netdev - Re: 10% regression in qperf tcp latency after introducing commit "4a61bf7f9b18 random: defer fast pool mixing to worker"

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Date:   Wed, 21 Sep 2022 13:54:43 -1000
From:   Tejun Heo <tj@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Sherry Yang <sherry.yang@...cle.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Jack Vogel <jack.vogel@...cle.com>,
        Tariq Toukan <tariqt@...dia.com>
Subject: Re: 10% regression in qperf tcp latency after introducing commit
 "4a61bf7f9b18 random: defer fast pool mixing to worker"

Hello,

On Thu, Sep 22, 2022 at 12:32:49AM +0200, Jason A. Donenfeld wrote:
> What are our options? Investigate queue_work_on() bottlenecks? Move back
> to the original pattern, but use raw spinlocks? Some thing else?

I doubt it's queue_work_on() itself if it's called at very high frequency as
the duplicate calls would just fail to claim the PENDING bit and return but
if it's being called at a high frequency, it'd be waking up a kthread over
and over again, which can get pretty expensive. Maybe that ends competing
with softirqd which is handling net rx or sth?

So, yeah, I'd try something which doesn't always involve scheduling and a
context switch whether that's softirq, tasklet, or irq work. I probably am
mistaken but I thought RT kernel pushes irq handling to threads so that
these things can be handled sanely. Is this some special case?

Thanks.

-- 
tejun