From: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
To: "Jürgen Groß" <jgross@suse.com>
Cc: George Dunlap <george.dunlap@citrix.com>,
Dario Faggioli <dfaggioli@suse.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [RFC PATCH 01/10] sched: core: save IRQ state during locking
Date: Tue, 23 Feb 2021 11:15:00 +0000 [thread overview]
Message-ID: <877dmz6nnv.fsf@epam.com> (raw)
In-Reply-To: <4f7b4788-3b2e-8501-6aec-948b70320af2@suse.com>
Hi Jurgen,
Jürgen Groß writes:
> On 23.02.21 03:34, Volodymyr Babchuk wrote:
>> With XEN preemption enabled, scheduler functions can be called with
>> IRQs disabled (for example, at end of IRQ handler), so we should
>> save and restore IRQ state in schedulers code.
>
> This breaks core scheduling.
Yes, thank you. I forgot to mention that this PoC is not compatible with
core scheduling. It is not used on ARM, so I could not test it anyways.
> Waiting for another sibling with interrupts disabled is an absolute
> no go, as deadlocks are the consequence.
>
> You could (in theory) make preemption and core scheduling mutually
> exclusive, but this would break the forward path to mutexes etc.
>
Well, I implemented the most naive way to enable hypervisor
preemption. I'm sure that with a bit more careful approach I can make it
compatible with core scheduling. There is no strict requirement to run
scheduler with IRQs disabled.
>
> Juergen
>
>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
>> ---
>> xen/common/sched/core.c | 33 ++++++++++++++++++---------------
>> 1 file changed, 18 insertions(+), 15 deletions(-)
>> diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
>> index 9745a77eee..7e075613d5 100644
>> --- a/xen/common/sched/core.c
>> +++ b/xen/common/sched/core.c
>> @@ -2470,7 +2470,8 @@ static struct vcpu *sched_force_context_switch(struct vcpu *vprev,
>> * sched_res_rculock has been dropped.
>> */
>> static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
>> - spinlock_t **lock, int cpu,
>> + spinlock_t **lock,
>> + unsigned long *flags, int cpu,
>> s_time_t now)
>> {
>> struct sched_unit *next;
>> @@ -2500,7 +2501,7 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
>> prev->rendezvous_in_cnt++;
>> atomic_set(&prev->rendezvous_out_cnt, 0);
>> - pcpu_schedule_unlock_irq(*lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(*lock, *flags, cpu);
>> sched_context_switch(vprev, v, false, now);
>> @@ -2530,7 +2531,7 @@ static struct sched_unit
>> *sched_wait_rendezvous_in(struct sched_unit *prev,
>> prev->rendezvous_in_cnt++;
>> atomic_set(&prev->rendezvous_out_cnt, 0);
>> - pcpu_schedule_unlock_irq(*lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(*lock, *flags, cpu);
>> raise_softirq(SCHED_SLAVE_SOFTIRQ);
>> sched_context_switch(vprev, vprev, false, now);
>> @@ -2538,11 +2539,11 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
>> return NULL; /* ARM only. */
>> }
>> - pcpu_schedule_unlock_irq(*lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(*lock, *flags, cpu);
>> cpu_relax();
>> - *lock = pcpu_schedule_lock_irq(cpu);
>> + *lock = pcpu_schedule_lock_irqsave(cpu, flags);
>> /*
>> * Check for scheduling resource switched. This happens when we are
>> @@ -2557,7 +2558,7 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
>> ASSERT(is_idle_unit(prev));
>> atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
>> prev->rendezvous_in_cnt = 0;
>> - pcpu_schedule_unlock_irq(*lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(*lock, *flags, cpu);
>> rcu_read_unlock(&sched_res_rculock);
>> return NULL;
>> }
>> @@ -2574,12 +2575,13 @@ static void sched_slave(void)
>> spinlock_t *lock;
>> bool do_softirq = false;
>> unsigned int cpu = smp_processor_id();
>> + unsigned long flags;
>> ASSERT_NOT_IN_ATOMIC();
>> rcu_read_lock(&sched_res_rculock);
>> - lock = pcpu_schedule_lock_irq(cpu);
>> + lock = pcpu_schedule_lock_irqsave(cpu, &flags);
>> now = NOW();
>> @@ -2590,7 +2592,7 @@ static void sched_slave(void)
>> if ( v )
>> {
>> - pcpu_schedule_unlock_irq(lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(lock, flags, cpu);
