Re: [PATCH 4/5] xen/cpupool: Create different cpupools at boot time

[Luca Fancellu <luca.fancellu@xxxxxxx>]

> On 16 Feb 2022, at 02:45, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> 
> On Tue, 15 Feb 2022, Luca Fancellu wrote:
>> Introduce an architecture specific way to create different cpupools
>> at boot time, this is particularly useful on ARM big.LITTLE system
>> where there might be the need to have different cpupools for each type
>> of core, but also systems using NUMA can have different cpu pools for
>> each node.
>> 
>> The feature on arm relies on a specification of the cpupools from the
>> device tree to build pools and assign cpus to them.
>> 
>> Documentation is created to explain the feature.
>> 
>> Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
>> ---
>> docs/misc/arm/device-tree/cpupools.txt | 118 +++++++++++++++++++++++++
>> xen/arch/arm/Kconfig                   |   9 ++
>> xen/arch/arm/Makefile                  |   1 +
>> xen/arch/arm/cpupool.c                 | 118 +++++++++++++++++++++++++
>> xen/common/sched/cpupool.c             |   4 +-
>> xen/include/xen/sched.h                |  11 +++
>> 6 files changed, 260 insertions(+), 1 deletion(-)
>> create mode 100644 docs/misc/arm/device-tree/cpupools.txt
>> create mode 100644 xen/arch/arm/cpupool.c
>> 
>> diff --git a/docs/misc/arm/device-tree/cpupools.txt 
>> b/docs/misc/arm/device-tree/cpupools.txt
>> new file mode 100644
>> index 000000000000..7298b6394332
>> --- /dev/null
>> +++ b/docs/misc/arm/device-tree/cpupools.txt
>> @@ -0,0 +1,118 @@
>> +Boot time cpupools
>> +==================
>> +
>> +On arm, when BOOT_TIME_CPUPOOLS is enabled in the Xen configuration, it is
>> +possible to create cpupools during boot phase by specifying them in the 
>> device
>> +tree.
>> +
>> +Cpupools specification nodes shall be direct childs of /chosen node.
>> +Each cpupool node contains the following properties:
>> +
>> +- compatible (mandatory)
>> +
>> +    Must always include the compatiblity string: "xen,cpupool".
>> +
>> +- cpupool-id (mandatory)
>> +
>> +    Must be a positive integer number.
> 

Hi Stefano,

Thank you for your review,

> Why is cpupool-id mandatory? It looks like it could be generated by Xen.
> Or is it actually better to have the user specify it anyway?
> 

Yes at first I thought to automatically generate that, however I needed a 
structure
to map the id to the cpupool DT node. Here my doubt was about the size of the
structure, because the user could even specify a cpupool for each cpu. I could 
allocate
It dynamically and free it after domUs creation in setup_xen.
What do you think could be the right way?
Or the dom0less guest could specify the id, but I like it more when using a 
phandle to the
Xen,cpupool node.

> 
>> +- cpupool-cpus (mandatory)
>> +
>> +    Must be a list of device tree phandle to nodes describing cpus (e.g. 
>> having
>> +    device_type = "cpu"), it can't be empty.
>> +
>> +- cpupool-sched (optional)
>> +
>> +    Must be a string having the name of a Xen scheduler, it has no effect 
>> when
>> +    used in conjunction of a cpupool-id equal to zero, in that case the
>> +    default Xen scheduler is selected (sched=<...> boot argument).
> 
> I don't get why cpupool-id == 0 should trigger a special cpupool-sched
> behavior.

Cpupool with id 0 is embedded in Xen, it has its own special case handling in 
cpupool_create
that is giving it the default scheduler. I thought it was better to leave it as 
it was, however the
cpupool0 scheduler can be modified using sched= boot args as it was before.

> 
> 
>> +Constraints
>> +===========
>> +
>> +The cpupool with id zero is implicitly created even if not specified, that 
>> pool
>> +must have at least one cpu assigned, otherwise Xen will stop.
>> +
>> +Every cpu brought up by Xen will be assigned to the cpupool with id zero if 
>> it's
>> +not assigned to any other cpupool.
>> +
>> +If a cpu is assigned to a cpupool, but it's not brought up correctly, Xen 
>> will
>> +stop.
> 
> Thank you for documenting the constraints, but why do we have them?
> Imagine a user specifying 3 cpu pools and imagine the cpupool-id is
> optional and missing. We could take care of the cpupool-id generation in
> Xen and we could also assign the default scheduler everywhere
> cpupool-sched is not specified. Maybe I am missing something?

