Re: [PATCH v2 4/6] xen/cpupool: Create different cpupools at boot time

[Luca Fancellu <Luca.Fancellu@xxxxxxx>]

Hi Stefano,

> On 11 Mar 2022, at 03:57, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> 
> On Thu, 10 Mar 2022, Luca Fancellu wrote:
>> Introduce a 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>
>> ---
>> Changes in v2:
>> - Move feature to common code (Juergen)
>> - Try to decouple dtb parse and cpupool creation to allow
>>  more way to specify cpupools (for example command line)
>> - Created standalone dt node for the scheduler so it can
>>  be used in future work to set scheduler specific
>>  parameters
>> - Use only auto generated ids for cpupools
>> ---
>> docs/misc/arm/device-tree/cpupools.txt | 156 ++++++++++++++++++
>> xen/common/Kconfig                     |   8 +
>> xen/common/Makefile                    |   1 +
>> xen/common/boot_cpupools.c             | 212 +++++++++++++++++++++++++
>> xen/common/sched/cpupool.c             |   6 +-
>> xen/include/xen/sched.h                |  19 +++
>> 6 files changed, 401 insertions(+), 1 deletion(-)
>> create mode 100644 docs/misc/arm/device-tree/cpupools.txt
>> create mode 100644 xen/common/boot_cpupools.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..d5a82ed0d45a
>> --- /dev/null
>> +++ b/docs/misc/arm/device-tree/cpupools.txt
>> @@ -0,0 +1,156 @@
>> +Boot time cpupools
>> +==================
>> +
>> +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-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 device tree phandle to a node having "xen,scheduler" 
>> compatible
>> +    (description below), it has no effect when the cpupool refers to the 
>> cpupool
>> +    number zero, in that case the default Xen scheduler is selected 
>> (sched=<...>
>> +    boot argument).
> 
> This is *a lot* better.
> 
> The device tree part is nice. I have only one question left on it: why
> do we need a separate scheduler node? Could the "cpupool-sched" property
> be a simple string with the scheduler name?
> 
> E.g.:
> 
>    cpupool_a {
>        compatible = "xen,cpupool";
>        cpupool-cpus = <&a53_1 &a53_2>;
>    };
>    cpupool_b {
>        compatible = "xen,cpupool";
>        cpupool-cpus = <&a72_1 &a72_2>;
>        cpupool-sched = "null";
>    };
> 
> 
> To me, it doesn't look like these new "scheduler specification nodes"
> bring any benefits. I would just get rid of them.

From a comment of Juergen on the second patch I thought someone sees the need to
have a way to set scheduling parameters:

“you are allowing to use another scheduler,
but what if someone wants to set non-standard scheduling parameters
(e.g. another time slice)?”

So I thought I could introduce a scheduler specification node that could in the 
future be
extended and used to set scheduling parameter.

If it is something that is not needed, I will get rid of it.