>> sched_context_switch(vprev, v, false, now);
>> @@ -2602,7 +2604,7 @@ static void sched_slave(void)
>> if ( !prev->rendezvous_in_cnt )
>> {
>> - pcpu_schedule_unlock_irq(lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(lock, flags, cpu);
>> rcu_read_unlock(&sched_res_rculock);
>> @@ -2615,11 +2617,11 @@ static void sched_slave(void)
>> stop_timer(&get_sched_res(cpu)->s_timer);
>> - next = sched_wait_rendezvous_in(prev, &lock, cpu, now);
>> + next = sched_wait_rendezvous_in(prev, &lock, &flags, cpu, now);
>> if ( !next )
>> return;
>> - pcpu_schedule_unlock_irq(lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(lock, flags, cpu);
>> sched_context_switch(vprev, sched_unit2vcpu_cpu(next, cpu),
>> is_idle_unit(next) && !is_idle_unit(prev), now);
>> @@ -2637,6 +2639,7 @@ static void schedule(void)
>> s_time_t now;
>> struct sched_resource *sr;
>> spinlock_t *lock;
>> + unsigned long flags;
>> int cpu = smp_processor_id();
>> unsigned int gran;
>> @@ -2646,7 +2649,7 @@ static void schedule(void)
>> rcu_read_lock(&sched_res_rculock);
>> - lock = pcpu_schedule_lock_irq(cpu);
>> + lock = pcpu_schedule_lock_irqsave(cpu, &flags);
>> sr = get_sched_res(cpu);
>> gran = sr->granularity;
>> @@ -2657,7 +2660,7 @@ static void schedule(void)
>> * We have a race: sched_slave() should be called, so raise a softirq
>> * in order to re-enter schedule() later and call sched_slave() now.
>> */
>> - pcpu_schedule_unlock_irq(lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(lock, flags, cpu);
>> rcu_read_unlock(&sched_res_rculock);
>> @@ -2676,7 +2679,7 @@ static void schedule(void)
>> prev->rendezvous_in_cnt = gran;
>> cpumask_andnot(mask, sr->cpus, cpumask_of(cpu));
>> cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ);
>> - next = sched_wait_rendezvous_in(prev, &lock, cpu, now);
>> + next = sched_wait_rendezvous_in(prev, &lock, &flags, cpu, now);
>> if ( !next )
>> return;
>> }
>> @@ -2687,7 +2690,7 @@ static void schedule(void)
>> atomic_set(&next->rendezvous_out_cnt, 0);
>> }
>> - pcpu_schedule_unlock_irq(lock, cpu);
>> + pcpu_schedule_unlock_irqrestore(lock, flags, cpu);
>> vnext = sched_unit2vcpu_cpu(next, cpu);
>> sched_context_switch(vprev, vnext,
>>
--
Volodymyr Babchuk at EPAM
next prev parent reply other threads:[~2021-02-23 11:15 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-23 2:34 [RFC PATCH 00/10] Preemption in hypervisor (ARM only) Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 01/10] sched: core: save IRQ state during locking Volodymyr Babchuk
2021-02-23 8:52 ` Jürgen Groß
2021-02-23 11:15 ` Volodymyr Babchuk [this message]
2021-02-24 18:29 ` Andrew Cooper
2021-02-23 2:34 ` [RFC PATCH 03/10] sched: credit2: " Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 02/10] sched: rt: " Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 04/10] preempt: use atomic_t to for preempt_count Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 05/10] preempt: add try_preempt() function Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 07/10] sched: core: remove ASSERT_NOT_IN_ATOMIC and disable preemption[!] Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 06/10] arm: setup: disable preemption during startup Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 08/10] arm: context_switch: allow to run with IRQs already disabled Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 10/10] [HACK] alloc pages: enable preemption early Volodymyr Babchuk
2021-02-23 2:34 ` [RFC PATCH 09/10] arm: traps: try to preempt before leaving IRQ handler Volodymyr Babchuk
2021-02-23 9:02 ` [RFC PATCH 00/10] Preemption in hypervisor (ARM only) Julien Grall
2021-02-23 12:06 ` Volodymyr Babchuk
2021-02-24 10:08 ` Julien Grall
2021-02-24 20:57 ` Volodymyr Babchuk
2021-02-24 22:31 ` Julien Grall
2021-02-24 23:58 ` Volodymyr Babchuk
2021-02-25 0:39 ` Andrew Cooper
2021-02-25 12:51 ` Volodymyr Babchuk
2021-03-05 9:31 ` Volodymyr Babchuk
2021-02-24 18:07 ` Andrew Cooper
2021-02-24 23:37 ` Volodymyr Babchuk
2021-03-01 14:39 ` George Dunlap
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