[RFC PATCH v3] sched: Fix performance regression introduced by mm_cid

From: Mathieu Desnoyers
Date: Wed Apr 05 2023 - 12:26:59 EST

Introduce per-mm/cpu current concurrency id (mm_cid) to fix a PostgreSQL
sysbench regression reported by Aaron Lu.

Keep track of the currently allocated mm_cid for each mm/cpu rather than
freeing them immediately on context switch. This eliminates most atomic
operations when context switching back and forth between threads
belonging to different memory spaces in multi-threaded scenarios (many
processes, each with many threads). The per-mm/per-cpu mm_cid values are
serialized by their respective runqueue locks.

Thread migration is handled by introducing invocation to
sched_mm_cid_migrate_from() in set_task_cpu() and to
sched_mm_cid_migrate_to() (with destination runqueue lock held) in
activate_task() for migrating tasks. set_task_cpu() is invoked with and
without source rq lock held: the wakeup path does not hold the source rq
lock.

sched_mm_cid_migrate_from() clears the mm_cid from the task's mm per-cpu
index corresponding to the source runqueue if it matches the last mm_cid
observed by the migrated task. This last mm_cid value is returned as a
hint to conditionally clear the mm's per-cpu mm_cid on the destination
cpu.

Then, in sched_mm_cid_migrate_to(), if the last mm_cid is smaller than
the mm's destination cpu current mm_cid, clear the mm's destination cpu
current mm_cid. If the migrated task's mm is in use on the destination
cpu, the reclaim of the mm_cid will be done lazily on the next
destination cpu context switch, else it is performed immediately.

The source cpu's mm_cid is _not_ simply moved to the destination cpu on
migration, because passing ownership of the mm_cid value to the
destination cpu while an actively running tasks also has its own
mm_cid value (in case of lazy reclaim on next context switch) would
over-allocate mm_cid values beyond the number of possible cpus.

Because we want to ensure the mm_cid converges towards the smaller
values as migrations happen, the prior optimization that was done when
context switching between threads belonging to the same mm is removed,
because it could delay the lazy release of the destination runqueue
mm_cid after it has been replaced by a migration. Removing this prior
optimization is not an issue performance-wise because the introduced
per-mm/per-cpu mm_cid tracking also covers this more specific case.

This patch is based on v6.3-rc5 with this patch applied:

("mm: Fix memory leak on mm_init error handling")

https://lore.kernel.org/lkml/20230330133822.66271-1-mathieu.desnoyers@xxxxxxxxxxxx/

Fixes: af7f588d8f73 ("sched: Introduce per-memory-map concurrency ID")
Link: https://lore.kernel.org/lkml/20230327080502.GA570847@ziqianlu-desk2/
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
include/linux/mm_types.h | 60 +++++++++++
include/linux/sched.h | 1 +
kernel/fork.c | 8 +-
kernel/sched/core.c | 218 +++++++++++++++++++++++++++++++++++----
kernel/sched/sched.h | 106 +++++++++++++++----
5 files changed, 352 insertions(+), 41 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0722859c3647..0304f098bd50 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -609,6 +609,14 @@ struct mm_struct {
* were being concurrently updated by the updaters.
*/
raw_spinlock_t cid_lock;
+ /**
+ * @pcpu_cid: Per-cpu current cid.
+ *
+ * Keep track of the currently allocated mm_cid for each cpu.
+ * The per-cpu mm_cid values are serialized by their respective
+ * runqueue locks.
+ */
+ int __percpu *pcpu_cid;
#endif
#ifdef CONFIG_MMU
atomic_long_t pgtables_bytes; /* size of all page tables */
@@ -872,6 +880,37 @@ static inline void vma_iter_init(struct vma_iterator *vmi,
}

#ifdef CONFIG_SCHED_MM_CID
+
+enum mm_cid_state {
+ MM_CID_UNSET = -1U, /* Unset state has lazy_put flag set. */
+ MM_CID_LAZY_PUT = (1U << 31),
+};
+
+static inline bool mm_cid_is_unset(int cid)
+{
+ return cid == MM_CID_UNSET;
+}
+
+static inline bool mm_cid_is_lazy_put(int cid)
+{
+ return !mm_cid_is_unset(cid) && (cid & MM_CID_LAZY_PUT);
+}
+
+static inline bool mm_cid_is_valid(int cid)
+{
+ return !(cid & MM_CID_LAZY_PUT);
+}
+
+static inline int mm_cid_set_lazy_put(int cid)
+{
+ return cid | MM_CID_LAZY_PUT;
+}
+
+static inline int mm_cid_clear_lazy_put(int cid)
+{
+ return cid & ~MM_CID_LAZY_PUT;
+}
+
/* Accessor for struct mm_struct's cidmask. */
static inline cpumask_t *mm_cidmask(struct mm_struct *mm)
{
@@ -885,16 +924,37 @@ static inline cpumask_t *mm_cidmask(struct mm_struct *mm)

static inline void mm_init_cid(struct mm_struct *mm)
{
+ int i;
+
raw_spin_lock_init(&mm->cid_lock);
+ for_each_possible_cpu(i)
+ *per_cpu_ptr(mm->pcpu_cid, i) = -1;
cpumask_clear(mm_cidmask(mm));
}

