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   | 55 ++++++++++++++++++++++++++++++++++++-------
 kernel/sched/sched.h  |  2 ++
 3 files changed, 51 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..c334c1971e81 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2999,15 +2999,49 @@ 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;
+		}
+
+		/*
+		 * In the unlikely event that sched_setaffinity() has been
+		 * called on a cpu with cpus_allowed_restricted flag set,
+		 * we have to store in user_cpus_ptr a restricted version
+		 * of cpu affinity mask that won't be restored when
+		 * relax_compatible_cpus_allowed_ptr() is called.
+		 *
+		 * 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.
+		 */
+		if (ctx->flags & SCA_USER)
+			cpumask_copy((struct cpumask *)ctx->new_mask, rq->scratch_mask);
+		else
+			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 +3059,8 @@ 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 +3071,7 @@ 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 +3103,8 @@ 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 +3159,14 @@ 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

On 1/20/23 16:08, Waiman Long wrote:
> 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   | 55 ++++++++++++++++++++++++++++++++++++-------
>   kernel/sched/sched.h  |  2 ++
>   3 files changed, 51 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..c334c1971e81 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2999,15 +2999,49 @@ 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;
> +		}
> +
> +		/*
> +		 * In the unlikely event that sched_setaffinity() has been
> +		 * called on a cpu with cpus_allowed_restricted flag set,
> +		 * we have to store in user_cpus_ptr a restricted version
> +		 * of cpu affinity mask that won't be restored when
> +		 * relax_compatible_cpus_allowed_ptr() is called.
> +		 *
> +		 * 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.
> +		 */
> +		if (ctx->flags & SCA_USER)
> +			cpumask_copy((struct cpumask *)ctx->new_mask, rq->scratch_mask);

Sorry, that part is wrong. I forgot there were two masks in ctx. Will 
send out a v2.

Cheers,
Longman

> +		else
> +			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 +3059,8 @@ 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 +3071,7 @@ 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 +3103,8 @@ 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 +3159,14 @@ 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
>