Messages in this thread | | | Date | Sat, 20 Apr 2024 11:13:25 +0800 | Subject | Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode | From | Rongwei Wang <> |
| |
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. > 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; >> >>
| |