Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode
From: zhangpeng (AS)
Date: Thu Apr 18 2024 - 23:32:59 EST
On 2024/4/19 10:30, Rongwei Wang wrote:
On 2024/4/18 22:20, Peng Zhang wrote:
From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
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@xxxxxxxxxx/
Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx>
Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
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