Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

["zhangpeng (AS)" <zhangpeng362@huawei.com>]

On 2024/4/20 11:13, Rongwei Wang wrote:

> On 2024/4/19 11:32, zhangpeng (AS) wrote:
>> On 2024/4/19 10:30, Rongwei Wang wrote:
>>
>>> On 2024/4/18 22:20, Peng Zhang wrote:
>>>> From: ZhangPeng <zhangpeng362@huawei.com>
>>>>
>>>> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
>>>> percpu_counter"), the rss_stats have converted into percpu_counter,
>>>> which convert the error margin from (nr_threads * 64) to approximately
>>>> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() 
>>>> causes a
>>>> performance regression on fork/exec/shell. Even after commit 
>>>> 14ef95be6f55
>>>> ("kernel/fork: group allocation/free of per-cpu counters for mm 
>>>> struct"),
>>>> the performance of fork/exec/shell is still poor compared to previous
>>>> kernel versions.
>>>>
>>>> To mitigate performance regression, we delay the allocation of percpu
>>>> memory for rss_stats. Therefore, we convert mm's rss stats to use
>>>> percpu_counter atomic mode. For single-thread processes, rss_stat 
>>>> is in
>>>> atomic mode, which reduces the memory consumption and performance
>>>> regression caused by using percpu. For multiple-thread processes,
>>>> rss_stat is switched to the percpu mode to reduce the error margin.
>>>> We convert rss_stats from atomic mode to percpu mode only when the
>>>> second thread is created.
>>> Hi, Zhang Peng
>>>
>>> This regression we also found it in lmbench these days. I have not 
>>> test your patch, but it seems will solve a lot for it.
>>> And I see this patch not fix the regression in multi-threads, that's 
>>> because of the rss_stat switched to percpu mode?
>>> (If I'm wrong, please correct me.) And It seems percpu_counter also 
>>> has a bad effect in exit_mmap().
>>>
>>> If so, I'm wondering if we can further improving it on the 
>>> exit_mmap() path in multi-threads scenario, e.g. to determine which 
>>> CPUs the process has run on (mm_cpumask()? I'm not sure).
>>>
>> Hi, Rongwei,
>>
>> Yes, this patch only fixes the regression in single-thread processes. 
>> How
>> much bad effect does percpu_counter have in exit_mmap()? IMHO, the 
>> addition
> Actually, I not sure, just found a little free percpu hotspot in 
> exit_mmap() path when comparing 4 core vs 32 cores.
>
> I can test more next.

Thanks, it would be better if there is test data.

