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Message-ID: <20210607172042.GB7650@willie-the-truck>
Date: Mon, 7 Jun 2021 18:20:42 +0100
From: Will Deacon <will@...nel.org>
To: Valentin Schneider <valentin.schneider@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
linux-kernel@...r.kernel.org,
Catalin Marinas <catalin.marinas@....com>,
Marc Zyngier <maz@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Morten Rasmussen <morten.rasmussen@....com>,
Qais Yousef <qais.yousef@....com>,
Suren Baghdasaryan <surenb@...gle.com>,
Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
kernel-team@...roid.com, Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH v8 06/19] cpuset: Don't use the cpu_possible_mask as a
last resort for cgroup v1
On Fri, Jun 04, 2021 at 06:11:03PM +0100, Valentin Schneider wrote:
> On 02/06/21 17:47, Will Deacon wrote:
> > @@ -3322,9 +3322,13 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
> >
> > void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
> > {
> > + const struct cpumask *cs_mask;
> > + const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
> > +
> > rcu_read_lock();
> > - do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> > - task_cs(tsk)->cpus_allowed : cpu_possible_mask);
> > + cs_mask = task_cs(tsk)->cpus_allowed;
> > + if (is_in_v2_mode() && cpumask_subset(cs_mask, possible_mask))
> > + do_set_cpus_allowed(tsk, cs_mask);
>
> Since the task will still go through the is_cpu_allowed() loop in
> select_fallback_rq() after this, is the subset check actually required
> here?
Yes, I think it's needed. do_set_cpus_allowed() doesn't do any checking
against the task_cpu_possible_mask, so if we returned to
select_fallback_rq() with a mask containing a mixture of 32-bit-capable and
64-bit-only CPUs then we'd end up setting an affinity mask for a 32-bit
task which contains 64-bit-only cores.
> It would have more merit if cpuset_cpus_allowed_fallback() returned whether
> it actually changed the allowed mask or not, in which case we could branch
> either to the is_cpu_allowed() loop (as we do unconditionally now), or to
> the 'state == possible' switch case.
I think this is a cleanup, so I can include it as a separate patch (see
below).
Will
--->8
diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 414a8e694413..d2b9c41c8edf 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -59,7 +59,7 @@ extern void cpuset_wait_for_hotplug(void);
extern void cpuset_read_lock(void);
extern void cpuset_read_unlock(void);
extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
-extern void cpuset_cpus_allowed_fallback(struct task_struct *p);
+extern bool cpuset_cpus_allowed_fallback(struct task_struct *p);
extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
#define cpuset_current_mems_allowed (current->mems_allowed)
void cpuset_init_current_mems_allowed(void);
@@ -188,8 +188,9 @@ static inline void cpuset_cpus_allowed(struct task_struct *p,
cpumask_copy(mask, task_cpu_possible_mask(p));
}
-static inline void cpuset_cpus_allowed_fallback(struct task_struct *p)
+static inline bool cpuset_cpus_allowed_fallback(struct task_struct *p)
{
+ return false;
}
static inline nodemask_t cpuset_mems_allowed(struct task_struct *p)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 4e7c271e3800..a6bab2259f98 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3327,17 +3327,22 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
* which will not contain a sane cpumask during cases such as cpu hotplugging.
* This is the absolute last resort for the scheduler and it is only used if
* _every_ other avenue has been traveled.
+ *
+ * Returns true if the affinity of @tsk was changed, false otherwise.
**/
-void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
+bool cpuset_cpus_allowed_fallback(struct task_struct *tsk)
{
const struct cpumask *cs_mask;
+ bool changed = false;
const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
rcu_read_lock();
cs_mask = task_cs(tsk)->cpus_allowed;
- if (is_in_v2_mode() && cpumask_subset(cs_mask, possible_mask))
+ if (is_in_v2_mode() && cpumask_subset(cs_mask, possible_mask)) {
do_set_cpus_allowed(tsk, cs_mask);
+ changed = true;
+ }
rcu_read_unlock();
/*
@@ -3357,6 +3362,7 @@ void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
* select_fallback_rq() will fix things ups and set cpu_possible_mask
* if required.
*/
+ return changed;
}
void __init cpuset_init_current_mems_allowed(void)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fc7de4f955cf..9d7a74a07632 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2951,8 +2951,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
/* No more Mr. Nice Guy. */
switch (state) {
case cpuset:
- if (IS_ENABLED(CONFIG_CPUSETS)) {
- cpuset_cpus_allowed_fallback(p);
+ if (cpuset_cpus_allowed_fallback(p)) {
state = possible;
break;
}
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