Date:   Fri, 4 Nov 2022 00:02:30 +0100
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: convert mm's rss stats into percpu_counter

Hi,

On 03.11.2022 18:14, Shakeel Butt wrote:
> On Wed, Nov 02, 2022 at 10:09:57PM +0100, Marek Szyprowski wrote:
>> On 24.10.2022 07:28, Shakeel Butt wrote:
>>> Currently mm_struct maintains rss_stats which are updated on page fault
>>> and the unmapping codepaths. For page fault codepath the updates are
>>> cached per thread with the batch of TASK_RSS_EVENTS_THRESH which is 64.
>>> The reason for caching is performance for multithreaded applications
>>> otherwise the rss_stats updates may become hotspot for such
>>> applications.
>>>
>>> However this optimization comes with the cost of error margin in the rss
>>> stats. The rss_stats for applications with large number of threads can
>>> be very skewed. At worst the error margin is (nr_threads * 64) and we
>>> have a lot of applications with 100s of threads, so the error margin can
>>> be very high. Internally we had to reduce TASK_RSS_EVENTS_THRESH to 32.
>>>
>>> Recently we started seeing the unbounded errors for rss_stats for
>>> specific applications which use TCP rx0cp. It seems like
>>> vm_insert_pages() codepath does not sync rss_stats at all.
>>>
>>> This patch converts the rss_stats into percpu_counter to convert the
>>> error margin from (nr_threads * 64) to approximately (nr_cpus ^ 2).
>>> However this conversion enable us to get the accurate stats for
>>> situations where accuracy is more important than the cpu cost. Though
>>> this patch does not make such tradeoffs.
>>>
>>> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
>> This patch landed recently in linux-next as commit d59f19a7a068 ("mm:
>> convert mm's rss stats into percpu_counter"). Unfortunately it causes a
>> regression on my test systems. I've noticed that it triggers a 'BUG: Bad
>> rss-counter state' warning from time to time for random processes. This
>> is somehow related to CPU hot-plug and/or system suspend/resume. The
>> easiest way to reproduce this issue (although not always) on my test
>> systems (ARM or ARM64 based) is to run the following commands:
>>
>> root@...get:~# for i in /sys/devices/system/cpu/cpu[1-9]; do echo 0
>>   >$i/online;
>> BUG: Bad rss-counter state mm:f04c7160 type:MM_FILEPAGES val:1
>> BUG: Bad rss-counter state mm:50f1f502 type:MM_FILEPAGES val:2
>> BUG: Bad rss-counter state mm:50f1f502 type:MM_ANONPAGES val:15
>> BUG: Bad rss-counter state mm:63660fd0 type:MM_FILEPAGES val:2
>> BUG: Bad rss-counter state mm:63660fd0 type:MM_ANONPAGES val:15
>>
>> Let me know if I can help debugging this somehow or testing a fix.
>>
> Hi Marek,
>
> Thanks for the report. It seems like there is a race between
> for_each_online_cpu() in __percpu_counter_sum() and
> percpu_counter_cpu_dead()/cpu-offlining. Normally this race is fine for
> percpu_counter users but for check_mm() is not happy with this race. Can
> you please try the following patch:
>
>
> From: Shakeel Butt <shakeelb@...gle.com>
> Date: Thu, 3 Nov 2022 06:05:13 +0000
> Subject: [PATCH] mm: percpu_counter: use race free percpu_counter sum
>   interface
>
> percpu_counter_sum can race with cpu offlining. Add a new interface
> which does not race with it and use that for check_mm().
> ---
>   include/linux/percpu_counter.h | 11 +++++++++++
>   kernel/fork.c                  |  2 +-
>   lib/percpu_counter.c           | 24 ++++++++++++++++++------
>   3 files changed, 30 insertions(+), 7 deletions(-)


Yes, this seems to fix the issue I've reported. Feel free to add:

Reported-by: Marek Szyprowski <m.szyprowski@...sung.com>

Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>


> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index bde6c4c1f405..3070c1043acf 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);
>   
> @@ -85,6 +86,11 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
>   	return __percpu_counter_sum(fbc);
>   }
>   
> +static inline s64 percpu_counter_sum_all(struct percpu_counter *fbc)
> +{
> +	return __percpu_counter_sum_all(fbc);
> +}
> +
>   static inline s64 percpu_counter_read(struct percpu_counter *fbc)
>   {
>   	return fbc->count;
> @@ -193,6 +199,11 @@ 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..f26a1a5df399 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 +126,30 @@ 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);
>   
> +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)
>   {

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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