| Message ID | 20200303123058.27210-1-jgross@suse.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | xen/sched: fix cpu offlining with core scheduling | expand |
On 03.03.2020 13:30, Juergen Gross wrote: > @@ -2538,7 +2552,10 @@ static void sched_slave(void) > > next = sched_wait_rendezvous_in(prev, &lock, cpu, now); > if ( !next ) > + { > + rcu_read_unlock(&sched_res_rculock); > return; > + } This and ... > @@ -2599,7 +2616,10 @@ static void schedule(void) > cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ); > next = sched_wait_rendezvous_in(prev, &lock, cpu, now); > if ( !next ) > + { > + rcu_read_unlock(&sched_res_rculock); > return; > + } ... this look like independent fixes, as on Arm, sched_wait_rendezvous_in() can already return NULL. If they get folded into here, I think the description should clarify that these are orthogonal to the rest. Jan
On 03.03.20 14:45, Jan Beulich wrote: > On 03.03.2020 13:30, Juergen Gross wrote: >> @@ -2538,7 +2552,10 @@ static void sched_slave(void) >> >> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >> if ( !next ) >> + { >> + rcu_read_unlock(&sched_res_rculock); >> return; >> + } > > This and ... > >> @@ -2599,7 +2616,10 @@ static void schedule(void) >> cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ); >> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >> if ( !next ) >> + { >> + rcu_read_unlock(&sched_res_rculock); >> return; >> + } > > ... this look like independent fixes, as on Arm, > sched_wait_rendezvous_in() can already return NULL. If they get > folded into here, I think the description should clarify that > these are orthogonal to the rest. Yeah, probably better to split the patch. Juergen
On 03.03.20 17:05, Jürgen Groß wrote: > On 03.03.20 14:45, Jan Beulich wrote: >> On 03.03.2020 13:30, Juergen Gross wrote: >>> @@ -2538,7 +2552,10 @@ static void sched_slave(void) >>> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >>> if ( !next ) >>> + { >>> + rcu_read_unlock(&sched_res_rculock); >>> return; >>> + } >> >> This and ... >> >>> @@ -2599,7 +2616,10 @@ static void schedule(void) >>> cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ); >>> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >>> if ( !next ) >>> + { >>> + rcu_read_unlock(&sched_res_rculock); >>> return; >>> + } >> >> ... this look like independent fixes, as on Arm, >> sched_wait_rendezvous_in() can already return NULL. If they get >> folded into here, I think the description should clarify that >> these are orthogonal to the rest. > > Yeah, probably better to split the patch. Oh, this patch was wrong: Up to now sched_wait_rendezvous_in() always was responsible for dropping sched_res_rculock, so I should do that in the new NULL return case, too. So no patch splitting, but V2. Juergen
On 03.03.2020 17:20, Jürgen Groß wrote: > On 03.03.20 17:05, Jürgen Groß wrote: >> On 03.03.20 14:45, Jan Beulich wrote: >>> On 03.03.2020 13:30, Juergen Gross wrote: >>>> @@ -2538,7 +2552,10 @@ static void sched_slave(void) >>>> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >>>> if ( !next ) >>>> + { >>>> + rcu_read_unlock(&sched_res_rculock); >>>> return; >>>> + } >>> >>> This and ... >>> >>>> @@ -2599,7 +2616,10 @@ static void schedule(void) >>>> cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ); >>>> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >>>> if ( !next ) >>>> + { >>>> + rcu_read_unlock(&sched_res_rculock); >>>> return; >>>> + } >>> >>> ... this look like independent fixes, as on Arm, >>> sched_wait_rendezvous_in() can already return NULL. If they get >>> folded into here, I think the description should clarify that >>> these are orthogonal to the rest. >> >> Yeah, probably better to split the patch. > > Oh, this patch was wrong: Up to now sched_wait_rendezvous_in() always > was responsible for dropping sched_res_rculock, so I should do that in > the new NULL return case, too. Oh, through calling of sched_context_switch(). I guess both functions want to gain a comment about this aspect of their behavior. Jan
