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Date:   Mon, 12 Jun 2023 16:47:17 -0700
From:   Brian Norris <briannorris@chromium.org>
To:     Pin-yen Lin <treapking@chromium.org>
Cc:     Kalle Valo <kvalo@kernel.org>,
        Amitkumar Karwar <amitkarwar@gmail.com>,
        Ganapathi Bhat <ganapathi017@gmail.com>,
        Sharvari Harisangam <sharvari.harisangam@nxp.com>,
        Xinming Hu <huxinming820@gmail.com>,
        linux-wireless@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] wifi: mwifiex: Replace RX workqueues with kthreads
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Hi,

Thanks Pin-yen for most of the investigation here and for pushing the
patch. With some additional information though, I might suggest *not*
landing this patch at the moment. More details appended:

On Sat, Jun 10, 2023 at 01:41:51AM +0800, Pin-yen Lin wrote:
> I realized that I might have over-simplified the background and the
> impact of this patch...
> 
> The short answer to the question is that the throughput improved from
> 100 mbps to 180 mbps. The test was run on ChromeOS's v5.15 kernel
> fork. More detailed test setting is mentioned in [1].
> 
> However, the throughput of the same test case on our v4.19 kernel is
> 320 mbps. That is, we observed a 320 mbps --> 100 mbps regression when
> we tried to update the kernel version. This patch is more like a
> mitigation of the regression. It improves the throughput, even though
> it is still not as good as the older kernel.
> 
> That being said, this patch does improve the throughput, so we think
> this patch can be landed into the mainline kernel.
> 
> Best regards,
> Pin-yen
> 
> [1]: https://lore.kernel.org/all/ZFvpJb9Dh0FCkLQA@google.com/

I (we?) was optimistic this would be an improvement (or at least, no
worse) due to some of the reasoning at [1]. And, the work here is just a
single work item, queued repeatedly to the same unbound workqueue. So
conceptually, it shouldn't be much different than a kthread_worker,
except for scheduler details -- where again, we'd think this should be
an improvement, as the scheduler would now better track load for the
task (mwifiex RX) in question.

But additional testing on other mwifiex-based systems (RK3399 + PCIE
8997) showed the inverse: some throughput drops on similar benchmarks,
from 110 Mbps to 80 Mbps. (Frankly, both numbers are significantly below
where we might like...)

Considering we still don't have a full explanation for all the
performance differences we've been seeing (on either test platform), and
that at least one of our platforms showed a (smaller) regression, I
think we might want to do more research before committing to this.

Brian