Commit 82241a83 authored Jun 05, 2025 by Baolin Wang Committed by Andrew Morton Jul 09, 2025

mm: fix the inaccurate memory statistics issue for users

On some large machines with a high number of CPUs running a 64K pagesize
kernel, we found that the 'RES' field is always 0 displayed by the top
command for some processes, which will cause a lot of confusion for users.

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
875525 root 20 0 12480 0 0 R 0.3 0.0 0:00.08 top
1 root 20 0 172800 0 0 S 0.0 0.0 0:04.52 systemd

The main reason is that the batch size of the percpu counter is quite
large on these machines, caching a significant percpu value, since
converting mm's rss stats into percpu_counter by commit f1a79412 ("mm:
convert mm's rss stats into percpu_counter"). Intuitively, the batch
number should be optimized, but on some paths, performance may take
precedence over statistical accuracy. Therefore, introducing a new
interface to add the percpu statistical count and display it to users,
which can remove the confusion. In addition, this change is not expected
to be on a performance-critical path, so the modification should be
acceptable.

In addition, the 'mm->rss_stat' is updated by using add_mm_counter() and
dec/inc_mm_counter(), which are all wrappers around
percpu_counter_add_batch(). In percpu_counter_add_batch(), there is
percpu batch caching to avoid 'fbc->lock' contention. This patch changes
task_mem() and task_statm() to get the accurate mm counters under the
'fbc->lock', but this should not exacerbate kernel 'mm->rss_stat' lock
contention due to the percpu batch caching of the mm counters. The
following test also confirm the theoretical analysis.

I run the stress-ng that stresses anon page faults in 32 threads on my 32
cores machine, while simultaneously running a script that starts 32
threads to busy-loop pread each stress-ng thread's /proc/pid/status
interface. From the following data, I did not observe any obvious impact
of this patch on the stress-ng tests.

w/o patch:
stress-ng: info: [6848] 4,399,219,085,152 CPU Cycles 67.327 B/sec
stress-ng: info: [6848] 1,616,524,844,832 Instructions 24.740 B/sec (0.367 instr. per cycle)
stress-ng: info: [6848] 39,529,792 Page Faults Total 0.605 M/sec
stress-ng: info: [6848] 39,529,792 Page Faults Minor 0.605 M/sec

w/patch:
stress-ng: info: [2485] 4,462,440,381,856 CPU Cycles 68.382 B/sec
stress-ng: info: [2485] 1,615,101,503,296 Instructions 24.750 B/sec (0.362 instr. per cycle)
stress-ng: info: [2485] 39,439,232 Page Faults Total 0.604 M/sec
stress-ng: info: [2485] 39,439,232 Page Faults Minor 0.604 M/sec

On comparing a very simple app which just allocates & touches some
memory against v6.1 (which doesn't have f1a79412) and latest Linus
tree (4c06e63b) I can see that on latest Linus tree the values for
VmRSS, RssAnon and RssFile from /proc/self/status are all zeroes while
they do report values on v6.1 and a Linus tree with this patch.

Link: https://lkml.kernel.org/r/f4586b17f66f97c174f7fd1f8647374fdb53de1c.1749119050.git.baolin.wang@linux.alibaba.com

Fixes: f1a79412 ("mm: convert mm's rss stats into percpu_counter")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Tested-by Donet Tom <donettom@linux.ibm.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: SeongJae Park <sj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent bd225b95

fs/proc/task_mmu.c

+7 −7

Original line number	Diff line number	Diff line
		@@ -36,9 +36,9 @@ void task_mem(struct seq_file m, struct mm_struct mm)
		unsigned long text, lib, swap, anon, file, shmem;
		unsigned long hiwater_vm, total_vm, hiwater_rss, total_rss;

		anon = get_mm_counter(mm, MM_ANONPAGES);
		file = get_mm_counter(mm, MM_FILEPAGES);
		shmem = get_mm_counter(mm, MM_SHMEMPAGES);
		anon = get_mm_counter_sum(mm, MM_ANONPAGES);
		file = get_mm_counter_sum(mm, MM_FILEPAGES);
		shmem = get_mm_counter_sum(mm, MM_SHMEMPAGES);

		/*
		* Note: to minimize their overhead, mm maintains hiwater_vm and
		@@ -59,7 +59,7 @@ void task_mem(struct seq_file m, struct mm_struct mm)
		text = min(text, mm->exec_vm << PAGE_SHIFT);
		lib = (mm->exec_vm << PAGE_SHIFT) - text;

		swap = get_mm_counter(mm, MM_SWAPENTS);
		swap = get_mm_counter_sum(mm, MM_SWAPENTS);
		SEQ_PUT_DEC("VmPeak:\t", hiwater_vm);
		SEQ_PUT_DEC(" kB\nVmSize:\t", total_vm);
		SEQ_PUT_DEC(" kB\nVmLck:\t", mm->locked_vm);
		@@ -92,12 +92,12 @@ unsigned long task_statm(struct mm_struct *mm,
		unsigned long shared, unsigned long text,
		unsigned long data, unsigned long resident)
		{
		*shared = get_mm_counter(mm, MM_FILEPAGES) +
		get_mm_counter(mm, MM_SHMEMPAGES);
		*shared = get_mm_counter_sum(mm, MM_FILEPAGES) +
		get_mm_counter_sum(mm, MM_SHMEMPAGES);
		*text = (PAGE_ALIGN(mm->end_code) - (mm->start_code & PAGE_MASK))
		>> PAGE_SHIFT;
		*data = mm->data_vm + mm->stack_vm;
		resident = shared + get_mm_counter(mm, MM_ANONPAGES);
		resident = shared + get_mm_counter_sum(mm, MM_ANONPAGES);
		return mm->total_vm;
		}

include/linux/mm.h

+5 −0

Original line number	Diff line number	Diff line
		@@ -2568,6 +2568,11 @@ static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
		return percpu_counter_read_positive(&mm->rss_stat[member]);
		}

		static inline unsigned long get_mm_counter_sum(struct mm_struct *mm, int member)
		{
		return percpu_counter_sum_positive(&mm->rss_stat[member]);
		}

		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)