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

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Message-ID: <096fafc8-f3fa-42d2-a374-101d4facbe86@kernel.dk>
Date: Wed, 14 Aug 2024 10:50:14 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Wen Yang <wen.yang@...ux.dev>, Mateusz Guzik <mjguzik@...il.com>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Alexander Viro <viro@...iv.linux.org.uk>, Dylan Yudaken <dylany@...com>,
 David Woodhouse <dwmw@...zon.co.uk>, Paolo Bonzini <pbonzini@...hat.com>,
 Dave Young <dyoung@...hat.com>, kernel test robot <lkp@...el.com>,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v2] eventfd: introduce ratelimited wakeup for
 non-semaphore eventfd

On 8/14/24 10:15 AM, Wen Yang wrote:
> 
> 
> 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:

This is hardly surprising - you've got probably the worst kind of
producer/consumer setup here, with the producer on one CPU, and the
consumer on another. You force this relationship by pinning both of
them. Then you have a queue in between, and locking that needs to be
acquired on both sides.

It's hard to come up with a WORSE way of doing that.

I'll have to agree with the notion that you're using the wrong tool for
the job, and hacking around it is not the right solution.

-- 
Jens Axboe