linux-kernel - Re: [PATCH] hrtimer: Update softirq_expires_next correctly after __hrtimer_get_next

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Date:   Tue, 20 Apr 2021 08:44:52 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Maciej Żenczykowski <maze@...gle.com>
Cc:     Lorenzo Colitti <lorenzo@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        lkml <linux-kernel@...r.kernel.org>, mikael.beckius@...driver.com
Subject: Re: [PATCH] hrtimer: Update softirq_expires_next correctly after __hrtimer_get_next_event()

On Mon, Apr 19 2021 at 20:12, Maciej Żenczykowski wrote:
> On Thu, Apr 15, 2021 at 9:47 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> Run the test on a kernels with and without that commit and collect trace
>> data for both.
>>
>> That should give me a pretty clear picture what's going on.
>
> Lorenzo is trying to get the traces you asked for, or rather he’s
> trying to get confirmation he can post them.
>
> Our initial observation of these results seems to suggest that
> updating the timer (hrtimer_start, which seems to also call
> hrtimer_cancel) takes twice as long as it used to.

Which contradicts my measurements. The change in complexity is marginal
and the overhead in cycles/instructions is close to noise. It's
measurable in a microbenchmark, but it's in the < 1% range which is far
away from the 60% you are seing.

> My gut feeling is that softirq_activated is usually false, and the old
> code in such a case calls just __hrtimer_get_next_event(,
> HRTIMER_ACTIVE_ALL).  While the new code will first call
> __hrtimer_get_next_event(, HRTIMER_ACTIVE_SOFT) and then
> __hrtimer_get_next_event(, HRTIMER_ACTIVE_HARD)
>
> Perhaps __hrtimer_get_next_event() should return both soft and hard
> event times in one function call?
> Or perhaps hrtimer_start should not call hrtimer_cancel?

Perhaps we do a proper analysis first :)

Thanks,

        tglx