Re: [RESEND PATCH v2] eventfd: introduce ratelimited wakeup for non-semaphore eventfd

From: Wen Yang
Date: Wed Aug 14 2024 - 12:20:36 EST



On 2024/8/11 18:26, Mateusz Guzik wrote:
On Sun, Aug 11, 2024 at 04:59:54PM +0800, Wen Yang wrote:
For the NON-SEMAPHORE eventfd, a write (2) call adds the 8-byte integer
value provided in its buffer to the counter, while a read (2) returns the
8-byte value containing the value and resetting the counter value to 0.
Therefore, the accumulated value of multiple writes can be retrieved by a
single read.

However, the current situation is to immediately wake up the read thread
after writing the NON-SEMAPHORE eventfd, which increases unnecessary CPU
overhead. By introducing a configurable rate limiting mechanism in
eventfd_write, these unnecessary wake-up operations are reduced.


[snip]

# ./a.out -p 2 -s 3
The original cpu usage is as follows:
09:53:38 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
09:53:40 PM 2 47.26 0.00 52.74 0.00 0.00 0.00 0.00 0.00 0.00 0.00
09:53:40 PM 3 44.72 0.00 55.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00

09:53:40 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
09:53:42 PM 2 45.73 0.00 54.27 0.00 0.00 0.00 0.00 0.00 0.00 0.00
09:53:42 PM 3 46.00 0.00 54.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

09:53:42 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
09:53:44 PM 2 48.00 0.00 52.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
09:53:44 PM 3 45.50 0.00 54.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Then enable the ratelimited wakeup, eg:
# ./a.out -p 2 -s 3 -r1000 -c2

Observing a decrease of over 20% in CPU utilization (CPU # 3, 54% ->30%), as shown below:
10:02:32 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
10:02:34 PM 2 53.00 0.00 47.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
10:02:34 PM 3 30.81 0.00 30.81 0.00 0.00 0.00 0.00 0.00 0.00 38.38

10:02:34 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
10:02:36 PM 2 48.50 0.00 51.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00
10:02:36 PM 3 30.20 0.00 30.69 0.00 0.00 0.00 0.00 0.00 0.00 39.11

10:02:36 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
10:02:38 PM 2 45.00 0.00 55.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
10:02:38 PM 3 27.08 0.00 30.21 0.00 0.00 0.00 0.00 0.00 0.00 42.71



Where are these stats from? Is this from your actual program you coded
the feature for?

The program you inlined here does next to nothing in userspace and
unsurprisingly the entire thing is dominated by kernel time, regardless
of what event rate can be achieved.

For example I got: /a.out -p 2 -s 3 5.34s user 60.85s system 99% cpu 66.19s (1:06.19) total

Even so, looking at perf top shows me that a significant chunk is
contention stemming from calls to poll -- perhaps the overhead will
sufficiently go down if you epoll instead?

We have two threads here, one publishing and one subscribing, running on CPUs 2 and 3 respectively. If we further refine and collect performance data on CPU 2, we will find that a large amount of CPU is consumed on the spin lock of the wake-up logic of event write, for example:

# perf top -C 2 -e cycles:k

65.80% [kernel] [k] do_syscall_64
14.71% [kernel] [k] _raw_spin_unlock_irq
7.54% [kernel] [k] __fget_light
4.52% [kernel] [k] ksys_write
1.94% [kernel] [k] vfs_write
1.43% [kernel] [k] _copy_from_user
0.87% [kernel] [k] common_file_perm
0.61% [kernel] [k] aa_file_perm
0.46% [kernel] [k] eventfd_write


One of its call stacks:

|--6.39%--vfs_write
| --5.46%--eventfd_write
| --4.73%--_raw_spin_unlock_irq


> I think the idea is pretty dodgey. If the consumer program can tolerate
some delay in event processing, this probably can be massaged entirely in
userspace.

If your real program has the wake up rate so high that it constitutes a
tangible problem I wonder if eventfd is even the right primitive to use
-- perhaps something built around shared memory and futexes would do the
trick significantly better?

Thank you for your feedback.

This demo comes from the real world: the test vehicle has sensors with multiple cycles (such as 1ms, 5ms, 10ms, etc.), and due to the large number of sensors, data is reported at all times. The publisher reported data through libzmq and went to the write logic of eventfd, frequently waking up the receiver. We collected flame graph and observed that a significant amount of CPU was consumed in this path: eventfd_write -> _raw_spin_unlock_irq.

We did modify a lot of code in user mode on the test vehicle to avoid this issue, such as not using wake-up, not using eventfd, the publisher writing shared memory directly, the receiver periodically extracting the content of shared memory, and so on.

However, since the eventfd mechanism of the kernel provides two different attributes, EFD_SEMAPHORE and EFD_NONSEMAPHORE, should we utilize both of them instead of default to only using EFD_SEMAPHORE?

By utilizing EFD_NONSEMAPHORE on the write side, it is indeed possible to avoid the problem of frequently waking up the read side process.

Since last year, in my spare time, I have released multiple versions of patches and received some feedback, such as:

https://lkml.org/lkml/2023/1/29/228
https://lkml.org/lkml/2023/4/16/149
https://lkml.org/lkml/2024/5/19/135


Fortunately, some small optimization patches around EFD_SEMAPHORE have already entered the mainline kernel, such as:

eventfd: add a BUILD_BUG_ON() to ensure consistency between EFD_SEMAPHORE and the uapi
eventfd: prevent underflow for eventfd semaphores
eventfd: show the EFD_SEMAPHORE flag in fdinfo


Looking forward to the final resolution of this issue, and we welcome your further suggestions.

--
Best wishes,
Wen