sched/isolation: add cpu_is_isolated() API

Patch series "memcg, cpuisol: do not interfere pcp cache charges draining
with cpuisol workloads".

Leonardo has reported [1] that pcp memcg charge draining can interfere
with cpu isolated workloads. The said draining is done from a WQ context
with a pcp worker scheduled on each CPU which holds any cached charges for
a specific memcg hierarchy. Operation is not really a common operation
[2]. It can be triggered from the userspace though so some care is
definitely due.

Leonardo has tried to address the issue by allowing remote charge draining
[3]. This approach requires an additional locking to synchronize pcp
caches sync from a remote cpu from local pcp consumers. Even though the
proposed lock was per-cpu there is still potential for contention and less
predictable behavior.

This patchset addresses the issue from a different angle. Rather than
dealing with a potential synchronization, cpus which are isolated are
simply never scheduled to be drained. This means that a small amount of
charges could be laying around and waiting for a later use or they are
flushed when a different memcg is charged from the same cpu. More details
are in patch 2. The first patch from Frederic is implementing an
abstraction to tell whether a specific cpu has been isolated and therefore
require a special treatment.


This patch (of 2):

Provide this new API to check if a CPU has been isolated either through
isolcpus= or nohz_full= kernel parameter.

It aims at avoiding kernel load deemed to be safely spared on CPUs running
sensitive workload that can't bear any disturbance, such as pcp cache
draining.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Frederic Weisbecker <[e-mail]>
Signed-off-by: Michal Hocko <[e-mail]>
Suggested-by: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Marcelo Tosatti <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Leonardo Bras <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/sched/isolation.h

···
 
 static inline void housekeeping_affine(struct task_struct *t,
 				       enum hk_type type) { }
+
+static inline bool housekeeping_test_cpu(int cpu, enum hk_type type)
+{
+	return true;
+}
+
 static inline void housekeeping_init(void) { }
 #endif /* CONFIG_CPU_ISOLATION */
 
···
 		return housekeeping_test_cpu(cpu, type);
 #endif
 	return true;
+}
+
+static inline bool cpu_is_isolated(int cpu)
+{
+	return !housekeeping_test_cpu(cpu, HK_TYPE_DOMAIN) ||
+		 !housekeeping_test_cpu(cpu, HK_TYPE_TICK);
 }
 
 #endif /* _LINUX_SCHED_ISOLATION_H */