Re: [RFC PATCH 6/6] cpuset: Add cpuset.isolation_mask file

[Marcelo Tosatti]

On Wed, Jul 14, 2021 at 03:54:20PM +0200, Frederic Weisbecker wrote:
> Add a new cpuset.isolation_mask file in order to be able to modify the
> housekeeping cpumask for each individual isolation feature on runtime.
> In the future this will include nohz_full, unbound timers,
> unbound workqueues, unbound kthreads, managed irqs, etc...
> 
> Start with supporting domain exclusion and CPUs passed through
> "isolcpus=".

It is possible to just add return -ENOTSUPPORTED for the features 
whose support is not present?

> The cpuset.isolation_mask defaults to 0. Setting it to 1 will exclude
> the given cpuset from the domains (they will be attached to NULL domain).
> As long as a CPU is part of any cpuset with cpuset.isolation_mask set to
> 1, it will remain isolated even if it overlaps with another cpuset that
> has cpuset.isolation_mask  set to 0. The same applies to parent and
> subdirectories.
> 
> If a cpuset is a subset of "isolcpus=", it automatically maps it and
> cpuset.isolation_mask will be set to 1. This subset is then cleared from
> the initial "isolcpus=" mask. The user is then free to override
> cpuset.isolation_mask to 0 in order to revert the effect of "isolcpus=".
> 
> Here is an example of use where the CPU 7 has been isolated on boot and
> get re-attached to domains later from cpuset:
> 
> 	$ cat /proc/cmdline
> 		isolcpus=7
> 	$ cd /sys/fs/cgroup/cpuset
> 	$ mkdir cpu7
> 	$ cd cpu7
> 	$ cat cpuset.cpus
> 		0-7
> 	$ cat cpuset.isolation_mask
> 		0
> 	$ ls /sys/kernel/debug/domains/cpu7	# empty because isolcpus=7
> 	$ echo 7 > cpuset.cpus
> 	$ cat cpuset.isolation_mask	# isolcpus subset automatically mapped
> 		1
> 	$ echo 0 > cpuset.isolation_mask
> 	$ ls /sys/kernel/debug/domains/cpu7/
> 		domain0  domain1
> 
> CHECKME: Should we have individual cpuset.isolation.$feature files for
>          each isolation feature instead of a single mask file?

Yes, guess that is useful, for example due to the -ENOTSUPPORTED
comment above.


Guarantees on updates
=====================

Perhaps start with a document with:

On return to the write to the cpumask file, what are the guarantees?

For example, for kthread it is that any kernel threads from that point
on should start with the new mask. Therefore userspace should 
respect the order:

1) Change kthread mask.
2) Move threads.


Updates to interface
====================

Also, thinking about updates to the interface (which today are one
cpumask per isolation feature) might be useful. What can happen:

1) New isolation feature is added, feature name added to the interface.

Userspace must support new filename. If not there, then thats an 
old kernel without support for it.

2) If an isolation feature is removed, a file will be gone. What should
be the behaviour there? Remove the file? (userspace should probably 
ignore the failure in that case?) (then features names should not be
reused, as that can confuse #1 above).

Or maybe have a versioned scheme?

