[PATCH] percpu_counter: add percpu_counter_sum_all interface

Shakeel Butt
2022-11-05T01:40:34Z

The percpu_counter is used for scenarios where performance is more
important than the accuracy. For percpu_counter users, who want more
accurate information in their slowpath, percpu_counter_sum is provided
which traverses all the online CPUs to accumulate the data. The reason
it only needs to traverse online CPUs is because percpu_counter does
implement CPU offline callback which syncs the local data of the
offlined CPU.

However there is a small race window between the online CPUs traversal
of percpu_counter_sum and the CPU offline callback. The offline callback
has to traverse all the percpu_counters on the system to flush the CPU
local data which can be a lot. During that time, the CPU which is going
offline has already been published as offline to all the readers. So, as
the offline callback is running, percpu_counter_sum can be called for
one counter which has some state on the CPU going offline. Since
percpu_counter_sum only traverses online CPUs, it will skip that
specific CPU and the offline callback might not have flushed the state
for that specific percpu_counter on that offlined CPU.

Normally this is not an issue because percpu_counter users can deal with
some inaccuracy for small time window. However a new user i.e. mm_struct
on the cleanup path wants to check the exact state of the percpu_counter
through check_mm(). For such users, this patch introduces
percpu_counter_sum_all() which traverses all possible CPUs.

Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
include/linux/percpu_counter.h | 6 ++++++
kernel/fork.c | 2 +-
lib/percpu_counter.c | 29 +++++++++++++++++++++++------
3 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index bde6c4c1f405..a3aae8d57a42 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -45,6 +45,7 @@ void percpu_counter_set(struct percpu_counter *fbc, s64 amount);
void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount,
s32 batch);
s64 __percpu_counter_sum(struct percpu_counter *fbc);
+s64 percpu_counter_sum_all(struct percpu_counter *fbc);
int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch);
void percpu_counter_sync(struct percpu_counter *fbc);

@@ -193,6 +194,11 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
return percpu_counter_read(fbc);
}

+static inline s64 percpu_counter_sum_all(struct percpu_counter *fbc)
+{
+ return percpu_counter_read(fbc);
+}
+
static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
{
return true;
diff --git a/kernel/fork.c b/kernel/fork.c
index 9c32f593ef11..7d6f510cf397 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -756,7 +756,7 @@ static void check_mm(struct mm_struct *mm)
"Please make sure 'struct resident_page_types[]' is updated as well");

for (i = 0; i < NR_MM_COUNTERS; i++) {
- long x = percpu_counter_sum(&mm->rss_stat[i]);
+ long x = percpu_counter_sum_all(&mm->rss_stat[i]);

if (unlikely(x))
pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index ed610b75dc32..42f729c8e56c 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -117,11 +117,8 @@ void percpu_counter_sync(struct percpu_counter *fbc)
}
EXPORT_SYMBOL(percpu_counter_sync);

-/*
- * Add up all the per-cpu counts, return the result. This is a more accurate
- * but much slower version of percpu_counter_read_positive()
- */
-s64 __percpu_counter_sum(struct percpu_counter *fbc)
+static s64 __percpu_counter_sum_mask(struct percpu_counter *fbc,
+ const struct cpumask *cpu_mask)
{
s64 ret;
int cpu;
@@ -129,15 +126,35 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)

raw_spin_lock_irqsave(&fbc->lock, flags);
ret = fbc->count;
- for_each_online_cpu(cpu) {
+ for_each_cpu(cpu, cpu_mask) {
s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
ret += *pcount;
}
raw_spin_unlock_irqrestore(&fbc->lock, flags);
return ret;
}
+
+/*
+ * Add up all the per-cpu counts, return the result. This is a more accurate
+ * but much slower version of percpu_counter_read_positive()
+ */
+s64 __percpu_counter_sum(struct percpu_counter *fbc)
+{
+ return __percpu_counter_sum_mask(fbc, cpu_online_mask);
+}
EXPORT_SYMBOL(__percpu_counter_sum);

+/*
+ * This is slower version of percpu_counter_sum as it traverses all possible
+ * cpus. Use this only in the cases where accurate data is needed in the
+ * presense of CPUs getting offlined.
+ */
+s64 percpu_counter_sum_all(struct percpu_counter *fbc)
+{
+ return __percpu_counter_sum_mask(fbc, cpu_possible_mask);
+}
+EXPORT_SYMBOL(percpu_counter_sum_all);
+
int __percpu_counter_init(struct percpu_counter *fbc, s64 amount, gfp_t gfp,
struct lock_class_key *key)
{
--
2.38.1.431.g37b22c650d-goog