Yes we could make the cpupool-id optional, my doubts are in the fist comment 
above.
Whenever the cpupool-sched is not specified, the current behaviour is to use 
the default scheduler.

> 
> Does cpupool0 has to exist? I guess the answer could be yes, but if it
> is specified as id of one of the pools we are fine, otherwise it could
> be automatically generated by Xen.

Yes cpupool0 needs to exists, however it is still generated by Xen regardless 
of the DT
specifications. In fact you could not specify in the DT any xen,cpupool 
compatible node
with the cpupool-id == 0 and Xen will generate the cpupool0 anyway
(Xen internals are tied with the existence of a cpupool0).

> 
> In any case, I don't think that cpupool0 has to have the default
> scheduler?

Ok I think I can create a function to assign a scheduler to the cpupool0 after 
its creation,
I would need to test it to be sure I don’t find something strange.

> 
> My suggestion would be:
> 
> - make cpupool-id optional
> - assign automatically cpupool-ids starting from 0
>    - respect cpupool-ids chosen by the user

Ok, it would start from 1 because cpupool0 always exists

> - if some CPUs are left out (not specified in any pool) add an extra cpupool
>    - the extra cpupool doesn't have to be cpupool-id == 0, it could be
>      cpupool-id == n
>    - the extra cpupool uses the default scheduler

I gave all the unassigned cpus to cpupool0 to reflect the current behaviour, so 
that
a user that doesn’t specify any xen,cpupool node ends up in a system reflecting 
the
current behaviour as the feature is not enabled.
However I can say, if no xen,cpupool nodes are found then assign cpus to 
cpupool0,
else assign them to a new cpupool and...

> 
> If the user created cpupools in device tree covering all CPUs and also
> specified all cpupool-ids everywhere, and none of them are 0 (no cpupool
> in the system is cpupool0) then panic. (Assuming that cpupool0 is
> required.)

… panic if cpupool0 has no cpus.


