linux-kernel - Re: [PATCH v2] timer: Use static_branch_likely() for timers_nohz

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Message-ID: <20210820181814.2761b051@xhacker.debian>
Date:   Fri, 20 Aug 2021 18:18:14 +0800
From:   Jisheng Zhang <Jisheng.Zhang@...aptics.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] timer: Use static_branch_likely() for
 timers_nohz_active

On Tue, 10 Aug 2021 18:00:16 +0200 Thomas Gleixner wrote:

> 
> 
> Jisheng,

Hi Thomas,

> 
> On Thu, Jun 24 2021 at 16:32, Jisheng Zhang wrote:
> 
> > The static key timers_nohz_active is likely to be true, so use
> > static_branch_likely() to reflect this fact.  
> 
> you still lack any justification for your statement that NOHZ active is
> likely.

This patch only affects NO_HZ_COMMON=y code path which is enabled if either
NO_HZ_IDLE or NO_HZ_FULL = y.
Per my understanding, when the system has oneshot tick device, NOHZ active
is likely true. After going through drivers/clocksource/*, I found there are
about 86 drivers, 69(occupies 80%) of which support CLOCK_EVT_FEAT_ONESHOT.
Then I browsed apic, hpet and xen timer source code in x86 directory, they
all support oneshot feature.

I'm not sure whether I misunderstood the code or not.

Thanks

> 
> It might be likely for your system, but is this true in general?
>