First time poster here:
I worry a bit about a coroutine approach as it may be challenging for some uses like ours. We have a purely event driven loop with a Reactor model written in D. The details are not specifically needed here, except to point out that an approach based on ucontext.h or something like that would very likely be utterly incompatible in our environment. While we don’t currently plan to integrate support for your hns3 device, I would have grave reservations about a general coroutine library making it’s way into DPDK drivers — it would almost certainly cause no end of grief for us at Weka.
I’m doubtful that we’re the only DPDK users in this situation.
On Apr 24, 2023 at 7:50 PM -0700, fengchengwen <fengchengwen@huawei.com>, wrote:
On 2023/4/25 10:16, Stephen Hemminger wrote:
On Tue, 25 Apr 2023 10:11:43 +0800
fengchengwen <fengchengwen@huawei.com> wrote:
On 2023/4/25 0:08, Stephen Hemminger wrote:
On Mon, 24 Apr 2023 13:02:05 +0000
Chengwen Feng <fengchengwen@huawei.com> wrote:
This patchset introduces the coroutine library which will help refactor
the hns3 PMD's reset process.
The hns3 single function reset process consists of the following steps:
1.stop_service();
2.prepare_reset();
3.delay(100ms);
4.notify_hw();
5.wait_hw_reset_done(); // multiple sleep waits are involved.
6.reinit();
7.restore_conf();
If the DPDK process take over multiple hns3 functions (e.g. 100),
it's impractical to reset and restore functions in sequence:
1.proc_func(001); // will completed in 100+ms range.
2.proc_func(002); // will completed in 100~200+ms range.
...
x.proc_func(100); // will completed in 9900~10000+ms range.
The later functions will process fail because it's too late to deal with.
One solution is that create a reset thread for each function, and it
will lead to large number of threads if the DPDK process take over
multiple hns3 functions.
So the current hns3 driver uses asynchronous mechanism, for examples, it
use rte_eal_alarm_set() when process delay(100ms), it splits a serial
process into multiple asynchronous processes, and the code is complex
and difficult to understand.
The coroutine is a good mechanism to provide programmers with the
simplicity of keeping serial processes within a limited number of
threads.
This patchset use <ucontext.h> to build the coroutine framework, and it
just provides a demo. More APIs maybe added in the future.
In addition, we would like to ask the community whether it it possible
to accept the library. If not, whether it is allowed to provide the
library in hns3 PMD.
Chengwen Feng (3):
lib/coroutine: add coroutine library
examples/coroutine: support coroutine examples
net/hns3: refactor reset process with coroutine
Interesting, but the DPDK really is not the right place for this.
Also, why so much sleeping. Can't this device be handled with an event based
model. Plus any complexity like this introduces more bugs into already fragile
interaction of DPDK userspace applications and threads.
A event base model will function as:
event-handler() {
for (...) {
event = get_next_event();
proc_event();
}
}
The root cause is that the proc_event() take too many time, and it will lead to other
function can't be processed timely.
For which proc_event() may wait a lot of time, the coroutine could also used to optimize
it.
Not only that, coroutines add to the pre-existing problems with locking.
If coroutine 1 acquires a lock, the coroutine 2 will deadlock itself.
And someone will spend days figuring that out. And the existing analyzer
tools will not know about the magic coroutine library.
Analyzer tools like lock annotations maybe a problem.
Locks in DPDK APIs are mostly no-blocking. We can add some restrictions(by reviewer), such
as once holding a lock, you can't invoke rte_co_yield() or rte_co_delay() API.
In addition, any technology has two sides, the greatest advantage of coroutine I think is
removes a large number of callbacks in asychronous programming. And also high-level languages
generally provide coroutines (e.g. C++/Python). With the development, the analyzer tools maybe
evolved to support detect.
And one more, if not acceptable as public library, whether it is allowed intergration of this
library in hns3 PMD ? Our internal evaluation solution (use coroutine refactor) is feasible,
but the code needs to be upstream, hope to listen to community's comments.
The standard DPDK architecture is to have dedicated threads.
Unless you convert the user application to some other model, there really is no other
Instead of adding a new running model, this coroutine library just adapts to the current
DPDK framework.
My visions:
1. DPDK launch a default thread run coroutine service, this thread just like interrupt thread,
it could provide server for PMD drivers.
2. Application could launch coroutine scheduler in lcore (just like 2/3 commit) if it want to
use this library.
useful work that can be done while waiting for your driver.
There was a previous DPDK library for lightweight threading, but it never got any
usage and was abandoned and dropped. Why is this better?
DPDK lightweight threading ? I didn't know before, but will take a closer look at.
This patchset is caused by a problem in the our driver reset process. The reset process is
complex and difficult to understand. We think the coroutine implementation is practical to
solve this problem, so we try to send this RFC.
.