Messages in this thread | | | Date | Fri, 19 Apr 2024 10:30:24 +0800 | Subject | Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode | From | Rongwei Wang <> |
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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).
> > 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?
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;
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