+static inline int mm_alloc_cid(struct mm_struct *mm)
+{
+ mm->pcpu_cid = alloc_percpu(int);
+ if (!mm->pcpu_cid)
+ return -ENOMEM;
+ mm_init_cid(mm);
+ return 0;
+}
+
+static inline void mm_destroy_cid(struct mm_struct *mm)
+{
+ free_percpu(mm->pcpu_cid);
+ mm->pcpu_cid = NULL;
+}
+
static inline unsigned int mm_cid_size(void)
{
return cpumask_size();
}
#else /* CONFIG_SCHED_MM_CID */
static inline void mm_init_cid(struct mm_struct *mm) { }
+static inline int mm_alloc_cid(struct mm_struct *mm) { return 0; }
+static inline void mm_destroy_cid(struct mm_struct *mm) { }
static inline unsigned int mm_cid_size(void)
{
return 0;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 63d242164b1a..48d48b2c73a5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1313,6 +1313,7 @@ struct task_struct {

#ifdef CONFIG_SCHED_MM_CID
int mm_cid; /* Current cid in mm */
+ int last_mm_cid; /* Most recent cid in mm */
int mm_cid_active; /* Whether cid bitmap is active */
#endif

diff --git a/kernel/fork.c b/kernel/fork.c
index c983c4fe3090..0a2b905c2ed5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -790,6 +790,7 @@ void __mmdrop(struct mm_struct *mm)
check_mm(mm);
put_user_ns(mm->user_ns);
mm_pasid_drop(mm);
+ mm_destroy_cid(mm);

for (i = 0; i < NR_MM_COUNTERS; i++)
percpu_counter_destroy(&mm->rss_stat[i]);
@@ -1054,6 +1055,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)

#ifdef CONFIG_SCHED_MM_CID
tsk->mm_cid = -1;
+ tsk->last_mm_cid = -1;
tsk->mm_cid_active = 0;
#endif
return tsk;
@@ -1159,18 +1161,22 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
if (init_new_context(p, mm))
goto fail_nocontext;

+ if (mm_alloc_cid(mm))
+ goto fail_cid;
+
for (i = 0; i < NR_MM_COUNTERS; i++)
if (percpu_counter_init(&mm->rss_stat[i], 0, GFP_KERNEL_ACCOUNT))
goto fail_pcpu;

mm->user_ns = get_user_ns(user_ns);
lru_gen_init_mm(mm);
- mm_init_cid(mm);
return mm;

fail_pcpu:
while (i > 0)
percpu_counter_destroy(&mm->rss_stat[--i]);
+ mm_destroy_cid(mm);
+fail_cid:
destroy_context(mm);
fail_nocontext:
mm_free_pgd(mm);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d18c3969f90..707a0a5e036e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2084,8 +2084,10 @@ static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)

void activate_task(struct rq *rq, struct task_struct *p, int flags)
{
- if (task_on_rq_migrating(p))
+ if (task_on_rq_migrating(p)) {
flags |= ENQUEUE_MIGRATED;
+ sched_mm_cid_migrate_to(rq, p);
+ }

enqueue_task(rq, p, flags);

@@ -3195,6 +3197,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
p->sched_class->migrate_task_rq(p, new_cpu);
p->se.nr_migrations++;
rseq_migrate(p);
+ sched_mm_cid_migrate_from(p);
perf_event_task_migrate(p);
}

@@ -11383,45 +11386,224 @@ void call_trace_sched_update_nr_running(struct rq *rq, int count)
}

