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Tue, 08 Nov 2022 17:20:14 -0800 (PST) Date: Wed, 9 Nov 2022 01:20:11 +0000 Mime-Version: 1.0 X-Mailer: git-send-email 2.38.1.431.g37b22c650d-goog Message-ID: <20221109012011.881058-1-shakeelb@google.com> Subject: [PATCH v2] percpu_counter: add percpu_counter_sum_all interface From: Shakeel Butt To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Shakeel Butt , Marek Szyprowski ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1667956863; a=rsa-sha256; cv=none; b=FogWmmaPZOBlBb8TyepsFfv8eIHwtFRrb5bpRv5uPAlTXHxW/DITXnBSL5z7awwDR8POBP OTIS3EpIhkxMavqk4sEgUKfl8rHr2kLzfrolFbxZZhKZQFX+XuCDvEaHn7AOkjhj6wcHRP 1x6nbfm2+8cC/RLtiTtANkwtuUPYgu8= ARC-Authentication-Results: i=1; imf08.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=G82DxUUK; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf08.hostedemail.com: domain of 3TgBrYwgKCB0LA3D77E49HH9E7.5HFEBGNQ-FFDO35D.HK9@flex--shakeelb.bounces.google.com designates 209.85.128.202 as permitted sender) smtp.mailfrom=3TgBrYwgKCB0LA3D77E49HH9E7.5HFEBGNQ-FFDO35D.HK9@flex--shakeelb.bounces.google.com ARC-Message-Signature: i=1; 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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. Signed-off-by: Shakeel Butt Reported-by: Marek Szyprowski Tested-by: Marek Szyprowski --- Changes since v1: - Only call percpu_counter_sum_all() if percpu_counter_sum() gives non-zero value in check_mm(). include/linux/percpu_counter.h | 6 ++++++ kernel/fork.c | 5 +++++ lib/percpu_counter.c | 29 +++++++++++++++++++++++------ 3 files changed, 34 insertions(+), 6 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..9f7fe3541897 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -758,6 +758,11 @@ 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); 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) {