> 
> CHECKME: The scheduler is unhappy when _every_ CPUs are isolated
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Nitesh Lal <nilal@xxxxxxxxxx>
> Cc: Nicolas Saenz <nsaenzju@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx>
> Cc: Alex Belits <abelits@xxxxxxxxxxx>
> ---
>  kernel/cgroup/cpuset.c | 111 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 107 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index adb5190c4429..ecb63be04408 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -82,6 +82,7 @@ struct cpuset {
>  	struct cgroup_subsys_state css;
>  
>  	unsigned long flags;		/* "unsigned long" so bitops work */
> +	unsigned long isol_flags;
>  
>  	/*
>  	 * On default hierarchy:
> @@ -258,6 +259,17 @@ static inline int is_spread_slab(const struct cpuset *cs)
>  	return test_bit(CS_SPREAD_SLAB, &cs->flags);
>  }
>  
> +/* bits in struct cpuset flags field */
> +typedef enum {
> +	CS_ISOL_DOMAIN,
> +	CS_ISOL_MAX
> +} isol_flagbits_t;
> +
> +static inline int is_isol_domain(const struct cpuset *cs)
> +{
> +	return test_bit(CS_ISOL_DOMAIN, &cs->isol_flags);
> +}
> +
>  static inline int is_partition_root(const struct cpuset *cs)
>  {
>  	return cs->partition_root_state > 0;
> @@ -269,6 +281,13 @@ static struct cpuset top_cpuset = {
>  	.partition_root_state = PRS_ENABLED,
>  };
>  
> +/*
> + * CPUs passed through "isolcpus=" on boot, waiting to be mounted
> + * as soon as we meet a cpuset directory whose cpus_allowed is a
> + * subset of "isolcpus="
> + */
> +static cpumask_var_t unmounted_isolcpus_mask;
> +
>  /**
>   * cpuset_for_each_child - traverse online children of a cpuset
>   * @child_cs: loop cursor pointing to the current child
> @@ -681,6 +700,39 @@ static inline int nr_cpusets(void)
>  	return static_key_count(&cpusets_enabled_key.key) + 1;
>  }
>  
> +static int update_domain_housekeeping_mask(void)
> +{
> +	struct cpuset *cp;	/* top-down scan of cpusets */
> +	struct cgroup_subsys_state *pos_css;
> +	cpumask_var_t domain_mask;
> +
> +	if (!zalloc_cpumask_var(&domain_mask, GFP_KERNEL))
> +		return -ENOMEM;
> +
> +	cpumask_andnot(domain_mask, cpu_possible_mask, unmounted_isolcpus_mask);
> +
> +	rcu_read_lock();
> +	cpuset_for_each_descendant_pre(cp, pos_css, &top_cpuset) {
> +		if (is_isol_domain(cp))
> +			cpumask_andnot(domain_mask, domain_mask, cp->cpus_allowed);
> +
> +		if (cpumask_subset(cp->cpus_allowed, unmounted_isolcpus_mask)) {
> +			unsigned long flags;
> +			cpumask_andnot(unmounted_isolcpus_mask, unmounted_isolcpus_mask,
> +				       cp->cpus_allowed);
> +			spin_lock_irqsave(&callback_lock, flags);
> +			cp->isol_flags |= BIT(CS_ISOL_DOMAIN);
> +			spin_unlock_irqrestore(&callback_lock, flags);
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	housekeeping_cpumask_set(domain_mask, HK_FLAG_DOMAIN);
> +	free_cpumask_var(domain_mask);
> +
> +	return 0;
> +}
> +
>  /*
>   * generate_sched_domains()
>   *
> @@ -741,6 +793,7 @@ static int generate_sched_domains(cpumask_var_t **domains,
>  	struct cpuset **csa;	/* array of all cpuset ptrs */
>  	int csn;		/* how many cpuset ptrs in csa so far */
>  	int i, j, k;		/* indices for partition finding loops */
> +	int err;
>  	cpumask_var_t *doms;	/* resulting partition; i.e. sched domains */
>  	struct sched_domain_attr *dattr;  /* attributes for custom domains */
>  	int ndoms = 0;		/* number of sched domains in result */
> @@ -752,6 +805,10 @@ static int generate_sched_domains(cpumask_var_t **domains,
>  	dattr = NULL;
>  	csa = NULL;
>  
> +	err = update_domain_housekeeping_mask();
> +	if (err < 0)
> +		pr_err("Can't update housekeeping cpumask\n");
> +
>  	/* Special case for the 99% of systems with one, full, sched domain */
>  	if (root_load_balance && !top_cpuset.nr_subparts_cpus) {
