Re: [PATCH 3/7] x86/intel_rdt: Support cache bit mask for Intel CAT

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Mar 12, 2015 at 04:16:03PM -0700, Vikas Shivappa wrote:
> Add support for cache bit mask manipulation. The change adds a file to
> the RDT cgroup which represents the CBM(cache bit mask) for the cgroup.
> 
> The RDT cgroup follows cgroup hierarchy ,mkdir and adding tasks to the
> cgroup never fails.  When a child cgroup is created it inherits the
> CLOSid and the CBM from its parent.  When a user changes the default
> CBM for a cgroup, a new CLOSid may be allocated if the CBM was not
> used before. If the new CBM is the one that is already used, the
> count for that CLOSid<->CBM is incremented. The changing of 'cbm'
> may fail with -ENOSPC once the kernel runs out of maximum CLOSids it
> can support.
> User can create as many cgroups as he wants but having different CBMs
> at the same time is restricted by the maximum number of CLOSids
> (multiple cgroups can have the same CBM).
> Kernel maintains a CLOSid<->cbm mapping which keeps count
> of cgroups using a CLOSid.
> 
> The tasks in the CAT cgroup would get to fill the LLC cache represented
> by the cgroup's 'cbm' file.
> 
> Reuse of CLOSids for cgroups with same bitmask also has following
> advantages:
> - This helps to use the scant CLOSids optimally.
> - This also implies that during context switch, write to PQR-MSR is done
> only when a task with a different bitmask is scheduled in.
> 
> Signed-off-by: Vikas Shivappa <vikas.shivappa@linux.intel.com>
> ---
>  arch/x86/include/asm/intel_rdt.h |   3 +
>  arch/x86/kernel/cpu/intel_rdt.c  | 205 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 208 insertions(+)
> 
> diff --git a/arch/x86/include/asm/intel_rdt.h b/arch/x86/include/asm/intel_rdt.h
> index 87af1a5..0ed28d9 100644
> --- a/arch/x86/include/asm/intel_rdt.h
> +++ b/arch/x86/include/asm/intel_rdt.h
> @@ -4,6 +4,9 @@
>  #ifdef CONFIG_CGROUP_RDT
>  
>  #include <linux/cgroup.h>
> +#define MAX_CBM_LENGTH			32
> +#define IA32_L3_CBM_BASE		0xc90
> +#define CBM_FROM_INDEX(x)		(IA32_L3_CBM_BASE + x)
>  
>  struct rdt_subsys_info {
>  	/* Clos Bitmap to keep track of available CLOSids.*/
> diff --git a/arch/x86/kernel/cpu/intel_rdt.c b/arch/x86/kernel/cpu/intel_rdt.c
> index 3726f41..495497a 100644
> --- a/arch/x86/kernel/cpu/intel_rdt.c
> +++ b/arch/x86/kernel/cpu/intel_rdt.c
> @@ -33,6 +33,9 @@ static struct rdt_subsys_info rdtss_info;
>  static DEFINE_MUTEX(rdt_group_mutex);
>  struct intel_rdt rdt_root_group;
>  
> +#define rdt_for_each_child(pos_css, parent_ir)		\
> +	css_for_each_child((pos_css), &(parent_ir)->css)
> +
>  static inline bool cat_supported(struct cpuinfo_x86 *c)
>  {
>  	if (cpu_has(c, X86_FEATURE_CAT_L3))
> @@ -83,6 +86,31 @@ static int __init rdt_late_init(void)
>  late_initcall(rdt_late_init);
>  
>  /*
> + * Allocates a new closid from unused closids.
> + * Called with the rdt_group_mutex held.
> + */
> +
> +static int rdt_alloc_closid(struct intel_rdt *ir)
> +{
> +	unsigned int id;
> +	unsigned int maxid;
> +
> +	lockdep_assert_held(&rdt_group_mutex);
> +
> +	maxid = boot_cpu_data.x86_cat_closs;
> +	id = find_next_zero_bit(rdtss_info.closmap, maxid, 0);
> +	if (id == maxid)
> +		return -ENOSPC;
> +
> +	set_bit(id, rdtss_info.closmap);
> +	WARN_ON(ccmap[id].cgrp_count);
> +	ccmap[id].cgrp_count++;
> +	ir->clos = id;
> +
> +	return 0;
> +}
> +
> +/*
>  * Called with the rdt_group_mutex held.
>  */
>  static int rdt_free_closid(struct intel_rdt *ir)
> @@ -133,8 +161,185 @@ static void rdt_css_free(struct cgroup_subsys_state *css)
>  	mutex_unlock(&rdt_group_mutex);
>  }
>  
> +/*
> + * Tests if atleast two contiguous bits are set.
> + */
> +
> +static inline bool cbm_is_contiguous(unsigned long var)
> +{
> +	unsigned long first_bit, zero_bit;
> +	unsigned long maxcbm = MAX_CBM_LENGTH;
> +
> +	if (bitmap_weight(&var, maxcbm) < 2)
> +		return false;
> +
> +	first_bit = find_next_bit(&var, maxcbm, 0);
> +	zero_bit = find_next_zero_bit(&var, maxcbm, first_bit);
> +
> +	if (find_next_bit(&var, maxcbm, zero_bit) < maxcbm)
> +		return false;
> +
> +	return true;
> +}
> +
> +static int cat_cbm_read(struct seq_file *m, void *v)
> +{
> +	struct intel_rdt *ir = css_rdt(seq_css(m));
> +
> +	seq_printf(m, "%08lx\n", ccmap[ir->clos].cbm);
> +	return 0;
> +}
> +
> +static int validate_cbm(struct intel_rdt *ir, unsigned long cbmvalue)
> +{
> +	struct intel_rdt *par, *c;
> +	struct cgroup_subsys_state *css;
> +	unsigned long *cbm_tmp;
> +
> +	if (!cbm_is_contiguous(cbmvalue)) {
> +		pr_info("cbm should have >= 2 bits and be contiguous\n");
> +		return -EINVAL;
> +	}
> +
> +	par = parent_rdt(ir);
> +	cbm_tmp = &ccmap[par->clos].cbm;
> +	if (!bitmap_subset(&cbmvalue, cbm_tmp, MAX_CBM_LENGTH))
> +		return -EINVAL;

