Re: [GIT PULL] First batch of KVM changes for 4.1

Andy Lutomirski
2015-04-17T19:17:04Z

On Fri, Apr 17, 2015 at 12:01 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote:
> On Fri, Apr 17, 2015 at 03:38:58PM +0200, Paolo Bonzini wrote:
>> On 17/04/2015 15:10, Peter Zijlstra wrote:
>> > On Fri, Apr 17, 2015 at 02:46:57PM +0200, Paolo Bonzini wrote:
>> >> On 17/04/2015 12:55, Peter Zijlstra wrote:
>> >>> Also, it looks like you already do exactly this for other things, look
>> >>> at:
>> >>>
>> >>> kvm_sched_in()
>> >>> kvm_arch_vcpu_load()
>> >>> if (unlikely(vcpu->cpu != cpu) ... )
>> >>>
>> >>> So no, I don't believe for one second you need this.
>> >
>> > This [...] brings us back to where we were last
>> > time. There is _0_ justification for this in the patches, that alone is
>> > grounds enough to reject it.
>>
>> Oh, we totally agree on that. I didn't commit that patch, but I already
>> said the commit message was insufficient.
>>
>> > Why should the guest task care about the physical cpu of the vcpu;
>> > that's a layering fail if ever there was one.
>>
>> It's totally within your right to not read the code, but then please
>> don't try commenting at it.
>>
>> This code:
>>
>> kvm_sched_in()
>> kvm_arch_vcpu_load()
>> if (unlikely(vcpu->cpu != cpu) ... )
>>
>> runs in the host. The hypervisor obviously cares if the physical CPU of
>> the VCPU changes. It has to tell the source processor (vcpu->cpu) to
>> release the VCPU's data structure and only then it can use it in the
>> target processor (cpu). No layering violation here.
>>
>> The task migration notifier runs in the guest, whenever the VCPU of
>> a task changes.
>>
>> > Furthermore, the only thing that migration handler seems to do is
>> > increment a variable that is not actually used in that file.
>>
>> It's used in the vDSO, so you cannot increment it in the file that uses it.
>>
>> >> And frankly, I think the static key is snake oil. The cost of task
>> >> migration in terms of cache misses and TLB misses is in no way
>> >> comparable to the cost of filling in a structure on the stack,
>> >> dereferencing the head of the notifiers list and seeing that it's NULL.
>> >
>> > The path this notifier is called from has nothing to do with those
>> > costs.
>>
>> How not? The task is going to incur those costs, it's not like half
>> a dozen extra instruction make any difference. But anyway...
>>
>> > And the fact you're inflicting these costs on _everyone_ for a
>> > single x86_64-paravirt case is insane.
>>
>> ... that's a valid objection. Please look at the patch below.
>>
>> > I've had enough of this, the below goes into sched/urgent and you can
>> > come back with sane patches if and when you're ready.
>>
>> Oh, please, cut the alpha male crap.
>>
>> Paolo
>>
>> ------------------- 8< ----------------
>> >From 4eb9d7132e1990c0586f28af3103675416d38974 Mon Sep 17 00:00:00 2001
>> From: Paolo Bonzini <pbonzini@redhat.com>
>> Date: Fri, 17 Apr 2015 14:57:34 +0200
>> Subject: [PATCH] sched: add CONFIG_TASK_MIGRATION_NOTIFIER
>>
>> The task migration notifier is only used in x86 paravirt. Make it
>> possible to compile it out.
>>
>> While at it, move some code around to ensure tmn is filled from CPU
>> registers.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>> arch/x86/Kconfig | 1 +
>> init/Kconfig | 3 +++
>> kernel/sched/core.c | 9 ++++++++-
>> 3 files changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index d43e7e1c784b..9af252c8698d 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -649,6 +649,7 @@ if HYPERVISOR_GUEST
>>
>> config PARAVIRT
>> bool "Enable paravirtualization code"
>> + select TASK_MIGRATION_NOTIFIER
>> ---help---
>> This changes the kernel so it can modify itself when it is run
>> under a hypervisor, potentially improving performance significantly
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 3b9df1aa35db..891917123338 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -2016,6 +2016,9 @@ source "block/Kconfig"
>> config PREEMPT_NOTIFIERS
>> bool
>>
>> +config TASK_MIGRATION_NOTIFIER
>> + bool
>> +
>> config PADATA
>> depends on SMP
>> bool
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index f9123a82cbb6..c07a53aa543c 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1016,12 +1016,14 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
>> rq_clock_skip_update(rq, true);
>> }
>>
>> +#ifdef CONFIG_TASK_MIGRATION_NOTIFIER
>> static ATOMIC_NOTIFIER_HEAD(task_migration_notifier);
>>
>> void register_task_migration_notifier(struct notifier_block *n)
>> {
>> atomic_notifier_chain_register(&task_migration_notifier, n);
>> }
>> +#endif
>>
>> #ifdef CONFIG_SMP
>> void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>> @@ -1053,18 +1055,23 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>> trace_sched_migrate_task(p, new_cpu);
>>
>> if (task_cpu(p) != new_cpu) {
>> +#ifdef CONFIG_TASK_MIGRATION_NOTIFIER
>> struct task_migration_notifier tmn;
>> + int from_cpu = task_cpu(p);
>> +#endif
>>
>> if (p->sched_class->migrate_task_rq)
>> p->sched_class->migrate_task_rq(p, new_cpu);
>> p->se.nr_migrations++;
>> perf_sw_event_sched(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 0);
>>
>> +#ifdef CONFIG_TASK_MIGRATION_NOTIFIER
>> tmn.task = p;
>> - tmn.from_cpu = task_cpu(p);
>> + tmn.from_cpu = from_cpu;
>> tmn.to_cpu = new_cpu;
>>
>> atomic_notifier_call_chain(&task_migration_notifier, 0, &tmn);
>> +#endif
>> }
>>
>> __set_task_cpu(p, new_cpu);
>> --
>> 2.3.5
>
> Paolo,
>
> Please revert the patch -- can fix properly in the host
> which also conforms the KVM guest/host documented protocol.
>
> Radim submitted a patch to kvm@ to split
> the kvm_write_guest in two with a barrier in between, i think.
>
> I'll review that patch.
>

Can you cc me on that?

Thanks,
Andy

--
Andy Lutomirski
AMA Capital Management, LLC