Cheers,
Luca


> 
> 
>> +Examples
>> +========
>> +
>> +A system having two types of core, the following device tree specification 
>> will
>> +instruct Xen to have two cpupools:
>> +
>> +- The cpupool with id 0 will have 4 cpus assigned.
>> +- The cpupool with id 1 will have 2 cpus assigned.
>> +
>> +As can be seen from the example, cpupool_a has only two cpus assigned, but 
>> since
>> +there are two cpus unassigned, they are automatically assigned to cpupool 
>> with
>> +id zero. The following example can work only if hmp-unsafe=1 is passed to 
>> Xen
>> +boot arguments, otherwise not all cores will be brought up by Xen and the
>> +cpupool creation process will stop Xen.
>> +
>> +
>> +a72_1: cpu@0 {
>> +        compatible = "arm,cortex-a72";
>> +        reg = <0x0 0x0>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +a72_2: cpu@1 {
>> +        compatible = "arm,cortex-a72";
>> +        reg = <0x0 0x1>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +a53_1: cpu@100 {
>> +        compatible = "arm,cortex-a53";
>> +        reg = <0x0 0x100>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +a53_2: cpu@101 {
>> +        compatible = "arm,cortex-a53";
>> +        reg = <0x0 0x101>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +cpu@102 {
>> +        compatible = "arm,cortex-a53";
>> +        reg = <0x0 0x102>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +cpu@103 {
>> +        compatible = "arm,cortex-a53";
>> +        reg = <0x0 0x103>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +chosen {
>> +
>> +    cpupool_a {
>> +        compatible = "xen,cpupool";
>> +        cpupool-id = <0>;
>> +        cpupool-cpus = <&a53_1 &a53_2>;
>> +    };
>> +    cpupool_b {
>> +        compatible = "xen,cpupool";
>> +        cpupool-id = <1>;
>> +        cpupool-cpus = <&a72_1 &a72_2>;
>> +        cpupool-sched = "credit2";
>> +    };
> 
> Question above notwithstanding, I like it!
> 
> 
>> +    [...]
>> +
>> +};
>> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
>> index ecfa6822e4d3..64c2879513b7 100644
>> --- a/xen/arch/arm/Kconfig
>> +++ b/xen/arch/arm/Kconfig
>> @@ -33,6 +33,15 @@ config ACPI
>>        Advanced Configuration and Power Interface (ACPI) support for Xen is
>>        an alternative to device tree on ARM64.
>> 
>> +config BOOT_TIME_CPUPOOLS
>> +    bool "Create cpupools at boot time"
>> +    depends on ARM
>> +    default n
>> +    help
>> +
>> +      Creates cpupools during boot time and assigns cpus to them. Cpupools
>> +      options can be specified in the device tree.
>> +
>> config GICV3
>>      bool "GICv3 driver"
>>      depends on ARM_64 && !NEW_VGIC
>> diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
>> index d0dee10102b6..6165da4e77b4 100644
>> --- a/xen/arch/arm/Makefile
>> +++ b/xen/arch/arm/Makefile
>> @@ -13,6 +13,7 @@ obj-$(CONFIG_HAS_ALTERNATIVE) += alternative.o
>> obj-y += bootfdt.init.o
>> obj-y += cpuerrata.o
>> obj-y += cpufeature.o
>> +obj-$(CONFIG_BOOT_TIME_CPUPOOLS) += cpupool.o
>> obj-y += decode.o
>> obj-y += device.o
>> obj-$(CONFIG_IOREQ_SERVER) += dm.o
>> diff --git a/xen/arch/arm/cpupool.c b/xen/arch/arm/cpupool.c
>> new file mode 100644
>> index 000000000000..a9d5b94635b9
>> --- /dev/null
>> +++ b/xen/arch/arm/cpupool.c
>> @@ -0,0 +1,118 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * xen/arch/arm/cpupool.c
>> + *
>> + * Code to create cpupools at boot time for arm architecture.
>> + *
>> + * Copyright (C) 2022 Arm Ltd.
>> + */
>> +
>> +#include <xen/sched.h>
>> +
>> +static struct cpupool *__initdata pool_cpu_map[NR_CPUS];
>> +
>> +void __init arch_allocate_cpupools(const cpumask_t *cpu_online_map)
>> +{
>> +    const struct dt_device_node *chosen, *node;
>> +    unsigned int cpu_num, cpupool0_cpu_count = 0;
>> +    cpumask_t cpus_to_assign;
>> +
>> +    chosen = dt_find_node_by_path("/chosen");
>> +    if ( !chosen )
>> +        return;
>> +
>> +    cpumask_copy(&cpus_to_assign, cpu_online_map);
>> +
>> +    dt_for_each_child_node(chosen, node)
>> +    {
>> +        const struct dt_device_node *cpu_node;
>> +        unsigned int pool_id;
>> +        int i = 0, sched_id = -1;
>> +        const char* scheduler_name;
>> +        struct cpupool *pool = cpupool0;
>> +
>> +        if ( !dt_device_is_compatible(node, "xen,cpupool") )
