[PATCH v3] sched: Store restrict_cpus_allowed_ptr() call state

Waiman Long
2023-01-27T02:03:10Z

The user_cpus_ptr field was originally added by commit b90ca8badbd1
("sched: Introduce task_struct::user_cpus_ptr to track requested
affinity"). It was used only by arm64 arch due to possible asymmetric
CPU setup.

Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
cpumask"), task_struct::user_cpus_ptr is repurposed to store user
requested cpu affinity specified in the sched_setaffinity().

This results in a slight performance regression on an arm64
system when booted with "allow_mismatched_32bit_el0"
on the command-line. The arch code will (amongst
other things) calls force_compatible_cpus_allowed_ptr() and
relax_compatible_cpus_allowed_ptr() when exec()'ing a 32-bit or a 64-bit
task respectively. Now a call to relax_compatible_cpus_allowed_ptr()
will always result in a __sched_setaffinity() call whether there is a
previous force_compatible_cpus_allowed_ptr() call or not.

In order to fix this regression, a new scheduler flag
task_struct::cpus_allowed_restricted is now added to track if
force_compatible_cpus_allowed_ptr() has been called before or not. This
patch also updates the comments in force_compatible_cpus_allowed_ptr()
and relax_compatible_cpus_allowed_ptr() and handles their interaction
with sched_setaffinity().

This patch also removes the task_user_cpus() helper. In the case of
relax_compatible_cpus_allowed_ptr(), cpu_possible_mask as user_cpu_ptr
masking will be performed within __sched_setaffinity() anyway.

Fixes: 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask")
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
include/linux/sched.h | 3 +++
kernel/sched/core.c | 25 +++++++++++++++++--------
kernel/sched/sched.h | 8 +-------
3 files changed, 21 insertions(+), 15 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 853d08f7562b..f93f62a1f858 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -886,6 +886,9 @@ struct task_struct {
unsigned sched_contributes_to_load:1;
unsigned sched_migrated:1;

+ /* restrict_cpus_allowed_ptr() bit, serialized by scheduler locks */
+ unsigned cpus_allowed_restricted:1;
+
/* Force alignment to the next boundary: */
unsigned :0;

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bb1ee6d7bdde..d7bc809c109e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2999,6 +2999,10 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
struct rq *rq;

rq = task_rq_lock(p, &rf);
+
+ if (ctx->flags & SCA_CLR_RESTRICT)
+ p->cpus_allowed_restricted = 0;
+
/*
* Masking should be skipped if SCA_USER or any of the SCA_MIGRATE_*
* flags are set.
@@ -3025,8 +3029,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
/*
* Change a given task's CPU affinity to the intersection of its current
* affinity mask and @subset_mask, writing the resulting mask to @new_mask.
- * If user_cpus_ptr is defined, use it as the basis for restricting CPU
- * affinity or use cpu_online_mask instead.
+ * The cpus_allowed_restricted bit is set to indicate to a later
+ * relax_compatible_cpus_allowed_ptr() call to relax the cpumask.
*
* If the resulting mask is empty, leave the affinity unchanged and return
* -EINVAL.
@@ -3044,6 +3048,7 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
int err;

rq = task_rq_lock(p, &rf);
+ p->cpus_allowed_restricted = 1;

/*
* Forcefully restricting the affinity of a deadline task is
@@ -3055,7 +3060,8 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
goto err_unlock;
}

- if (!cpumask_and(new_mask, task_user_cpus(p), subset_mask)) {
+ if (p->user_cpu_ptr &&
+ !cpumask_and(new_mask, p->user_cpu_ptr, subset_mask)) {
err = -EINVAL;
goto err_unlock;
}
@@ -3069,9 +3075,8 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,

/*
* Restrict the CPU affinity of task @p so that it is a subset of
- * task_cpu_possible_mask() and point @p->user_cpus_ptr to a copy of the
- * old affinity mask. If the resulting mask is empty, we warn and walk
- * up the cpuset hierarchy until we find a suitable mask.
+ * task_cpu_possible_mask(). If the resulting mask is empty, we warn
+ * and walk up the cpuset hierarchy until we find a suitable mask.
*/
void force_compatible_cpus_allowed_ptr(struct task_struct *p)
{
@@ -3125,11 +3130,15 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx);
void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
{
struct affinity_context ac = {
- .new_mask = task_user_cpus(p),
- .flags = 0,
+ .new_mask = cpu_possible_mask;
+ .flags = SCA_CLR_RESTRICT,
};
int ret;

+ /* Return if no previous force_compatible_cpus_allowed_ptr() call */
+ if (!data_race(p->cpus_allowed_restricted))
+ return;
+
/*
* Try to restore the old affinity mask with __sched_setaffinity().
* Cpuset masking will be done there too.
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 771f8ddb7053..e32ba5d9bb54 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1881,13 +1881,6 @@ static inline void dirty_sched_domain_sysctl(int cpu)
#endif

extern int sched_update_scaling(void);
-
-static inline const struct cpumask *task_user_cpus(struct task_struct *p)
-{
- if (!p->user_cpus_ptr)
- return cpu_possible_mask; /* &init_task.cpus_mask */
- return p->user_cpus_ptr;
-}
#endif /* CONFIG_SMP */

#include "stats.h"
@@ -2293,6 +2286,7 @@ extern struct task_struct *pick_next_task_idle(struct rq *rq);
#define SCA_MIGRATE_DISABLE 0x02
#define SCA_MIGRATE_ENABLE 0x04
#define SCA_USER 0x08
+#define SCA_CLR_RESTRICT 0x10

#ifdef CONFIG_SMP

--
2.31.1