> 
> 
>> +A scheduler specification node is a device tree node that contains the 
>> following
>> +properties:
>> +
>> +- compatible (mandatory)
>> +
>> +    Must always include the compatiblity string: "xen,scheduler".
>> +
>> +- sched-name (mandatory)
>> +
>> +    Must be a string having the name of a Xen scheduler, check the 
>> sched=<...>
>> +    boot argument for allowed values.
>> +
>> +
>> +Constraints
>> +===========
>> +
>> +If no cpupools are specified, all cpus will be assigned to one cpupool
>> +implicitly created (Pool-0).
>> +
>> +If cpupools node are specified, but not every cpu brought up by Xen is 
>> assigned,
>> +all the not assigned cpu will be assigned to an additional cpupool.
>> +
>> +If a cpu is assigned to a cpupool, but it's not brought up correctly, Xen 
>> will
>> +stop.
>> +
>> +
>> +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.
>> +
>> +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";
>> +        [...]
>> +};
>> +
>> +a53_3: cpu@102 {
>> +        compatible = "arm,cortex-a53";
>> +        reg = <0x0 0x102>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +a53_4: cpu@103 {
>> +        compatible = "arm,cortex-a53";
>> +        reg = <0x0 0x103>;
>> +        device_type = "cpu";
>> +        [...]
>> +};
>> +
>> +chosen {
>> +
>> +    sched: sched_a {
>> +        compatible = "xen,scheduler";
>> +        sched-name = "credit2";
>> +    };
>> +    cpupool_a {
>> +        compatible = "xen,cpupool";
>> +        cpupool-cpus = <&a53_1 &a53_2 &a53_3 &a53_4>;
>> +    };
>> +    cpupool_b {
>> +        compatible = "xen,cpupool";
>> +        cpupool-cpus = <&a72_1 &a72_2>;
>> +        cpupool-sched = <&sched>;
>> +    };
>> +
>> +    [...]
>> +
>> +};
>> +
>> +
>> +A system having the cpupools specification below will instruct Xen to have 
>> three
>> +cpupools:
>> +
>> +- The cpupool Pool-0 will have 2 cpus assigned.
>> +- The cpupool Pool-1 will have 2 cpus assigned.
>> +- The cpupool Pool-2 will have 2 cpus assigned (created by Xen with all the 
>> not
>> +  assigned cpus a53_3 and a53_4).
>> +
>> +chosen {
>> +
>> +    sched: sched_a {
>> +        compatible = "xen,scheduler";
>> +        sched-name = "null";
>> +    };
>> +    cpupool_a {
>> +        compatible = "xen,cpupool";
>> +        cpupool-cpus = <&a53_1 &a53_2>;
>> +    };
>> +    cpupool_b {
>> +        compatible = "xen,cpupool";
>> +        cpupool-cpus = <&a72_1 &a72_2>;
>> +        cpupool-sched = <&sched>;
>> +    };
>> +
>> +    [...]
>> +
>> +};
>> \ No newline at end of file
>> diff --git a/xen/common/Kconfig b/xen/common/Kconfig
>> index 64439438891c..dc9eed31682f 100644
>> --- a/xen/common/Kconfig
>> +++ b/xen/common/Kconfig
>> @@ -22,6 +22,14 @@ config GRANT_TABLE
>> 
>>        If unsure, say Y.
>> 
>> +config BOOT_TIME_CPUPOOLS
>> +    bool "Create cpupools at boot time"
>> +    depends on HAS_DEVICE_TREE
>> +    default n
>> +    help
>> +      Creates cpupools during boot time and assigns cpus to them. Cpupools
>> +      options can be specified in the device tree.
>> +
>> config ALTERNATIVE_CALL
>>      bool
>> 
>> diff --git a/xen/common/Makefile b/xen/common/Makefile
>> index dc8d3a13f5b8..c5949785ab28 100644
>> --- a/xen/common/Makefile
>> +++ b/xen/common/Makefile
>> @@ -1,5 +1,6 @@
>> obj-$(CONFIG_ARGO) += argo.o
>> obj-y += bitmap.o
>> +obj-$(CONFIG_BOOT_TIME_CPUPOOLS) += boot_cpupools.o
>> obj-$(CONFIG_HYPFS_CONFIG) += config_data.o
>> obj-$(CONFIG_CORE_PARKING) += core_parking.o
>> obj-y += cpu.o
>> diff --git a/xen/common/boot_cpupools.c b/xen/common/boot_cpupools.c
>> new file mode 100644
>> index 000000000000..e8529a902d21
>> --- /dev/null
>> +++ b/xen/common/boot_cpupools.c
>> @@ -0,0 +1,212 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * xen/common/boot_cpupools.c
>> + *
>> + * Code to create cpupools at boot time for arm architecture.
>> + *
>> + * Copyright (C) 2022 Arm Ltd.
>> + */
>> +
>> +#include <xen/sched.h>
>> +
>> +#define BTCPUPOOLS_DT_NODE_NO_REG     (-1)
>> +#define BTCPUPOOLS_DT_NODE_NO_LOG_CPU (-2)
>> +
>> +struct pool_map {
>> +    int pool_id;
>> +    int sched_id;
>> +    struct cpupool *pool;
>> +};
>> +
>> +static struct pool_map __initdata pool_cpu_map[NR_CPUS] =
>> +    { [0 ... NR_CPUS-1] = {.pool_id = -1, .sched_id = -1, .pool = NULL} };
>> +static unsigned int __initdata next_pool_id;
>> +
>> +#ifdef CONFIG_ARM
>> +static int __init get_logical_cpu_from_hw_id(unsigned int hwid)
>> +{
>> +    unsigned int i;
>> +
>> +    for ( i = 0; i < nr_cpu_ids; i++ )
>> +        if ( cpu_logical_map(i) == hwid )
