linux-kernel - Re: [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64

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Message-ID: <875zheclzm.fsf@nanos.tec.linutronix.de>
Date:   Tue, 14 Jan 2020 12:31:57 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, arnd@...db.de,
        vincenzo.frascino@....com, luto@...nel.org
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time

Christophe Leroy <christophe.leroy@....fr> writes:

> Using __iter_div_ulong_rem() is suboptimal on 32 bits.
> Nanoseconds are only 32 bits, and VDSO data is updated every 10ms
> so nsec will never overflow 32 bits.

That's theory and perhaps true for bare metal, but there is no guarantee
on VIRT that the CPU which has the timekeeping duty assigned is not
scheduled out for longer than 4 seconds.

Thanks,

        tglx