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client-ip=165.204.84.17; helo=satlexmb08.amd.com; pr=C Received: from satlexmb08.amd.com (165.204.84.17) by DS3PEPF000099DB.mail.protection.outlook.com (10.167.17.197) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9564.3 via Frontend Transport; Wed, 28 Jan 2026 07:18:44 +0000 Received: from satlexmb08.amd.com (10.181.42.217) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.17; Wed, 28 Jan 2026 01:18:43 -0600 Received: from [10.136.38.16] (10.180.168.240) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server id 15.2.2562.17 via Frontend Transport; Wed, 28 Jan 2026 01:18:40 -0600 Message-ID: Date: Wed, 28 Jan 2026 12:48:39 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7] sched/clock: Avoid false sharing for sched_clock_irqtime To: Shrikanth Hegde , "Guo, Wangyang" CC: , Benjamin Lei , Tim Chen , Tianyou Li , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider References: <20260127072509.2627346-1-wangyang.guo@intel.com> <7c6236a5-82b7-4d1b-9bb2-3f0d2ac186f8@linux.ibm.com> <3b69443c-f82c-4b73-8384-2a039d730213@intel.com> <5e001aeb-50a8-475f-bff2-6d101e15d078@amd.com> <868b9993-34a2-4b33-b47a-989b2c680689@linux.ibm.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <868b9993-34a2-4b33-b47a-989b2c680689@linux.ibm.com> Content-Type: text/plain; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 Jan 2026 07:18:44.1104 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 19118e56-9929-4d34-dac0-08de5e3d788f X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb08.amd.com] X-MS-Exchange-CrossTenant-AuthSource: DS3PEPF000099DB.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN7PPF62A0C9A68 On 1/28/2026 11:56 AM, Shrikanth Hegde wrote: > > > On 1/28/26 8:35 AM, K Prateek Nayak wrote: >> On 1/28/2026 7:49 AM, Guo, Wangyang wrote: >>> Yes, when clock mark unstable through tsc_.*mark_unstable() with non-native_sched_clock, clear_sched_clock_stable won't be called, thus sched_clock_irqtime still keep enabled. >>> >>> Maybe the dedicated workqueue for sched_clock_irqtime is still needed considering this case. >> >> In that case, shouldn't tsc_init() only enable irqtime when >> using_native_sched_clock()? How can tsc_init() make a call on irqtime if >> TSC isn't being used as the sched_clock() ultimately? >> >> For kvmclock, if PVCLOCK_TSC_STABLE_BIT is not set, it'll call >> clear_sched_clock_stable() at kvm_sched_clock_init() but none of the >> other clocksources do so we can assume once we override the sched_clock() >> it is up to the sched_clock() provider to deal with the clock stability. >> > > I think this would depend if mark_tsc_unstable happens after system boot, > specially while running kvm guest? I don't see anything on the guest side that would mark the kvmclock as unstable if host's TSC turns unstable post init and since kvmclock doesn't set CLOCK_SOURCE_MUST_VERIFY, I doubt if a watchdog runs to verify it in the guest. I have the following in the guest: $ sudo dmesg | grep -i clock [ 0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00 [ 0.000000] kvm-clock: using sched offset of 423259259 cycles [ 0.000002] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns [ 0.071675] clocksource: refined-jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645519600211568 ns [ 0.378467] clocksource: hpet: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604467 ns [ 0.388678] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x398cb1e4d56, max_idle_ns: 881590790753 ns [ 0.679262] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645041785100000 ns [ 0.903121] PTP clock support registered [ 0.927243] clocksource: Switched to clocksource kvm-clock [ 0.944986] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff, max_idle_ns: 2085701024 ns [ 0.993198] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x398cb1e4d56, max_idle_ns: 881590790753 ns [ 1.123796] rtc_cmos 00:05: setting system clock to 2026-01-28T07:03:45 UTC (1769583825) [ 1.155755] sched_clock: Marking stable (940009972, 212965288)->(1171254846, -18279586) [ 1.712598] clk: Disabling unused clocks Then I mark TSC unstable on the host tsc: Marking TSC unstable due to Faking unreliable TSC! TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'. clocksource: Checking clocksource tsc synchronization from CPU 93 to CPUs 0,2,26,75,101,114,118,195. sched_clock: Marking unstable (945948313746, 69389667)<-(947618130068, -1600430832) clocksource: CPU 93 check durations 3436ns - 25277ns for clocksource tsc. clocksource: Switched to clocksource hpet And nothing happens in the guest. cat /sys/devices/system/clocksource/clocksource0/current_clocksource kvm-clock If I launch the guest after marking host TSC unstable, I see: Unstable clock detected, switching default tracing clock to "global" and I don't get any "sched_clock: Marking stable" messages. -- Thanks and Regards, Prateek