#ifdef CONFIG_SCHED_MM_CID
+/*
+ * Migration from src cpu. Called from set_task_cpu(). There are no guarantees
+ * that the rq lock is held.
+ */
+void sched_mm_cid_migrate_from(struct task_struct *t)
+{
+ int src_cid, *src_pcpu_cid, last_mm_cid;
+ struct mm_struct *mm = t->mm;
+ struct rq *src_rq;
+ struct task_struct *src_task;
+
+ if (!mm)
+ return;
+
+ last_mm_cid = t->last_mm_cid;
+ /*
+ * If the migrated task has no last cid, or if the current
+ * task on src rq uses the cid, it means the destination cpu
+ * does not have to reallocate its cid to keep the cid allocation
+ * compact.
+ */
+ if (last_mm_cid == -1)
+ return;
+
+ src_rq = task_rq(t);
+ src_pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu_of(src_rq));
+ src_cid = READ_ONCE(*src_pcpu_cid);
+
+ if (!mm_cid_is_valid(src_cid) || last_mm_cid != src_cid)
+ return;
+
+ /*
+ * If we observe an active task using the mm on this rq, it means we
+ * are not the last task to be migrated from this cpu for this mm, so
+ * there is no need to clear the src_cid.
+ */
+ rcu_read_lock();
+ src_task = rcu_dereference(src_rq->curr);
+ if (src_task->mm_cid_active && src_task->mm == mm) {
+ rcu_read_unlock();
+ t->last_mm_cid = -1;
+ return;
+ }
+ rcu_read_unlock();
+
+ /*
+ * If the source cpu cid is set, and matches the last cid of the
+ * migrated task, clear the source cpu cid to keep cid allocation
+ * compact to cover the case where this task is the last task using
+ * this mm on the source cpu. If there happens to be other tasks left
+ * on the source cpu using this mm, the next task using this mm will
+ * reallocate its cid on context switch.
+ *
+ * We cannot keep ownership of concurrency ID without runqueue
+ * lock held when it is not used by a current task, because it
+ * would lead to allocation of more concurrency ids than there
+ * are possible cpus in the system. The last_mm_cid is used as
+ * a hint to conditionally unset the dst cpu cid, keeping
+ * allocated concurrency ids compact.
+ */
+ if (cmpxchg(src_pcpu_cid, src_cid, mm_cid_set_lazy_put(src_cid)) != src_cid)
+ return;
+
+ /*
+ * The implicit barrier after cmpxchg per-mm/cpu cid before loading
+ * rq->curr->mm matches the scheduler barrier in context_switch()
+ * between store to rq->curr and load of prev and next task's
+ * per-mm/cpu cid.
+ *
+ * The implicit barrier after cmpxchg per-mm/cpu cid before loading
+ * rq->curr->mm_cid_active matches the barrier in
+ * sched_mm_cid_exit_signals(), sched_mm_cid_before_execve(), and
+ * sched_mm_cid_after_execve() between store to t->mm_cid_active and
+ * load of per-mm/cpu cid.
+ */
+
+ /*
+ * If we observe an active task using the mm on this rq after setting the lazy-put
+ * flag, this task will be responsible for transitioning from lazy-put
+ * flag set to MM_CID_UNSET.
+ */
+ rcu_read_lock();
+ src_task = rcu_dereference(src_rq->curr);
+ if (src_task->mm_cid_active && src_task->mm == mm) {
+ rcu_read_unlock();
+ /*
+ * We observed an active task for this mm, clearing the destination
+ * cpu mm_cid is not relevant for compactness.
+ */
+ t->last_mm_cid = -1;
+ return;
+ }
+ rcu_read_unlock();
+
+ /*
+ * The src_cid is unused, so it can be unset.
+ */
+ if (cmpxchg(src_pcpu_cid, mm_cid_set_lazy_put(src_cid), MM_CID_UNSET) != mm_cid_set_lazy_put(src_cid))
+ return;
+ __mm_cid_put(mm, src_cid);
+}
+
+/*
+ * Migration to dst cpu. Called with dst_rq lock held.
+ */
+void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t)
+{
+ struct mm_struct *mm = t->mm;
+ int src_cid, dst_cid, *dst_pcpu_cid;
+ struct task_struct *dest_task;
+
+ lockdep_assert_rq_held(dst_rq);
+
+ src_cid = t->last_mm_cid;
+ if (!mm || src_cid == -1)
+ return;
+
+ dst_pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu_of(dst_rq));
+
+ /*
+ * If destination cpu cid is greater than the source cpu cid, unset it
+ * so it can be reallocated.
+ */
+ dst_cid = READ_ONCE(*dst_pcpu_cid);
+ if (!mm_cid_is_valid(dst_cid) || dst_cid <= src_cid)
+ return;
+ /*
+ * Put dst_cid if it is not currently in use, else it will be lazy put
+ * on the next context switch.
+ */
+ dest_task = dst_rq->curr;
+ if (dest_task->mm_cid_active && dest_task->mm == mm) {
+ WARN_ON_ONCE(dest_task->mm_cid != dst_cid);
+ /*
+ * Attempt to set lazy-put flag. Can fail due to concurrent
+ * migrate-from, which sets lazy-put flag or MM_CID_UNSET.
+ */
+ (void) cmpxchg(dst_pcpu_cid, dst_cid, mm_cid_set_lazy_put(dst_cid));
+ return;
+ }
+ /*
+ * Attempt to set MM_CID_UNSET. Can fail due to concurrent
+ * migrate-from, which sets lazy-put flag or MM_CID_UNSET.
+ */
+ if (cmpxchg(dst_pcpu_cid, dst_cid, MM_CID_UNSET) != dst_cid)
+ return;
+ __mm_cid_put(mm, dst_cid);
+}
+
void sched_mm_cid_exit_signals(struct task_struct *t)
{
struct mm_struct *mm = t->mm;
- unsigned long flags;
+ struct rq_flags rf;
+ struct rq *rq;

