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Wed, 28 Jan 2026 07:33:05 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8779C20043; Wed, 28 Jan 2026 07:33:05 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0CF7120040; Wed, 28 Jan 2026 07:33:01 +0000 (GMT) Received: from [9.111.60.95] (unknown [9.111.60.95]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 28 Jan 2026 07:33:00 +0000 (GMT) Message-ID: <2a7521d9-e4d8-4dc6-8e5a-122796f0d1ab@linux.ibm.com> Date: Wed, 28 Jan 2026 13:02:59 +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: K Prateek Nayak , "Guo, Wangyang" Cc: linux-kernel@vger.kernel.org, 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: Shrikanth Hegde In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: d0V8-cpBKNUjbhr9IbwsceVc2rMLWkw9 X-Proofpoint-ORIG-GUID: sqXLFblXVtaAZtItRV-MZ5Y1dDiViDAX X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTI4MDA1OCBTYWx0ZWRfX2mXiFSYPVYTo 9zSTsS9iEx/k7EXhJLro8TbmryopRGZvqzxZGmcUlqajv6J1JXFseechWQmaaXkf9VG5Vuo0izu FimOxJsJv/jes8Kiny7IsEmTL31setpWhdVyQd9NhTGzSRBpP6XgN8nMc3Yi0s+WXKf0OHjW3IB 0Omu4LuWEFjqDIvdg5x5j2ZnsXr3URvxpK0yZ+cE+dqM5LyUyscV3Mv26yZxp2D3Omj+YEh3kN4 t8LIhGeD16jZjuJ1GV/T960hZHSDSGW+GqRywSHAKiWH3VdISRUf7KB5z+hm/RrPTeI3f4mVfJO lyPYuwmTLZ+fbsVvIAIFsmFkci1svWuj3nXdygs3p43OuQCdMdB7+UzJL+5H7zMMpkwh3an4KiD OnkH9HLwywF9BiP1FPqBeucVSaAlKC+IyEYQAKN+LZL8er8nFmHr9bOICe2My8H5PcGcdk7AIDp gomirwtKm7VpuGI/82A== X-Authority-Analysis: v=2.4 cv=AMiVTGgp c=1 sm=1 tr=0 ts=6979bbb5 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=32R5OCWjW300CMAS1oQA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-01-28_01,2026-01-27_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 spamscore=0 phishscore=0 bulkscore=0 suspectscore=0 adultscore=0 clxscore=1015 priorityscore=1501 lowpriorityscore=0 impostorscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2601150000 definitions=main-2601280058 On 1/28/26 12:48 PM, K Prateek Nayak wrote: > 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 This means pv_sched_clock is kvm_sched_clock_read from now. and irqtime is enabled in the guest. right? > [ 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 > so now, using_native_sched_clock should fail in guest? If so, with the patch, irqtime won't be disabled no? > 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. > Maybe kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) won't be set.