>> +            return i;
>> +
>> +    return -1;
>> +}
>> +
>> +static int __init
>> +get_logical_cpu_from_cpu_node(const struct dt_device_node *cpu_node)
>> +{
>> +    unsigned int cpu_reg, cpu_num;
>> +    const __be32 *prop;
>> +
>> +    prop = dt_get_property(cpu_node, "reg", NULL);
>> +    if ( !prop )
>> +        return BTCPUPOOLS_DT_NODE_NO_REG;
>> +
>> +    cpu_reg = dt_read_number(prop, dt_n_addr_cells(cpu_node));
>> +
>> +    cpu_num = get_logical_cpu_from_hw_id(cpu_reg);
>> +    if ( cpu_num < 0 )
>> +        return BTCPUPOOLS_DT_NODE_NO_LOG_CPU;
>> +
>> +    return cpu_num;
>> +}
>> +
>> +static int __init check_and_get_sched_id(const char* scheduler_name)
>> +{
>> +    int sched_id = sched_get_id_by_name(scheduler_name);
>> +
>> +    if ( sched_id < 0 )
>> +        panic("Scheduler %s does not exists!\n", scheduler_name);
>> +
>> +    return sched_id;
>> +}
>> +
>> +void __init btcpupools_dtb_parse(void)
>> +{
>> +    const struct dt_device_node *chosen, *node;
>> +
>> +    chosen = dt_find_node_by_path("/chosen");
>> +    if ( !chosen )
>> +        return;
>> +
>> +    dt_for_each_child_node(chosen, node)
>> +    {
>> +        const struct dt_device_node *phandle_node;
>> +        int sched_id = -1;
>> +        const char* scheduler_name;
>> +        unsigned int i = 0;
>> +
>> +        if ( !dt_device_is_compatible(node, "xen,cpupool") )
>> +            continue;
>> +
>> +        phandle_node = dt_parse_phandle(node, "cpupool-sched", 0);
>> +        if ( phandle_node )
>> +        {
>> +            if ( !dt_device_is_compatible(phandle_node, "xen,scheduler") )
>> +                panic("cpupool-sched must be a xen,scheduler compatible"
>> +                      "node!\n");
>> +            if ( !dt_property_read_string(phandle_node, "sched-name",
>> +                                          &scheduler_name) )
>> +                sched_id = check_and_get_sched_id(scheduler_name);
>> +            else
>> +                panic("Error trying to read sched-name in %s!\n",
>> +                      dt_node_name(phandle_node));
>> +        }
> 
> it doesn't look like the "xen,scheduler" nodes are very useful from a dt
> parsing perspective either
> 
> 
>> +        phandle_node = dt_parse_phandle(node, "cpupool-cpus", i++);
>> +        if ( !phandle_node )
>> +            panic("Missing or empty cpupool-cpus property!\n");
>> +
>> +        while ( phandle_node )
>> +        {
>> +            int cpu_num;
>> +
>> +            cpu_num = get_logical_cpu_from_cpu_node(phandle_node);
>> +
>> +            if ( cpu_num < 0 )
>> +                panic("Error retrieving logical cpu from node %s (%d)\n",
>> +                      dt_node_name(node), cpu_num);
>> +
>> +            if ( pool_cpu_map[cpu_num].pool_id != -1 )
>> +                panic("Logical cpu %d already added to a cpupool!\n", 
>> cpu_num);
>> +
>> +            pool_cpu_map[cpu_num].pool_id = next_pool_id;
>> +            pool_cpu_map[cpu_num].sched_id = sched_id;
>> +
>> +            phandle_node = dt_parse_phandle(node, "cpupool-cpus", i++);
>> +        }
>> +
>> +        /* Let Xen generate pool ids */
>> +        next_pool_id++;
>> +    }
>> +}
>> +#endif
>> +
>> +void __init btcpupools_allocate_pools(const cpumask_t *cpu_online_map)
>> +{
>> +    unsigned int cpu_num;
>> +
>> +    /*
>> +     * If there are no cpupools, the value of next_pool_id is zero, so the 
>> code
>> +     * below will assign every cpu to cpupool0 as the default behavior.
>> +     * When there are cpupools, the code below is assigning all the not
>> +     * assigned cpu to a new pool (next_pool_id value is the last id + 1).
>> +     * In the same loop we check if there is any assigned cpu that is not
>> +     * online.
>> +     */
>> +    for ( cpu_num = 0; cpu_num < nr_cpu_ids; cpu_num++ )
>> +        if ( cpumask_test_cpu(cpu_num, cpu_online_map) )
>> +        {
>> +            if ( pool_cpu_map[cpu_num].pool_id < 0 )
>> +                pool_cpu_map[cpu_num].pool_id = next_pool_id;
>> +        }
>> +        else
> 
> Please add { }
> 
> 
>> +            if ( pool_cpu_map[cpu_num].pool_id >= 0 )
>> +                panic("Pool-%d contains cpu%u that is not online!\n",
>> +                      pool_cpu_map[cpu_num].pool_id, cpu_num);
> 
> 
> 
>> +#ifdef CONFIG_X86
>> +    /* Cpu0 must be in cpupool0 for x86 */
>> +    if ( pool_cpu_map[0].pool_id != 0 )
> 
> Is that even possible on x86 given that btcpupools_dtb_parse cannot even
> run on x86?
> 
> If it is not possible, I would remove the code below and simply panic
> instead.