if (!mm)
return;
- local_irq_save(flags);
- mm_cid_put(mm, t->mm_cid);
- t->mm_cid = -1;
- t->mm_cid_active = 0;
- local_irq_restore(flags);
+
+ preempt_disable();
+ rq = this_rq();
+ rq_lock_irqsave(rq, &rf);
+ preempt_enable_no_resched(); /* holding spinlock */
+ WRITE_ONCE(t->mm_cid_active, 0);
+ /*
+ * Store t->mm_cid_active before loading per-mm/cpu cid.
+ * Matches barrier in sched_mm_cid_migrate_from().
+ */
+ smp_mb();
+ mm_cid_put(t);
+ t->last_mm_cid = t->mm_cid = -1;
+ rq_unlock_irqrestore(rq, &rf);
}

void sched_mm_cid_before_execve(struct task_struct *t)
{
struct mm_struct *mm = t->mm;
- unsigned long flags;
+ struct rq_flags rf;
+ struct rq *rq;

if (!mm)
return;
- local_irq_save(flags);
- mm_cid_put(mm, t->mm_cid);
- t->mm_cid = -1;
- t->mm_cid_active = 0;
- local_irq_restore(flags);
+
+ preempt_disable();
+ rq = this_rq();
+ rq_lock_irqsave(rq, &rf);
+ preempt_enable_no_resched(); /* holding spinlock */
+ WRITE_ONCE(t->mm_cid_active, 0);
+ /*
+ * Store t->mm_cid_active before loading per-mm/cpu cid.
+ * Matches barrier in sched_mm_cid_migrate_from().
+ */
+ smp_mb();
+ mm_cid_put(t);
+ t->last_mm_cid = t->mm_cid = -1;
+ rq_unlock_irqrestore(rq, &rf);
}

void sched_mm_cid_after_execve(struct task_struct *t)
{
struct mm_struct *mm = t->mm;
- unsigned long flags;
+ struct rq_flags rf;
+ struct rq *rq;

if (!mm)
return;
- local_irq_save(flags);
- t->mm_cid = mm_cid_get(mm);
- t->mm_cid_active = 1;
- local_irq_restore(flags);
+
+ preempt_disable();
+ rq = this_rq();
+ rq_lock_irqsave(rq, &rf);
+ preempt_enable_no_resched(); /* holding spinlock */
+ WRITE_ONCE(t->mm_cid_active, 1);
+ /*
+ * Store t->mm_cid_active before loading per-mm/cpu cid.
+ * Matches barrier in sched_mm_cid_migrate_from().
+ */
+ smp_mb();
+ t->last_mm_cid = t->mm_cid = mm_cid_get(mm);
+ rq_unlock_irqrestore(rq, &rf);
rseq_set_notify_resume(t);
}

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3e8df6d31c1e..bc0e1cd0d6ac 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3249,7 +3249,66 @@ static inline void update_current_exec_runtime(struct task_struct *curr,
}

