). Not doing so, is (at least) racy. In fact, for instance, if we switch cpu X from, Credit2 to Credit, we do: schedule_cpu_switch(x, csched2 --> csched): //scheduler[x] is csched2 //schedule_lock[x] is csched2_lock csched_alloc_pdata(x) csched_init_pdata(x) pcpu_schedule_lock(x) ----> takes csched2_lock scheduler[X] = csched pcpu_schedule_unlock(x) --> unlocks csched2_lock [1] csched2_free_pdata(x) pcpu_schedule_lock(x) --> takes csched2_lock schedule_lock[x] = csched_lock spin_unlock(csched2_lock) While, if we switch cpu X from, Credit to Credit2, we do: schedule_cpu_switch(X, csched --> csched2): //scheduler[x] is csched //schedule_lock[x] is csched_lock csched2_alloc_pdata(x) csched2_init_pdata(x) pcpu_schedule_lock(x) --> takes csched_lock schedule_lock[x] = csched2_lock spin_unlock(csched_lock) [2] pcpu_schedule_lock(x) ----> takes csched2_lock scheduler[X] = csched2 pcpu_schedule_unlock(x) --> unlocks csched2_lock csched_free_pdata(x) And if we switch cpu X from RTDS to Credit2, we do: schedule_cpu_switch(X, RTDS --> csched2): //scheduler[x] is rtds //schedule_lock[x] is rtds_lock csched2_alloc_pdata(x) csched2_init_pdata(x) pcpu_schedule_lock(x) --> takes rtds_lock schedule_lock[x] = csched2_lock spin_unlock(rtds_lock) pcpu_schedule_lock(x) ----> takes csched2_lock scheduler[x] = csched2 pcpu_schedule_unlock(x) --> unlocks csched2_lock rtds_free_pdata(x) spin_lock(rtds_lock) ASSERT(schedule_lock[x] == rtds_lock) [3] schedule_lock[x] = DEFAULT_SCHEDULE_LOCK [4] spin_unlock(rtds_lock) So, the first problem is that, if anything related to scheduling, and involving CPU, happens at [1] or [2], we: - take csched2_lock, - operate on Credit1 functions and data structures, which is no good! The second problem is that the ASSERT at [3] triggers, and the third that at [4], we screw up the lock remapping we've done for ourself in csched2_init_pdata()! The first problem arises because there is a window during which the lock is already the new one, but the scheduler is still the old one. The other two, becase we let schedulers mess with the lock (re)mapping done by others. This patch, therefore, introduces a new hook in the scheduler interface, called switch_sched, meant at being used when switching scheduler on a CPU, and implements it for the various schedulers (that needs it: i.e., all except ARINC653), so that things are done in the proper order and under the protection of the best suited (set of) lock(s). It is necessary to add the hook (as compared to keep doing things in generic code), because different schedulers may have different locking schemes. Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com> --- Cc: George Dunlap <george.dunlap@eu.citrix.com> Cc: Meng Xu <mengxu@cis.upenn.edu> Cc: Tianyang Chen <tiche@seas.upenn.edu> --- Changes from v1: new patch, basically, coming from squashing what were 4 patches in v1. In any case, with respect to those 4 patches: - runqueue lock is back being taken in schedule_cpu_switch(), as suggested during review; - add barriers for making sure all initialization is done when the new lock is assigned, as sugested during review; - add comments and ASSERT-s about how and why the adopted locking scheme is safe, as suggested during review. --- xen/common/sched_credit.c | 44 ++++++++++++++++++++++++ xen/common/sched_credit2.c | 81 +++++++++++++++++++++++++++++++++----------- xen/common/sched_rt.c | 45 +++++++++++++++++------- xen/common/schedule.c | 41 +++++++++++++++++----- xen/include/xen/sched-if.h | 3 ++ 5 files changed, 172 insertions(+), 42 deletions(-)