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Date: Fri, 19 Apr 2024 11:32:41 +0800
From: "zhangpeng (AS)" <zhangpeng362@...wei.com>
To: Rongwei Wang <rongwei.wrw@...il.com>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>
CC: <akpm@...ux-foundation.org>, <dennisszhou@...il.com>,
<shakeelb@...gle.com>, <jack@...e.cz>, <surenb@...gle.com>,
<kent.overstreet@...ux.dev>, <mhocko@...e.cz>, <vbabka@...e.cz>,
<yuzhao@...gle.com>, <yu.ma@...el.com>, <wangkefeng.wang@...wei.com>,
<sunnanyong@...wei.com>
Subject: Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode
On 2024/4/19 10:30, Rongwei Wang wrote:
> On 2024/4/18 22:20, Peng Zhang wrote:
>> From: ZhangPeng <zhangpeng362@...wei.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
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@...e.cz>
>> Signed-off-by: ZhangPeng <zhangpeng362@...wei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.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
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