I am asking this question again to include the fs-devel list.
We have issues with the workqueue of the kernel overloading the CPU 0
when we we disconnect a USB stick.
This results in other items on the shared workqueue being delayed by
around 6.5 seconds with a default kernel configuration and 2.3 seconds
on a config tailored for our RCar embedded platform.
We first noticed this issue on custom hardware and we have recreated it
on an RCar Starter Kit using a test module [1] to replicate the
behaviour, the test module outputs any delays of greater then 9ms.
To run the test we have a 4GB random file on a USB stick and perform
the following test.
The stick is mounted as R/O and we are copying data from the stick:
- Mount the stick.
mount -o ro,remount /dev/sda1
- Load the Module:
# taskset -c 0 modprobe latency-mon
- Copy large amount of data from the stick:
# dd if=/run/media/sda1/sample.txt of=/dev/zero
[ 1437.517603] DELAY: 10
8388607+1 records in
8388607+1 records out
- Disconnect the USB stick:
[ 1551.796792] usb 2-1: USB disconnect, device number 2
[ 1558.625517] DELAY: 6782
The Delay output 6782 is in milliseconds.
Using umount stops the issue occurring but is unfortunately not guaranteed
in our particular system.
>From my analysis the hub_event workqueue kworker/0:1+usb thread uses around
98% of the CPU.
I have traced the workqueue:workqueue_queue_work function while unplugging the USB
and there is no particular workqueue function being executed a lot more then the
others for the kworker/0:1+usb thread.
Using perf I identified the hub_events workqueue was spending a lot of time in
invalidate_partition(), I have included a cut down the captured data from perf in
[2] which shows the additional functions where the kworker spends most of its time.
I am aware there will be delays on the shared workqueue, are the delays
we are seeing considered normal?
Is there any way to mitigate or identify where the delay is?
I am unsure if this is a memory or filesystem subsystem issue.
Thank you for you help.
Thanks,
Jim Baxter
[1] Test Module:
// SPDX-License-Identifier: GPL-2.0
/*
* Simple WQ latency monitoring
*
* Copyright (C) 2020 Advanced Driver Information Technology.
*/
#include <linux/init.h>
#include <linux/ktime.h>
#include <linux/module.h>
#define PERIOD_MS 100
static struct delayed_work wq;
static u64 us_save;
static void wq_cb(struct work_struct *work)
{
u64 us = ktime_to_us(ktime_get());
u64 us_diff = us - us_save;
u64 us_print = 0;
if (!us_save)
goto skip_print;
us_print = us_diff / 1000 - PERIOD_MS;
if (us_print > 9)
pr_crit("DELAY: %lld\n", us_print);
skip_print:
us_save = us;
schedule_delayed_work(&wq, msecs_to_jiffies(PERIOD_MS));
}
static int latency_mon_init(void)
{
us_save = 0;
INIT_DELAYED_WORK(&wq, wq_cb);
schedule_delayed_work(&wq, msecs_to_jiffies(PERIOD_MS));
return 0;
}
static void latency_mon_exit(void)
{
cancel_delayed_work_sync(&wq);
pr_info("%s\n", __func__);
}
module_init(latency_mon_init);
module_exit(latency_mon_exit);
MODULE_AUTHOR("Eugeniu Rosca <erosca@xxxxxxxxxxxxxx>");
MODULE_LICENSE("GPL");
[2] perf trace:
95.22% 0.00% kworker/0:2-eve [kernel.kallsyms]
|
---ret_from_fork
kthread
worker_thread
|
--95.15%--process_one_work
|
--94.99%--hub_event
|
--94.99%--usb_disconnect
<snip>
|
--94.90%--invalidate_partition
__invalidate_device
|
|--64.55%--invalidate_bdev
| |
| --64.13%--invalidate_mapping_pages
| |
| |--24.09%--invalidate_inode_page
| | |
| | --23.44%--remove_mapping
| | |
| | --23.20%--__remove_mapping
| | |
| | --21.90%--arch_local_irq_restore
| |
| |--22.44%--arch_local_irq_enable
|
--30.35%--shrink_dcache_sb
<snip>
|
--30.17%--truncate_inode_pages_range
