Re: Re: [PATCH v6] io_uring: Statistics of the true utilization of sq threads.

[Xiaobing Li <xiaobing.li@samsung.com>]

On 1/5/24 04:02 AM, Pavel Begunkov wrote:
>On 1/3/24 05:49, Xiaobing Li wrote:
>> On 12/30/23 9:27 AM, Pavel Begunkov wrote:
>>> Why it uses jiffies instead of some task run time?
>>> Consequently, why it's fine to account irq time and other
>>> preemption? (hint, it's not)
>>>
>>> Why it can't be done with userspace and/or bpf? Why
>>> can't it be estimated by checking and tracking
>>> IORING_SQ_NEED_WAKEUP in userspace?
>>>
>>> What's the use case in particular? Considering that
>>> one of the previous revisions was uapi-less, something
>>> is really fishy here. Again, it's a procfs file nobody
>>> but a few would want to parse to use the feature.
>>>
>>> Why it just keeps aggregating stats for the whole
>>> life time of the ring? If the workload changes,
>>> that would either totally screw the stats or would make
>>> it too inert to be useful. That's especially relevant
>>> for long running (days) processes. There should be a
>>> way to reset it so it starts counting anew.
>> 
>> Hi, Jens and Pavel,
>> I carefully read the questions you raised.
>> First of all, as to why I use jiffies to statistics time, it
>> is because I have done some performance tests and found that
>> using jiffies has a relatively smaller loss of performance
>> than using task run time. Of course, using task run time is
>
>How does taking a measure for task runtime looks like? I expect it to
>be a simple read of a variable inside task_struct, maybe with READ_ONCE,
>in which case the overhead shouldn't be realistically measurable. Does
>it need locking?

The task runtime I am talking about is similar to this:
start = get_system_time(current);
do_io_part();
sq->total_time += get_system_time(current) - start;

Currently, it is not possible to obtain the execution time of a piece of 
code by a simple read of a variable inside task_struct. 
Or do you have any good ideas?

>> indeed more accurate.  But in fact, our requirements for
>> accuracy are not particularly high, so after comprehensive
>
>I'm looking at it as a generic feature for everyone, and the
>accuracy behaviour is dependent on circumstances. High load
>networking spends quite a good share of CPU in softirq, and
>preemption would be dependent on config, scheduling, pinning,
>etc.

Yes, I quite agree that the accuracy behaviour is dependent on circumstances.
In fact, judging from some test results we have done, the current solution 
can basically meet everyone's requirements, and the error in the calculation 
result of utilization is estimated to be within 0.5%.


>> consideration, we finally chose to use jiffies.
>> Of course, if you think that a little more performance loss
>> here has no impact, I can use task run time instead, but in
>> this case, does the way of calculating sqpoll thread timeout
>> also need to be changed, because it is also calculated through
>> jiffies.
>
>That's a good point. It doesn't have to change unless you're
>directly inferring the idle time parameter from those two
>time values rather than using the ratio. E.g. a simple
>bisection of the idle time based on the utilisation metric
>shouldn't change. But that definitely raises the question
>what idle_time parameter should exactly mean, and what is
>more convenient for algorithms.

We think that idle_time represents the time spent by the sqpoll thread 
except for submitting IO.

In a ring, it may take time M to submit IO, or it may not submit IO in the 
entire cycle. Then we can optimize the efficiency of the sqpoll thread in 
two directions. The first is to reduce the number of rings that no IO submit,
The second is to increase the time M to increase the proportion of time 
submitted IO in the ring.
In order to observe the CPU ratio of sqthread's actual processing IO part, 
we need this patch.