The patch titled
Subject: percpu_counter: add percpu_counter_sum_all interface
has been added to the -mm mm-unstable branch. Its filename is
percpu_counter-add-percpu_counter_sum_all-interface.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/percpu_counter-add-percpu_counter_sum_all-interface.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Shakeel Butt <shakeelb@xxxxxxxxxx>
Subject: percpu_counter: add percpu_counter_sum_all interface
Date: Wed, 9 Nov 2022 01:20:11 +0000
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 and it is used
in fork.c:check_mm() to avoid the potential race.
This issue is exposed by the later patch "mm: convert mm's rss stats into
percpu_counter".
Link: https://lkml.kernel.org/r/20221109012011.881058-1-shakeelb@xxxxxxxxxx
Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/percpu_counter.h | 6 ++++++
kernel/fork.c | 5 +++++
lib/percpu_counter.c | 29 +++++++++++++++++++++++------
3 files changed, 34 insertions(+), 6 deletions(-)
--- a/include/linux/percpu_counter.h~percpu_counter-add-percpu_counter_sum_all-interface
+++ a/include/linux/percpu_counter.h
@@ -46,6 +46,7 @@ void percpu_counter_set(struct percpu_co
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(str
{
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)
{
--- a/kernel/fork.c~percpu_counter-add-percpu_counter_sum_all-interface
+++ a/kernel/fork.c
@@ -755,6 +755,11 @@ static void check_mm(struct mm_struct *m
for (i = 0; i < NR_MM_COUNTERS; i++) {
long x = atomic_long_read(&mm->rss_stat.count[i]);
+ if (likely(!x))
+ continue;
+
+ /* Making sure this is not due to race with CPU offlining. */
+ 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",
mm, resident_page_types[i], x);
--- a/lib/percpu_counter.c~percpu_counter-add-percpu_counter_sum_all-interface
+++ a/lib/percpu_counter.c
@@ -117,11 +117,8 @@ void percpu_counter_sync(struct percpu_c
}
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_c
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)
{
_
Patches currently in -mm which might be from shakeelb@xxxxxxxxxx are
percpu_counter-add-percpu_counter_sum_all-interface.patch
mm-convert-mms-rss-stats-into-percpu_counter.patch
mm-convert-mms-rss-stats-into-percpu_counter-fix.patch
