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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: Shakeel Butt <shakeelb@google.com> Reported-by: Marek Szyprowski <m.szyprowski@samsung.com> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -45,6 +45,7 @@ void percpu_counter_set(struct percpu_counter *fbc, s64 amount);
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void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount,
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s32 batch);
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s64 __percpu_counter_sum(struct percpu_counter *fbc);
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s64 percpu_counter_sum_all(struct percpu_counter *fbc);
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int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch);
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void percpu_counter_sync(struct percpu_counter *fbc);
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@ -193,6 +194,11 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
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return percpu_counter_read(fbc);
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}
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static inline s64 percpu_counter_sum_all(struct percpu_counter *fbc)
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{
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return percpu_counter_read(fbc);
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}
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static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
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{
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return true;
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@ -755,6 +755,11 @@ static void check_mm(struct mm_struct *mm)
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for (i = 0; i < NR_MM_COUNTERS; i++) {
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long x = percpu_counter_sum(&mm->rss_stat[i]);
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if (likely(!x))
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continue;
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/* Making sure this is not due to race with CPU offlining. */
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x = percpu_counter_sum_all(&mm->rss_stat[i]);
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if (unlikely(x))
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pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
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mm, resident_page_types[i], x);
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@ -117,11 +117,8 @@ void percpu_counter_sync(struct percpu_counter *fbc)
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}
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EXPORT_SYMBOL(percpu_counter_sync);
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/*
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* Add up all the per-cpu counts, return the result. This is a more accurate
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* but much slower version of percpu_counter_read_positive()
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*/
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s64 __percpu_counter_sum(struct percpu_counter *fbc)
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static s64 __percpu_counter_sum_mask(struct percpu_counter *fbc,
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const struct cpumask *cpu_mask)
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{
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s64 ret;
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int cpu;
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@ -129,15 +126,35 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)
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raw_spin_lock_irqsave(&fbc->lock, flags);
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ret = fbc->count;
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for_each_online_cpu(cpu) {
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for_each_cpu(cpu, cpu_mask) {
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s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
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ret += *pcount;
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}
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raw_spin_unlock_irqrestore(&fbc->lock, flags);
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return ret;
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}
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/*
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* Add up all the per-cpu counts, return the result. This is a more accurate
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* but much slower version of percpu_counter_read_positive()
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*/
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s64 __percpu_counter_sum(struct percpu_counter *fbc)
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{
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return __percpu_counter_sum_mask(fbc, cpu_online_mask);
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}
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EXPORT_SYMBOL(__percpu_counter_sum);
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/*
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* This is slower version of percpu_counter_sum as it traverses all possible
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* cpus. Use this only in the cases where accurate data is needed in the
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* presense of CPUs getting offlined.
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*/
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s64 percpu_counter_sum_all(struct percpu_counter *fbc)
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{
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return __percpu_counter_sum_mask(fbc, cpu_possible_mask);
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}
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EXPORT_SYMBOL(percpu_counter_sum_all);
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int __percpu_counter_init(struct percpu_counter *fbc, s64 amount, gfp_t gfp,
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struct lock_class_key *key)
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{
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