[PATCH v2] sched: Store restrict_cpus_allowed_ptr() call state

Waiman Long
2023-01-21T02:18:58Z

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 performance regression in 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().

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 | 46 ++++++++++++++++++++++++++++++++++---------
kernel/sched/sched.h | 2 ++
3 files changed, 42 insertions(+), 9 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..48234dc9005b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2999,15 +2999,40 @@ 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 | SCA_SET_RESTRICT)) {
+ p->cpus_allowed_restricted = 0;
+ } else if (p->cpus_allowed_restricted) {
+ /*
+ * If force_compatible_cpus_allowed_ptr() has been called,
+ * we can't extend cpumask to beyond what is in cpus_mask.
+ */
+ if (!cpumask_and(rq->scratch_mask, ctx->new_mask,
+ &p->cpus_mask)) {
+ task_rq_unlock(rq, p, &rf);
+ return -EINVAL;
+ }
+
+ /*
+ * Note that we don't need to do further user_cpus_ptr
+ * masking below as cpus_mask should be a subset of
+ * user_cpus_ptr if set.
+ */
+ ctx->new_mask = rq->scratch_mask;
+ }
+
/*
* Masking should be skipped if SCA_USER or any of the SCA_MIGRATE_*
- * flags are set.
+ * flags are set or when cpus_allowed_restricted flag has been set.
*/
- if (p->user_cpus_ptr &&
+ if (p->user_cpus_ptr && !p->cpus_allowed_restricted &&
!(ctx->flags & (SCA_USER | SCA_MIGRATE_ENABLE | SCA_MIGRATE_DISABLE)) &&
cpumask_and(rq->scratch_mask, ctx->new_mask, p->user_cpus_ptr))
ctx->new_mask = rq->scratch_mask;

+ if (ctx->flags & SCA_SET_RESTRICT)
+ p->cpus_allowed_restricted = 1;
+
return __set_cpus_allowed_ptr_locked(p, ctx, rq, &rf);
}

@@ -3025,8 +3050,8 @@ 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.
@@ -3037,7 +3062,7 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
{
struct affinity_context ac = {
.new_mask = new_mask,
- .flags = 0,
+ .flags = SCA_SET_RESTRICT,
};
struct rq_flags rf;
struct rq *rq;
@@ -3069,9 +3094,8 @@ 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)
{
@@ -3126,10 +3150,14 @@ void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
{
struct affinity_context ac = {
.new_mask = task_user_cpus(p),
- .flags = 0,
+ .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..adcef29d5479 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2293,6 +2293,8 @@ 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
+#define SCA_SET_RESTRICT 0x20

#ifdef CONFIG_SMP

--
2.31.1