>  		ndoms = 1;
> @@ -1449,7 +1506,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp)
>  		 * root as well.
>  		 */
>  		if (!cpumask_empty(cp->cpus_allowed) &&
> -		    is_sched_load_balance(cp) &&
> +		    (is_sched_load_balance(cp) || is_isol_domain(cs)) &&
>  		   (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
>  		    is_partition_root(cp)))
>  			need_rebuild_sched_domains = true;
> @@ -1935,6 +1992,30 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
>  	return err;
>  }
>  
> +/*
> + * update_isol_flags - read a 0 or a 1 in a file and update associated isol flag
> + * mask:	the new mask value to apply (see isol_flagbits_t)
> + * cs:		the cpuset to update
> + *
> + * Call with cpuset_mutex held.
> + */
> +static int update_isol_flags(struct cpuset *cs, u64 mask)
> +{
> +	unsigned long old_mask = cs->isol_flags;
> +
> +	if (mask & ~(BIT_ULL(CS_ISOL_MAX) - 1))
> +		return -EINVAL;
> +
> +	spin_lock_irq(&callback_lock);
> +	cs->isol_flags = (unsigned long)mask;
> +	spin_unlock_irq(&callback_lock);
> +
> +	if (mask ^ old_mask)
> +		rebuild_sched_domains_locked();
> +
> +	return 0;
> +}
> +
>  /*
>   * update_prstate - update partititon_root_state
>   * cs:	the cpuset to update
> @@ -2273,6 +2354,9 @@ typedef enum {
>  	FILE_MEMORY_PRESSURE,
>  	FILE_SPREAD_PAGE,
>  	FILE_SPREAD_SLAB,
> +//CHECKME: should we have individual cpuset.isolation.$feature files
> +//instead of a mask of features in a single file?
> +	FILE_ISOLATION_MASK,
>  } cpuset_filetype_t;
>  
>  static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
> @@ -2314,6 +2398,9 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
>  	case FILE_SPREAD_SLAB:
>  		retval = update_flag(CS_SPREAD_SLAB, cs, val);
>  		break;
> +	case FILE_ISOLATION_MASK:
> +		retval = update_isol_flags(cs, val);
> +		break;
>  	default:
>  		retval = -EINVAL;
>  		break;
> @@ -2481,6 +2568,8 @@ static u64 cpuset_read_u64(struct cgroup_subsys_state *css, struct cftype *cft)
>  		return is_spread_page(cs);
>  	case FILE_SPREAD_SLAB:
>  		return is_spread_slab(cs);
> +	case FILE_ISOLATION_MASK:
> +		return cs->isol_flags;
>  	default:
>  		BUG();
>  	}
> @@ -2658,6 +2747,13 @@ static struct cftype legacy_files[] = {
>  		.private = FILE_MEMORY_PRESSURE_ENABLED,
>  	},
>  
> +	{
> +		.name = "isolation_mask",
> +		.read_u64 = cpuset_read_u64,
> +		.write_u64 = cpuset_write_u64,
> +		.private = FILE_ISOLATION_MASK,
> +	},
> +
>  	{ }	/* terminate */
>  };
>  
> @@ -2834,9 +2930,12 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
>  	if (is_partition_root(cs))
>  		update_prstate(cs, 0);
>  
> -	if (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
> -	    is_sched_load_balance(cs))
> -		update_flag(CS_SCHED_LOAD_BALANCE, cs, 0);
> +	if (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys)) {
> +		if (is_sched_load_balance(cs))
> +			update_flag(CS_SCHED_LOAD_BALANCE, cs, 0);
> +		if (is_isol_domain(cs))
> +			update_isol_flags(cs, cs->isol_flags & ~BIT(CS_ISOL_DOMAIN));
> +	}
>  
>  	if (cs->use_parent_ecpus) {
>  		struct cpuset *parent = parent_cs(cs);
> @@ -2873,6 +2972,9 @@ static void cpuset_bind(struct cgroup_subsys_state *root_css)
>  		top_cpuset.mems_allowed = top_cpuset.effective_mems;
>  	}
>  
> +	cpumask_andnot(unmounted_isolcpus_mask, cpu_possible_mask,
> +		       housekeeping_cpumask(HK_FLAG_DOMAIN));
> +
>  	spin_unlock_irq(&callback_lock);
>  	percpu_up_write(&cpuset_rwsem);
>  }
> @@ -2932,6 +3034,7 @@ int __init cpuset_init(void)
>  	top_cpuset.relax_domain_level = -1;
>  
>  	BUG_ON(!alloc_cpumask_var(&cpus_attach, GFP_KERNEL));
> +	BUG_ON(!alloc_cpumask_var(&unmounted_isolcpus_mask, GFP_KERNEL));
>  
>  	return 0;
>  }
> -- 
> 2.25.1
> 
>