From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-19.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A6958C433B4 for ; Mon, 19 Apr 2021 04:54:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 82EED61077 for ; Mon, 19 Apr 2021 04:54:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233135AbhDSExz (ORCPT ); Mon, 19 Apr 2021 00:53:55 -0400 Received: from mail.kernel.org ([198.145.29.99]:49350 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231489AbhDSExm (ORCPT ); Mon, 19 Apr 2021 00:53:42 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id D723361078; Mon, 19 Apr 2021 04:53:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1618807981; bh=Fm8ha9NsD+vzNmpqwn+UflASmpASREXCUpZYkaCfmu0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=lw3V5qdzkOnctN5BY3hHg1k6EzlcHdinzyhDLaN+/tv6yoAAjzmepxvwnp+F7dPQx OPdwRH+W4NNP3vEoLkBjmD/P/cMSIuDmuinEIVlN7iPscF43wKuuUsQKXbPvjd4OVx 4HwQfQ14Cql3cGkuOY1UsUNIWOQKTD0EHNfXtbXSahoF9Xf5CuYHUqVMtw0mD/c8eI uC39IlFP9Nwh0Ufv8dJtEYzTxWiuZ878VpHJvEjEJ0xvVIKfWJKM3N38ni1448GQGA QEu0tUpB6zMbxrB1yBsr+mMTYhUx75v6nWwokMuSr/eX1EajHDA0Y5XDRI2e0bBnIv Cx3QWv6HB8yBg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 9A5925C011C; Sun, 18 Apr 2021 21:53:01 -0700 (PDT) From: "Paul E. McKenney" To: tglx@linutronix.de Cc: linux-kernel@vger.kernel.org, john.stultz@linaro.org, sboyd@kernel.org, corbet@lwn.net, Mark.Rutland@arm.com, maz@kernel.org, kernel-team@fb.com, neeraju@codeaurora.org, ak@linux.intel.com, "Paul E. McKenney" , Chris Mason Subject: [PATCH v9 clocksource 2/6] clocksource: Retry clock read if long delays detected Date: Sun, 18 Apr 2021 21:52:56 -0700 Message-Id: <20210419045300.596332-2-paulmck@kernel.org> X-Mailer: git-send-email 2.31.1.189.g2e36527f23 In-Reply-To: <20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1> References: <20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When the clocksource watchdog marks a clock as unstable, this might be due to that clock being unstable or it might be due to delays that happen to occur between the reads of the two clocks. Yes, interrupts are disabled across those two reads, but there are no shortage of things that can delay interrupts-disabled regions of code ranging from SMI handlers to vCPU preemption. It would be good to have some indication as to why the clock was marked unstable. Therefore, re-read the watchdog clock on either side of the read from the clock under test. If the watchdog clock shows an excessive time delta between its pair of reads, the reads are retried. The maximum number of retries is specified by a new kernel boot parameter clocksource.max_read_retries, which defaults to three, that is, up to four reads, one initial and up to three retries. If more than one retry was required, a message is printed on the console (the occasional single retry is expected behavior, especially in guest OSes). If the maximum number of retries is exceeded, the clock under test will be marked unstable. However, the probability of this happening due to various sorts of delays is quite small. In addition, the reason (clock-read delays) for the unstable marking will be apparent. Cc: John Stultz Cc: Thomas Gleixner Cc: Stephen Boyd Cc: Jonathan Corbet Cc: Mark Rutland Cc: Marc Zyngier Cc: Andi Kleen Reported-by: Chris Mason [ paulmck: Per-clocksource retries per Neeraj Upadhyay feedback. ] [ paulmck: Don't reset injectfail per Neeraj Upadhyay feedback. ] [ paulmck: Apply Thomas Gleixner feedback. ] Signed-off-by: Paul E. McKenney --- .../admin-guide/kernel-parameters.txt | 6 +++ kernel/time/clocksource.c | 53 ++++++++++++++++--- 2 files changed, 52 insertions(+), 7 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 4a372037b49f..7fff95bd5504 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -599,6 +599,12 @@ will be five delay-free reads followed by three delayed reads. + clocksource.max_read_retries= [KNL] + Number of clocksource_watchdog() retries due to + external delays before the clock will be marked + unstable. Defaults to three retries, that is, + four attempts to read the clock under test. + clearcpuid=BITNUM[,BITNUM...] [X86] Disable CPUID feature X for the kernel. See arch/x86/include/asm/cpufeatures.h for the valid bit diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index d77700c45f4e..facce83fcb4a 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -125,6 +125,13 @@ static void __clocksource_change_rating(struct clocksource *cs, int rating); #define WATCHDOG_INTERVAL (HZ >> 1) #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4) +/* + * Maximum permissible delay between two readouts of the watchdog + * clocksource surrounding a read of the clocksource being validated. + * This delay could be due to SMIs, NMIs, or to VCPU preemptions. + */ +#define WATCHDOG_MAX_SKEW (100 * NSEC_PER_USEC) + static void clocksource_watchdog_work(struct work_struct *work) { /* @@ -189,6 +196,8 @@ static ulong inject_delay_period; module_param(inject_delay_period, ulong, 0644); static ulong inject_delay_repeat = 1; module_param(inject_delay_repeat, ulong, 0644); +static ulong max_read_retries = 3; +module_param(max_read_retries, ulong, 0644); static void clocksource_watchdog_inject_delay(void) { @@ -206,12 +215,42 @@ static void clocksource_watchdog_inject_delay(void) invocations++; } +static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow) +{ + unsigned int nretries; + u64 wd_end, wd_delta; + int64_t wd_delay; + + for (nretries = 0; nretries <= max_read_retries; nretries++) { + local_irq_disable(); + *wdnow = watchdog->read(watchdog); + clocksource_watchdog_inject_delay(); + *csnow = cs->read(cs); + wd_end = watchdog->read(watchdog); + local_irq_enable(); + + wd_delta = clocksource_delta(wd_end, *wdnow, watchdog->mask); + wd_delay = clocksource_cyc2ns(wd_delta, watchdog->mult, watchdog->shift); + if (wd_delay <= WATCHDOG_MAX_SKEW) { + if (nretries > 1 || nretries >= max_read_retries) { + pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n", + smp_processor_id(), watchdog->name, nretries); + } + return true; + } + } + + pr_warn("timekeeping watchdog on CPU%d: %s read-back delay of %lldns, attempt %d, marking unstable\n", + smp_processor_id(), watchdog->name, wd_delay, nretries); + return false; +} + static void clocksource_watchdog(struct timer_list *unused) { - struct clocksource *cs; u64 csnow, wdnow, cslast, wdlast, delta; - int64_t wd_nsec, cs_nsec; int next_cpu, reset_pending; + int64_t wd_nsec, cs_nsec; + struct clocksource *cs; spin_lock(&watchdog_lock); if (!watchdog_running) @@ -228,11 +267,11 @@ static void clocksource_watchdog(struct timer_list *unused) continue; } - local_irq_disable(); - csnow = cs->read(cs); - clocksource_watchdog_inject_delay(); - wdnow = watchdog->read(watchdog); - local_irq_enable(); + if (!cs_watchdog_read(cs, &csnow, &wdnow)) { + /* Clock readout unreliable, so give it up. */ + __clocksource_unstable(cs); + continue; + } /* Clocksource initialized ? */ if (!(cs->flags & CLOCK_SOURCE_WATCHDOG) || -- 2.31.1.189.g2e36527f23