>> of mm counter is already in batch mode, maybe I miss something?
>>
>>>>
>>>> After lmbench test, we can get 2% ~ 4% performance improvement
>>>> for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
>>>> improvement for lmbench page_fault (before batch mode[1]).
>>>>
>>>> The test results are as follows:
>>>>
>>>>               base           base+revert        base+this patch
>>>>
>>>> fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms (4.2%)
>>>> exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
>>>> shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
>>>> page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
>>> I think the regression will becomes more obvious if more cores. How 
>>> about your test machine?
>>>
>> Maybe multi-core is not a factor in the performance of the lmbench 
>> test here.
>> Both of my test machines have 96 cores.
>>
>>> Thanks,
>>> -wrw
>>>>
>>>> [1] 
>>>> https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
>>>>
>>>> Suggested-by: Jan Kara <jack@suse.cz>
>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>> ---
>>>>   include/linux/mm.h          | 50 
>>>> +++++++++++++++++++++++++++++++------
>>>>   include/trace/events/kmem.h |  4 +--
>>>>   kernel/fork.c               | 18 +++++++------
>>>>   3 files changed, 56 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index d261e45bb29b..8f1bfbd54697 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -2631,30 +2631,66 @@ static inline bool 
>>>> get_user_page_fast_only(unsigned long addr,
>>>>    */
>>>>   static inline unsigned long get_mm_counter(struct mm_struct *mm, 
>>>> int member)
>>>>   {
>>>> -    return percpu_counter_read_positive(&mm->rss_stat[member]);
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        return percpu_counter_read_positive(fbc);
>>>> +
>>>> +    return percpu_counter_atomic_read(fbc);
>>>>   }
>>>>     void mm_trace_rss_stat(struct mm_struct *mm, int member);
>>>>     static inline void add_mm_counter(struct mm_struct *mm, int 
>>>> member, long value)
>>>>   {
>>>> -    percpu_counter_add(&mm->rss_stat[member], value);
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        percpu_counter_add(fbc, value);
>>>> +    else
>>>> +        percpu_counter_atomic_add(fbc, value);
>>>>         mm_trace_rss_stat(mm, member);
>>>>   }
>>>>     static inline void inc_mm_counter(struct mm_struct *mm, int 
>>>> member)
>>>>   {
>>>> -    percpu_counter_inc(&mm->rss_stat[member]);
>>>> -
>>>> -    mm_trace_rss_stat(mm, member);
>>>> +    add_mm_counter(mm, member, 1);
>>>>   }
>>>>     static inline void dec_mm_counter(struct mm_struct *mm, int 
>>>> member)
>>>>   {
>>>> -    percpu_counter_dec(&mm->rss_stat[member]);
>>>> +    add_mm_counter(mm, member, -1);
>>>> +}
>>>>   -    mm_trace_rss_stat(mm, member);
>>>> +static inline s64 mm_counter_sum(struct mm_struct *mm, int member)
>>>> +{
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        return percpu_counter_sum(fbc);
>>>> +
>>>> +    return percpu_counter_atomic_read(fbc);
>>>> +}
>>>> +
>>>> +static inline s64 mm_counter_sum_positive(struct mm_struct *mm, 
>>>> int member)
>>>> +{
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        return percpu_counter_sum_positive(fbc);
>>>> +
>>>> +    return percpu_counter_atomic_read(fbc);
>>>> +}
>>>> +
>>>> +static inline int mm_counter_switch_to_pcpu_many(struct mm_struct 
>>>> *mm)
>>>> +{
>>>> +    return percpu_counter_switch_to_pcpu_many(mm->rss_stat, 
>>>> NR_MM_COUNTERS);
>>>> +}
>>>> +
>>>> +static inline void mm_counter_destroy_many(struct mm_struct *mm)
>>>> +{
>>>> +    percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
>>>>   }
>>>>     /* Optimized variant when folio is already known not to be anon */
>>>> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
>>>> index 6e62cc64cd92..a4e40ae6a8c8 100644
>>>> --- a/include/trace/events/kmem.h
>>>> +++ b/include/trace/events/kmem.h
>>>> @@ -399,8 +399,8 @@ TRACE_EVENT(rss_stat,
>>>>           __entry->mm_id = mm_ptr_to_hash(mm);
>>>>           __entry->curr = !!(current->mm == mm);
>>>>           __entry->member = member;
>>>> -        __entry->size = 
>>>> (percpu_counter_sum_positive(&mm->rss_stat[member])
>>>> -                                << PAGE_SHIFT);
>>>> +        __entry->size = (mm_counter_sum_positive(mm, member)
>>>> +                            << PAGE_SHIFT);
>>>>       ),
>>>>         TP_printk("mm_id=%u curr=%d type=%s size=%ldB",
>>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>>> index 99076dbe27d8..0214273798c5 100644
>>>> --- a/kernel/fork.c
>>>> +++ b/kernel/fork.c
>>>> @@ -823,7 +823,7 @@ 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 = mm_counter_sum(mm, i);
>>>>             if (unlikely(x))
>>>>               pr_alert("BUG: Bad rss-counter state mm:%p type:%s 
>>>> val:%ld\n",
>>>> @@ -1301,16 +1301,10 @@ static struct mm_struct *mm_init(struct 
>>>> mm_struct *mm, struct task_struct *p,
>>>>       if (mm_alloc_cid(mm))
>>>>           goto fail_cid;
>>>>   -    if (percpu_counter_init_many(mm->rss_stat, 0, 
>>>> GFP_KERNEL_ACCOUNT,
>>>> -                     NR_MM_COUNTERS))
>>>> -        goto fail_pcpu;
>>>> -
>>>>       mm->user_ns = get_user_ns(user_ns);
>>>>       lru_gen_init_mm(mm);
>>>>       return mm;
>>>>   -fail_pcpu:
>>>> -    mm_destroy_cid(mm);
>>>>   fail_cid:
>>>>       destroy_context(mm);
>>>>   fail_nocontext:
>>>> @@ -1730,6 +1724,16 @@ static int copy_mm(unsigned long 
>>>> clone_flags, struct task_struct *tsk)
>>>>       if (!oldmm)
>>>>           return 0;
>>>>   +    /*
>>>> +     * For single-thread processes, rss_stat is in atomic mode, which
>>>> +     * reduces the memory consumption and performance regression 
>>>> caused by
>>>> +     * using percpu. For multiple-thread processes, rss_stat is 
>>>> switched to
>>>> +     * the percpu mode to reduce the error margin.
>>>> +     */
>>>> +    if (clone_flags & CLONE_THREAD)
>>>> +        if (mm_counter_switch_to_pcpu_many(oldmm))
>>>> +            return -ENOMEM;
>>>> +
>>>>       if (clone_flags & CLONE_VM) {
>>>>           mmget(oldmm);
>>>>           mm = oldmm;
>>>
>>>
>
>
-- 
Best Regards,
Peng