Commit f689054a authored by Shakeel Butt's avatar Shakeel Butt Committed by Andrew Morton
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percpu_counter: add percpu_counter_sum_all interface 

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@google.com


Signed-off-by: default avatarShakeel Butt <shakeelb@google.com>
Reported-by: default avatarMarek Szyprowski <m.szyprowski@samsung.com>
Tested-by: default avatarMarek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent f1a79412

include/linux/percpu_counter.h

+6 −0
Original line number Diff line number Diff line
@@ -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;

kernel/fork.c

+5 −0
Original line number Diff line number Diff line
@@ -755,6 +755,11 @@ static void check_mm(struct mm_struct *mm) 
	for (i = 0; i < NR_MM_COUNTERS; i++) {

		long x = percpu_counter_sum(&mm->rss_stat[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);

lib/percpu_counter.c

+23 −6
Original line number Diff line number Diff line
@@ -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)

{