Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753271AbcDRRF1 (ORCPT ); Mon, 18 Apr 2016 13:05:27 -0400 Received: from mail-ob0-f171.google.com ([209.85.214.171]:35118 "EHLO mail-ob0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753113AbcDRRFX (ORCPT ); Mon, 18 Apr 2016 13:05:23 -0400 MIME-Version: 1.0 In-Reply-To: <09c4f19409012995595db6fd0a12f326c292af1a.1460422356.git.shli@fb.com> References: <09c4f19409012995595db6fd0a12f326c292af1a.1460422356.git.shli@fb.com> Date: Mon, 18 Apr 2016 10:05:22 -0700 Message-ID: Subject: Re: [RFC 1/2] time: workaround crappy hpet From: John Stultz To: Shaohua Li Cc: lkml , Thomas Gleixner , calvinowens@fb.com Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1544 Lines: 33 On Mon, Apr 11, 2016 at 5:57 PM, Shaohua Li wrote: > Calvin found 'perf record -a --call-graph dwarf -- sleep 5' making clocksource > switching to hpet. We found similar symptom in another machine. Here is an example: > > [8224517.520885] timekeeping watchdog: Marking clocksource 'tsc' as unstable, because the skew is too large: > [8224517.540032] 'hpet' wd_now: ffffffff wd_last: b39c0bd mask: ffffffff > [8224517.553092] 'tsc' cs_now: 48ceac7013714e cs_last: 48ceac25be34ac mask: ffffffffffffffff > [8224517.569849] Switched to clocksource hpet > > In both machines, wd_now is 0xffffffff. The tsc time looks correct, the cpu is 2.5G > (0x48ceac7013714e - 0x48ceac25be34ac)/2500000 = 0.4988s > 0.4988s matches WATCHDOG_INTERVAL. Since hpet reads to 0xffffffff in both > machines, this sounds not coincidence, hept is crappy. > > This patch tries to workaround this issue. We do retry if hpet has 0xffffff value. > In the relevant machine, the hpet counter doesn't read to 0xffffffff later. > The chance hpet has 0xffffffff counter is very small, this patch should have no > impact for good hpet. > > I'm open if there is better solution. Hrm.. So can you characterize this bad behavior a bit more for us? Does every read of the HPET return 0xFFFFFFFF ? Or does it just occasionally start returning -1 values? Or once it trips and starts returning -1 does it always return -1? I'm trying to understand if there is a way to catch and disqualify that clocksource earlier then in the watchdog logic. thanks -john