netdev - Re: [PATCH 2/3] softirq: avoid spurious stalls due to need

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Date:   Fri, 03 Mar 2023 16:18:03 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jakub Kicinski <kuba@...nel.org>, peterz@...radead.org
Cc:     jstultz@...gle.com, edumazet@...gle.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH 2/3] softirq: avoid spurious stalls due to need_resched()

Jakub!

On Fri, Mar 03 2023 at 14:30, Thomas Gleixner wrote:
> On Thu, Dec 22 2022 at 14:12, Jakub Kicinski wrote:
> But without the sched_clock() changes the actual defer time depends on
> HZ and the point in time where limit is set. That means it ranges from 0
> to 1/HZ, i.e. the 2ms defer time ends up with close to 10ms on HZ=100 in
> the worst case, which perhaps explains the 8ms+ stalls you are still
> observing. Can you test with that sched_clock change applied, i.e. the
> first two commits from
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git core/softirq
>
> 59be25c466d9 ("softirq: Use sched_clock() based timeout")
> bd5a5bd77009 ("softirq: Rewrite softirq processing loop")
>
> whether that makes a difference? Those two can be applied with some
> minor polishing. The rest of that series is broken by f10020c97f4c
> ("softirq: Allow early break").

WHile staring I noticed that the current jiffies based time limit
handling has the exact same problem. For HZ=100 and HZ=250
MAX_SOFTIRQ_TIME resolves to 1 jiffy. So the window is between 0 and
1/HZ. Not really useful.

Thanks,

        tglx