Currently x86 doesn’t have a way to specify cpupools, so for now on x86 there 
will
be only cpupool 0 with every cpu attached, I thought I had to handle the case 
if in
the future someone adds a way to specify cpupools (cmdline?).
If you think this should be handled only by who implements that feature, I will 
remove
completely the block.

> 
> 
>> +    {
>> +        /* The cpupool containing cpu0 will become cpupool0 */
>> +        unsigned int swap_id = pool_cpu_map[0].pool_id;
>> +        for_each_cpu ( cpu_num, cpu_online_map )
>> +            if ( pool_cpu_map[cpu_num].pool_id == swap_id )
>> +                pool_cpu_map[cpu_num].pool_id = 0;
>> +            else if ( pool_cpu_map[cpu_num].pool_id == 0 )
>> +                pool_cpu_map[cpu_num].pool_id = swap_id;
>> +    }
>> +#endif
>> +
>> +    for_each_cpu ( cpu_num, cpu_online_map )
>> +    {
>> +        struct cpupool *pool = NULL;
>> +        int pool_id, sched_id;
>> +
>> +        pool_id = pool_cpu_map[cpu_num].pool_id;
>> +        sched_id = pool_cpu_map[cpu_num].sched_id;
>> +
>> +        if ( pool_id )
>> +        {
>> +            unsigned int i;
>> +
>> +            /* Look for previously created pool with id pool_id */
>> +            for ( i = 0; i < cpu_num; i++ )
> 
> Please add { }
> 
> But actually, the double loop seems a bit excessive for this. Could we
> just have a single loop to cpupool_create_pool from 0 to next_pool_id?
> 
> We could get rid of pool_cpu_map[i].pool and just rely on
> pool_cpu_map[i].pool_id. No need to update pool_cpu_map[i].pool if we
> get rid of it: it doesn't look like it is very useful anyway?

Yes we could create all the cpupools in a loop easily, but to retrieve the 
pointer
from the cpupool list I would need something, I can make public this function:

static struct cpupool *cpupool_find_by_id(unsigned int poolid)

from cpupool.c to get the pointer from the pool id, do you think it can be ok?

I will address your other findings in the next serie.

Thank you for your review.

Cheers,
Luca

> 
> 
>> +                if ( (pool_cpu_map[i].pool_id == pool_id) &&
>> +                     pool_cpu_map[i].pool )
>> +                {
>> +                    pool = pool_cpu_map[i].pool;
>> +                    break;
>> +                }
>> +
>> +            /* If no pool was created before, create it */
>> +            if ( !pool )
>> +                pool = cpupool_create_pool(pool_id, sched_id);
>> +            if ( !pool )
>> +                panic("Error creating pool id %u!\n", pool_id);
>> +        }
>> +        else
>> +            pool = cpupool0;
>> +
>> +        pool_cpu_map[cpu_num].pool = pool;
>> +        printk(XENLOG_INFO "Logical CPU %u in Pool-%u.\n", cpu_num, 
>> pool_id);
>> +    }
>> +}
>> +
>> +struct cpupool *__init btcpupools_get_cpupool(unsigned int cpu)
>> +{
>> +    return pool_cpu_map[cpu].pool;
>> +}
>> +
>> +/*
>> + * 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 89a891af7076..b2495ad6d03e 100644
>> --- a/xen/common/sched/cpupool.c
>> +++ b/xen/common/sched/cpupool.c
>> @@ -1247,12 +1247,16 @@ static int __init cf_check cpupool_init(void)
>>     cpupool_put(cpupool0);
>>     register_cpu_notifier(&cpu_nfb);
>> 
>> +    btcpupools_dtb_parse();
>> +
>> +    btcpupools_allocate_pools(&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(btcpupools_get_cpupool(cpu), cpu);
>> 
>>     spin_unlock(&cpupool_lock);
>> 
>> diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
>> index 2c10303f0187..de4e8feea399 100644
>> --- a/xen/include/xen/sched.h
>> +++ b/xen/include/xen/sched.h
>> @@ -1176,6 +1176,25 @@ extern void cf_check dump_runq(unsigned char key);
>> 
>> void arch_do_physinfo(struct xen_sysctl_physinfo *pi);
>> 
>> +#ifdef CONFIG_BOOT_TIME_CPUPOOLS
>> +void btcpupools_allocate_pools(const cpumask_t *cpu_online_map);
>> +struct cpupool *btcpupools_get_cpupool(unsigned int cpu);
>> +
>> +#ifdef CONFIG_ARM
>> +void btcpupools_dtb_parse(void);
>> +#else
>> +static inline void btcpupools_dtb_parse(void) {}
>> +#endif
>> +
>> +#else
>> +static inline void btcpupools_allocate_pools(const cpumask_t 
>> *cpu_online_map) {}
>> +static inline void btcpupools_dtb_parse(void) {}
>> +static inline struct cpupool *btcpupools_get_cpupool(unsigned int cpu)
>> +{
>> +    return cpupool0;
>> +}
>> +#endif
>> +
>> #endif /* __SCHED_H__ */
>> 
>> /*
>> -- 
>> 2.17.1