On 03.03.20 17:27, Jan Beulich wrote: > On 03.03.2020 17:20, Jürgen Groß wrote: >> On 03.03.20 17:05, Jürgen Groß wrote: >>> On 03.03.20 14:45, Jan Beulich wrote: >>>> On 03.03.2020 13:30, Juergen Gross wrote: >>>>> @@ -2538,7 +2552,10 @@ static void sched_slave(void) >>>>> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >>>>> if ( !next ) >>>>> + { >>>>> + rcu_read_unlock(&sched_res_rculock); >>>>> return; >>>>> + } >>>> >>>> This and ... >>>> >>>>> @@ -2599,7 +2616,10 @@ static void schedule(void) >>>>> cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ); >>>>> next = sched_wait_rendezvous_in(prev, &lock, cpu, now); >>>>> if ( !next ) >>>>> + { >>>>> + rcu_read_unlock(&sched_res_rculock); >>>>> return; >>>>> + } >>>> >>>> ... this look like independent fixes, as on Arm, >>>> sched_wait_rendezvous_in() can already return NULL. If they get >>>> folded into here, I think the description should clarify that >>>> these are orthogonal to the rest. >>> >>> Yeah, probably better to split the patch. >> >> Oh, this patch was wrong: Up to now sched_wait_rendezvous_in() always >> was responsible for dropping sched_res_rculock, so I should do that in >> the new NULL return case, too. > > Oh, through calling of sched_context_switch(). I guess both functions > want to gain a comment about this aspect of their behavior. Yes, already done. :-) Juergen
diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c index 7e8e7d2c39..723283ed00 100644 --- a/xen/common/sched/core.c +++ b/xen/common/sched/core.c @@ -2415,7 +2415,8 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev, { struct sched_unit *next; struct vcpu *v; - unsigned int gran = get_sched_res(cpu)->granularity; + struct sched_resource *sr = get_sched_res(cpu); + unsigned int gran = sr->granularity; if ( !--prev->rendezvous_in_cnt ) { @@ -2482,6 +2483,19 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev, atomic_set(&prev->next_task->rendezvous_out_cnt, 0); prev->rendezvous_in_cnt = 0; } + + /* + * Check for scheduling resourced switched. This happens when we are + * moved away from our cpupool and cpus are subject of the idle + * scheduler now. + */ + if ( unlikely(sr != get_sched_res(cpu)) ) + { + ASSERT(is_idle_unit(prev)); + atomic_set(&prev->next_task->rendezvous_out_cnt, 0); + prev->rendezvous_in_cnt = 0; + return NULL; + } } return prev->next_task; @@ -2538,7 +2552,10 @@ static void sched_slave(void) next = sched_wait_rendezvous_in(prev, &lock, cpu, now); if ( !next ) + { + rcu_read_unlock(&sched_res_rculock); return; + } pcpu_schedule_unlock_irq(lock, cpu); @@ -2567,11 +2584,11 @@ static void schedule(void) rcu_read_lock(&sched_res_rculock); + lock = pcpu_schedule_lock_irq(cpu); + sr = get_sched_res(cpu); gran = sr->granularity; - lock = pcpu_schedule_lock_irq(cpu); - if ( prev->rendezvous_in_cnt ) { /* @@ -2599,7 +2616,10 @@ static void schedule(void) cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ); next = sched_wait_rendezvous_in(prev, &lock, cpu, now); if ( !next ) + { + rcu_read_unlock(&sched_res_rculock); return; + } } else { @@ -3151,7 +3171,10 @@ int schedule_cpu_rm(unsigned int cpu) per_cpu(sched_res_idx, cpu_iter) = 0; if ( cpu_iter == cpu ) { - idle_vcpu[cpu_iter]->sched_unit->priv = NULL; + unit = idle_vcpu[cpu_iter]->sched_unit; + unit->priv = NULL; + atomic_set(&unit->next_task->rendezvous_out_cnt, 0); + unit->rendezvous_in_cnt = 0; } else { @@ -3182,6 +3205,8 @@ int schedule_cpu_rm(unsigned int cpu) } sr->scheduler = &sched_idle_ops; sr->sched_priv = NULL; + sr->granularity = 1; + sr->cpupool = NULL; smp_mb(); sr->schedule_lock = &sched_free_cpu_lock; @@ -3194,9 +3219,6 @@ int schedule_cpu_rm(unsigned int cpu) sched_free_udata(old_ops, vpriv_old); sched_free_pdata(old_ops, ppriv_old, cpu); - sr->granularity = 1; - sr->cpupool = NULL; - out: rcu_read_unlock(&sched_res_rculock); xfree(sr_new);
Offlining a cpu with core scheduling active can result in a hanging system. Reason is the scheduling resource and unit of the to be removed cpus needs to be split in order to remove the cpu from its cpupool and move it to the idle scheduler. In case one of the involved cpus happens to have received a sched slave event due to a vcpu former having been running on that cpu being woken up again, it can happen that this cpu will enter sched_wait_rendezvous_in() while its scheduling resource is just about to be split. It might wait for ever for the other sibling to join, which will never happen due to the resources already being modified. This can easily be avoided by: - resetting the rendezvous counters of the idle unit which is kept - checking for a new scheduling resource in sched_wait_rendezvous_in() after reacquiring the scheduling lock and resetting the counters in that case without scheduling another vcpu - moving schedule resource modifications (in schedule_cpu_rm()) and retrieving (schedule(), sched_slave() is fine already, others are not critical) into locked regions Reported-by: Igor Druzhinin <igor.druzhinin@citrix.com> Signed-off-by: Juergen Gross <jgross@suse.com> --- xen/common/sched/core.c | 36 +++++++++++++++++++++++++++++------- 1 file changed, 29 insertions(+), 7 deletions(-)