#ifdef CONFIG_SCHED_MM_CID
-static inline int __mm_cid_get(struct mm_struct *mm)
+extern void sched_mm_cid_migrate_from(struct task_struct *t);
+extern void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t);
+
+static inline void __mm_cid_put(struct mm_struct *mm, int cid)
+{
+ if (cid < 0)
+ return;
+ cpumask_clear_cpu(cid, mm_cidmask(mm));
+}
+
+/*
+ * The per-mm/cpu cid can have the MM_CID_LAZY_PUT flag set or transition to the
+ * MM_CID_UNSET state without holding the rq lock, but the rq lock needs to be
+ * held to transition to other states.
+ */
+static inline void mm_cid_put_lazy(struct task_struct *t)
+{
+ struct mm_struct *mm = t->mm;
+ int *pcpu_cid, cid;
+
+ lockdep_assert_rq_held(this_rq());
+ pcpu_cid = this_cpu_ptr(mm->pcpu_cid);
+ cid = READ_ONCE(*pcpu_cid);
+ if (!mm_cid_is_lazy_put(cid))
+ return;
+ if (cmpxchg(pcpu_cid, cid, MM_CID_UNSET) != cid)
+ return;
+ __mm_cid_put(mm, mm_cid_clear_lazy_put(cid));
+}
+
+static inline void mm_cid_put(struct task_struct *t)
+{
+ struct mm_struct *mm = t->mm;
+ int *pcpu_cid, cid, res;
+
+ lockdep_assert_rq_held(this_rq());
+ WARN_ON_ONCE(t->mm_cid < 0);
+ pcpu_cid = this_cpu_ptr(mm->pcpu_cid);
+ cid = READ_ONCE(*pcpu_cid);
+ for (;;) {
+ if (mm_cid_is_unset(cid))
+ return;
+ WARN_ON_ONCE(mm_cid_clear_lazy_put(cid) != t->mm_cid);
+ /*
+ * Attempt transition from valid or lazy-put to unset.
+ */
+ res = cmpxchg(pcpu_cid, cid, MM_CID_UNSET);
+ if (res == cid)
+ break;
+ cid = res;
+ }
+ __mm_cid_put(mm, mm_cid_clear_lazy_put(cid));
+}
+
+/*
+ * Lookup/set of mm_cidmask are serialized by cid_lock. Clear of mm_cidmask
+ * uses atomic operations, therefore requiring the set to be done with atomic
+ * operations as well.
+ */
+static inline int __mm_cid_get_locked(struct mm_struct *mm)
{
struct cpumask *cpumask;
int cid;
@@ -3258,52 +3317,55 @@ static inline int __mm_cid_get(struct mm_struct *mm)
cid = cpumask_first_zero(cpumask);
if (cid >= nr_cpu_ids)
return -1;
- __cpumask_set_cpu(cid, cpumask);
+ cpumask_set_cpu(cid, cpumask);
return cid;
}

-static inline void mm_cid_put(struct mm_struct *mm, int cid)
+static inline int __mm_cid_get(struct mm_struct *mm)
{
- lockdep_assert_irqs_disabled();
- if (cid < 0)
- return;
+ int ret;
+
+ lockdep_assert_rq_held(this_rq());
raw_spin_lock(&mm->cid_lock);
- __cpumask_clear_cpu(cid, mm_cidmask(mm));
+ ret = __mm_cid_get_locked(mm);
raw_spin_unlock(&mm->cid_lock);
+ return ret;
}

static inline int mm_cid_get(struct mm_struct *mm)
{
- int ret;
+ int *pcpu_cid, cid;

- lockdep_assert_irqs_disabled();
+ lockdep_assert_rq_held(this_rq());
+ pcpu_cid = this_cpu_ptr(mm->pcpu_cid);
+ cid = READ_ONCE(*pcpu_cid);
+ if (mm_cid_is_valid(cid))
+ return cid;
+ if (mm_cid_is_lazy_put(cid)) {
+ if (cmpxchg(pcpu_cid, cid, MM_CID_UNSET) == cid)
+ __mm_cid_put(mm, mm_cid_clear_lazy_put(cid));
+ }
raw_spin_lock(&mm->cid_lock);
- ret = __mm_cid_get(mm);
+ cid = __mm_cid_get_locked(mm);
raw_spin_unlock(&mm->cid_lock);
- return ret;
+ WRITE_ONCE(*pcpu_cid, cid);
+ return cid;
}

static inline void switch_mm_cid(struct task_struct *prev, struct task_struct *next)
{
if (prev->mm_cid_active) {
- if (next->mm_cid_active && next->mm == prev->mm) {
- /*
- * Context switch between threads in same mm, hand over
- * the mm_cid from prev to next.
- */
- next->mm_cid = prev->mm_cid;
- prev->mm_cid = -1;
- return;
- }
- mm_cid_put(prev->mm, prev->mm_cid);
+ mm_cid_put_lazy(prev);
prev->mm_cid = -1;
}
if (next->mm_cid_active)
- next->mm_cid = mm_cid_get(next->mm);
+ next->last_mm_cid = next->mm_cid = mm_cid_get(next->mm);
}

#else
static inline void switch_mm_cid(struct task_struct *prev, struct task_struct *next) { }
+static inline void sched_mm_cid_migrate_from(struct task_struct *t) { }
+static inline void sched_mm_cid_migrate_to(struct rq *src_rq, struct task_struct *t) { }
#endif

#endif /* _KERNEL_SCHED_SCHED_H */
--
2.25.1