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Message-ID: <20140722075729.GX3935@laptop> Date: Tue, 22 Jul 2014 09:57:29 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Andy Lutomirski <luto@...capital.net> Cc: Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org> Subject: Re: [PATCH] x86, TSC: Add a software TSC offset On Mon, Jul 21, 2014 at 02:56:49PM -0700, Andy Lutomirski wrote: > > Remember, this is only attempting to be a hardware workaround for a > > smallish number of systems out there. Most of current machines should > > have stable and synched TSCs. > > I actually own one of these systems. It's a Sandy Bridge Core-i7 > Extreme or something like that. If it is (and it sounds like it is) a single socket, then your unsynced TSC is likely due to SMM fuckery and the TSCs will drift further and further apart as (run)time increases due to SMM activity. The problem Borislav is talking about is multi socket systems, which due to (failed) board layout get the CPUs powered up 'wrong' and the TSCs between sockets is offset because of this, its a fixed offset and stable forever after (until power cycle etc..). I have one WSM-EP that does this (occasionally). His initial idea was to re-write the TSC value to match, but since writing the TSC is expensive (in the 1000s of cycles range) getting an offset adjustment of 10s of cycles in just right is nigh on impossible. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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