Hi
On 24.10.2022 07:28, Shakeel Butt wrote:
> Currently mm_struct maintains rss_stats which are updated on page fault
> and the unmapping codepaths. For page fault codepath the updates are
> cached per thread with the batch of TASK_RSS_EVENTS_THRESH which is 64.
> The reason for caching is performance for multithreaded applications
> otherwise the rss_stats updates may become hotspot for such
> applications.
>
> However this optimization comes with the cost of error margin in the rss
> stats. The rss_stats for applications with large number of threads can
> be very skewed. At worst the error margin is (nr_threads * 64) and we
> have a lot of applications with 100s of threads, so the error margin can
> be very high. Internally we had to reduce TASK_RSS_EVENTS_THRESH to 32.
>
> Recently we started seeing the unbounded errors for rss_stats for
> specific applications which use TCP rx0cp. It seems like
> vm_insert_pages() codepath does not sync rss_stats at all.
>
> This patch converts the rss_stats into percpu_counter to convert the
> error margin from (nr_threads * 64) to approximately (nr_cpus ^ 2).
> However this conversion enable us to get the accurate stats for
> situations where accuracy is more important than the cpu cost. Though
> this patch does not make such tradeoffs.
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
This patch landed recently in linux-next as commit d59f19a7a068 ("mm:
convert mm's rss stats into percpu_counter"). Unfortunately it causes a
regression on my test systems. I've noticed that it triggers a 'BUG: Bad
rss-counter state' warning from time to time for random processes. This
is somehow related to CPU hot-plug and/or system suspend/resume. The
easiest way to reproduce this issue (although not always) on my test
systems (ARM or ARM64 based) is to run the following commands:
root@target:~# for i in /sys/devices/system/cpu/cpu[1-9]; do echo 0
>$i/online;
BUG: Bad rss-counter state mm:f04c7160 type:MM_FILEPAGES val:1
BUG: Bad rss-counter state mm:50f1f502 type:MM_FILEPAGES val:2
BUG: Bad rss-counter state mm:50f1f502 type:MM_ANONPAGES val:15
BUG: Bad rss-counter state mm:63660fd0 type:MM_FILEPAGES val:2
BUG: Bad rss-counter state mm:63660fd0 type:MM_ANONPAGES val:15
Let me know if I can help debugging this somehow or testing a fix.
> ---
> include/linux/mm.h | 26 ++++--------
> include/linux/mm_types.h | 7 +---
> include/linux/mm_types_task.h | 13 ------
> include/linux/percpu_counter.h | 1 -
> include/linux/sched.h | 3 --
> include/trace/events/kmem.h | 8 ++--
> kernel/fork.c | 16 +++++++-
> mm/memory.c | 73 +++++-----------------------------
> 8 files changed, 40 insertions(+), 107 deletions(-)
>
> ...
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