Can you have different errors for the different cases?

> +	rcu_read_lock();
> +	rdt_for_each_child(css, ir) {
> +		c = css_rdt(css);
> +		cbm_tmp = &ccmap[c->clos].cbm;
> +		if (!bitmap_subset(cbm_tmp, &cbmvalue, MAX_CBM_LENGTH)) {
> +			pr_info("Children's mask not a subset\n");
> +			rcu_read_unlock();
> +			return -EINVAL;
> +		}
> +	}
> +
> +	rcu_read_unlock();
> +	return 0;
> +}
> +
> +static bool cbm_search(unsigned long cbm, int *closid)
> +{
> +	int maxid = boot_cpu_data.x86_cat_closs;
> +	unsigned int i;
> +
> +	for (i = 0; i < maxid; i++)
> +		if (bitmap_equal(&cbm, &ccmap[i].cbm, MAX_CBM_LENGTH)) {
> +			*closid = i;
> +			return true;
> +		}
> +
> +	return false;
> +}
> +
> +static void cbmmap_dump(void)
> +{
> +	int i;
> +
> +	pr_debug("CBMMAP\n");
> +	for (i = 0; i < boot_cpu_data.x86_cat_closs; i++)
> +		pr_debug("cbm: 0x%x,cgrp_count: %u\n",
> +		 (unsigned int)ccmap[i].cbm, ccmap[i].cgrp_count);
> +}
> +
> +static void cpu_cbm_update(void *info)
> +{
> +	unsigned int closid = *((unsigned int *)info);
> +
> +	wrmsrl(CBM_FROM_INDEX(closid), ccmap[closid].cbm);
> +}
> +
> +static inline void cbm_update(unsigned int closid)
> +{
> +	int pkg_id = -1;
> +	int cpu;
> +
> +	for_each_online_cpu(cpu) {
> +		if (pkg_id == topology_physical_package_id(cpu))
> +			continue;
> +		smp_call_function_single(cpu, cpu_cbm_update, &closid, 1);
> +		pkg_id = topology_physical_package_id(cpu);
> +
> +

Can use smp_call_function_many, once, more efficient.

Can this race with CPU hotplug? BTW, on CPU hotplug, where are
the IA32_L3_MASK_n initialized for the new CPU ?