>> +            continue;
>> +
>> +        if ( !dt_property_read_u32(node, "cpupool-id", &pool_id) )
>> +            panic("Missing cpupool-id property!\n");
>> +
>> +        if ( !dt_property_read_string(node, "cpupool-sched", 
>> &scheduler_name) )
>> +        {
>> +            sched_id = sched_get_id_by_name(scheduler_name);
>> +            if ( sched_id < 0 )
>> +                panic("Scheduler %s does not exists!\n", scheduler_name);
>> +        }
>> +
>> +        if ( pool_id )
>> +        {
>> +            pool = cpupool_create_pool(pool_id, sched_id);
>> +            if ( !pool )
>> +                panic("Error creating pool id %u!\n", pool_id);
>> +        }
>> +
>> +        cpu_node = dt_parse_phandle(node, "cpupool-cpus", 0);
>> +        if ( !cpu_node )
>> +            panic("Missing or empty cpupool-cpus property!\n");
>> +
>> +        while ( cpu_node )
>> +        {
>> +            register_t cpu_reg;
>> +            const __be32 *prop;
>> +
>> +            prop = dt_get_property(cpu_node, "reg", NULL);
>> +            if ( !prop )
>> +                panic("cpupool-cpus pointed node has no reg property!\n");
>> +
>> +            cpu_reg = dt_read_number(prop, dt_n_addr_cells(cpu_node));
>> +
>> +            /* Check if the cpu is online and in the set to be assigned */
>> +            for_each_cpu ( cpu_num, &cpus_to_assign )
>> +                if ( cpu_logical_map(cpu_num) == cpu_reg )
>> +                    break;
>> +
>> +            if ( cpu_num >= nr_cpu_ids )
>> +                panic("Cpu found in %s is not online or it's assigned 
>> twice!\n",
>> +                      dt_node_name(node));
>> +
>> +            pool_cpu_map[cpu_num] = pool;
>> +            cpumask_clear_cpu(cpu_num, &cpus_to_assign);
>> +
>> +            printk(XENLOG_INFO "CPU with MPIDR %"PRIregister" in 
>> Pool-%u.\n",
>> +                   cpu_reg, pool_id);
>> +
>> +            /* Keep track of how many cpus are assigned to Pool-0 */
>> +            if ( !pool_id )
>> +                cpupool0_cpu_count++;
>> +
>> +            cpu_node = dt_parse_phandle(node, "cpupool-cpus", ++i);
>> +        }
>> +    }
>> +
>> +    /* Assign every non assigned cpu to Pool-0 */
>> +    for_each_cpu ( cpu_num, &cpus_to_assign )
>> +    {
>> +        pool_cpu_map[cpu_num] = cpupool0;
>> +        cpupool0_cpu_count++;
>> +        printk(XENLOG_INFO "CPU with MPIDR %"PRIregister" in Pool-0.\n",
>> +               cpu_logical_map(cpu_num));
>> +    }
>> +
>> +    if ( !cpupool0_cpu_count )
>> +        panic("No cpu assigned to cpupool0!\n");
>> +}
>> +
>> +struct cpupool *__init arch_get_cpupool(unsigned int cpu)
>> +{
>> +    return pool_cpu_map[cpu];
>> +}
>> +
>> +/*
>> + * Local variables:
>> + * mode: C
>> + * c-file-style: "BSD"
>> + * c-basic-offset: 4
>> + * tab-width: 4
>> + * indent-tabs-mode: nil
>> + * End:
>> + */
>> diff --git a/xen/common/sched/cpupool.c b/xen/common/sched/cpupool.c
>> index 4da12528d6b9..6013d75e2edd 100644
>> --- a/xen/common/sched/cpupool.c
>> +++ b/xen/common/sched/cpupool.c
>> @@ -1257,12 +1257,14 @@ static int __init cpupool_init(void)
>>     cpupool_put(cpupool0);
>>     register_cpu_notifier(&cpu_nfb);
>> 
>> +    arch_allocate_cpupools(&cpu_online_map);
>> +
>>     spin_lock(&cpupool_lock);
>> 
>>     cpumask_copy(&cpupool_free_cpus, &cpu_online_map);
>> 
>>     for_each_cpu ( cpu, &cpupool_free_cpus )
>> -        cpupool_assign_cpu_locked(cpupool0, cpu);
>> +        cpupool_assign_cpu_locked(arch_get_cpupool(cpu), cpu);
>> 
>>     spin_unlock(&cpupool_lock);
>> 
>> diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
>> index a67a9eb2fe9d..dda7db2ba51f 100644
>> --- a/xen/include/xen/sched.h
>> +++ b/xen/include/xen/sched.h
>> @@ -1177,6 +1177,17 @@ extern void dump_runq(unsigned char key);
>> 
>> void arch_do_physinfo(struct xen_sysctl_physinfo *pi);
>> 
>> +#ifdef CONFIG_BOOT_TIME_CPUPOOLS
>> +void arch_allocate_cpupools(const cpumask_t *cpu_online_map);
>> +struct cpupool *arch_get_cpupool(unsigned int cpu);
>> +#else
>> +static inline void arch_allocate_cpupools(const cpumask_t *cpu_online_map) 
>> {}
>> +static inline struct cpupool *arch_get_cpupool(unsigned int cpu)
>> +{
>> +    return cpupool0;
>> +}
>> +#endif
>> +
>> #endif /* __SCHED_H__ */
>> 
>> /*
>> -- 
>> 2.17.1