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From: Thomas Gleixner <tglx@linutronix.de>
To: Christophe Leroy <christophe.leroy@c-s.fr>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	arnd@arndb.de, vincenzo.frascino@arm.com, luto@kernel.org
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
	x86@kernel.org
Subject: Re: [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time
Date: Tue, 14 Jan 2020 12:31:57 +0100	[thread overview]
Message-ID: <875zheclzm.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <5b38617a2ca4f719760aafbdb6115eaad28c0640.1578934751.git.christophe.leroy@c-s.fr>

Christophe Leroy <christophe.leroy@c-s.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

WARNING: multiple messages have this Message-ID (diff)
From: Thomas Gleixner <tglx@linutronix.de>
To: Christophe Leroy <christophe.leroy@c-s.fr>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	arnd@arndb.de, vincenzo.frascino@arm.com, luto@kernel.org
Cc: x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org
Subject: Re: [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time
Date: Tue, 14 Jan 2020 12:31:57 +0100	[thread overview]
Message-ID: <875zheclzm.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <5b38617a2ca4f719760aafbdb6115eaad28c0640.1578934751.git.christophe.leroy@c-s.fr>

Christophe Leroy <christophe.leroy@c-s.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

WARNING: multiple messages have this Message-ID (diff)
From: Thomas Gleixner <tglx@linutronix.de>
To: Christophe Leroy <christophe.leroy@c-s.fr>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	arnd@arndb.de, vincenzo.frascino@arm.com, luto@kernel.org
Cc: x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org
Subject: Re: [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time
Date: Tue, 14 Jan 2020 12:31:57 +0100	[thread overview]
Message-ID: <875zheclzm.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <5b38617a2ca4f719760aafbdb6115eaad28c0640.1578934751.git.christophe.leroy@c-s.fr>

Christophe Leroy <christophe.leroy@c-s.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

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linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2020-01-14 11:32 UTC|newest]

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-13 17:08 [RFC PATCH v3 00/12] powerpc: switch VDSO to C implementation Christophe Leroy
2020-01-13 17:08 ` Christophe Leroy
2020-01-13 17:08 ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 01/12] powerpc/64: Don't provide time functions in compat VDSO32 Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 02/12] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 03/12] lib: vdso: mark __cvdso_clock_getres() as static Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 04/12] lib: vdso: inline do_hres() and do_coarse() Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 05/12] lib: vdso: Avoid duplication in __cvdso_clock_getres() Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-14 11:31   ` Thomas Gleixner [this message]
2020-01-14 11:31     ` Thomas Gleixner
2020-01-14 11:31     ` Thomas Gleixner
2020-01-13 17:08 ` [RFC PATCH v3 07/12] powerpc/vdso: simplify __get_datapage() Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-14 23:06   ` Thomas Gleixner
2020-01-14 23:06     ` Thomas Gleixner
2020-01-14 23:06     ` Thomas Gleixner
2020-01-15  6:15     ` Christophe Leroy
2020-01-15  6:15       ` Christophe Leroy
2020-01-15  6:15       ` Christophe Leroy
2020-01-16  9:16       ` Christophe Leroy
2020-01-16  9:16         ` Christophe Leroy
2020-01-16  9:16         ` Christophe Leroy
2020-01-16 10:35         ` Thomas Gleixner
2020-01-16 10:35           ` Thomas Gleixner
2020-01-16 10:35           ` Thomas Gleixner
2020-01-16 20:22           ` Andy Lutomirski
2020-01-16 20:22             ` Andy Lutomirski
2020-01-16 20:22             ` Andy Lutomirski
2020-01-13 17:08 ` [RFC PATCH v3 09/12] powerpc/vdso: provide inline alternative to __get_datapage() Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 10/12] powerpc/vdso: provide vdso data pointer from the ASM caller Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 11/12] lib: vdso: split clock verification out of __arch_get_hw_counter() Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 12/12] powerpc/vdso: provide __arch_is_hw_counter_valid() Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy
2020-01-13 17:08   ` Christophe Leroy

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