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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Pavel Tatashin <pasha.tatashin@oracle.com>
Subject: Re: [RFC] tick_sched_timer() is not properly aligned, fixed by chance
In-Reply-To: <20230406150254.ZrawA2Y-@linutronix.de>
References: <20230406095735.0_14edn3@linutronix.de>
 <20230406105529.GB392176@hirez.programming.kicks-ass.net>
 <87mt3lqnde.ffs@tglx> <20230406150254.ZrawA2Y-@linutronix.de>
Date:   Thu, 06 Apr 2023 19:13:41 +0200
Message-ID: <87jzypq6gq.ffs@tglx>
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On Thu, Apr 06 2023 at 17:02, Sebastian Andrzej Siewior wrote:
> On 2023-04-06 13:08:29 [+0200], Thomas Gleixner wrote:
> The first one added is hpet. It points to "jiffies" as name and
> jiffies_read() as ->read().
>
> Before the change/ with PeterZ change I get from ktime_get() for
> base/nsec something like:
> |[    0.004000] tick_next_period Name: jiffies jiffies_read+0x0/0x10
> |[    0.004000] tick_next_period base/nsec: -401771248 405771248
> |[    0.004000] tick_next_period hpet 4000000
>
> base/nsec is different on each boot but it always ends up with 4000000.
> With the optimisation, ktime_get() is:
> | [    1.179079] tick_next_period base/nsec: 647439581 518613145
> | [    1.179646] tick_next_period hpet 1166052726
>
> so something is using it during init. And this is
> 	read_persistent_wall_and_boot_offset()

Duh, yes. I remember now. That was done to take the TSC "uptime" into
account, which is sensible.

So making tick_next_period aligned to TICK_NSEC in tick_setup_device()
is the right thing to do in general then.

Thanks,

        tglx