Messages in this thread Patch in this message | | | From | Feng Tang <> | Subject | [RFC 2/2] x86/tsc: mark tsc reliable for qualified platforms | Date | Tue, 30 Mar 2021 16:25:47 +0800 |
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There are cases that tsc clocksource are wrongly judged as unstable by clocksource watchdogs like hpet, acpi_pm or 'refined-jiffies'. While there is hardly a general reliable way to check the validity of a watchdog, and to protect the innocent tsc, Thomas Gleixner proposed [1]:
"I'm inclined to lift that requirement when the CPU has:
1) X86_FEATURE_CONSTANT_TSC 2) X86_FEATURE_NONSTOP_TSC 3) X86_FEATURE_NONSTOP_TSC_S3 4) X86_FEATURE_TSC_ADJUST 5) At max. 4 sockets
After two decades of horrors we're finally at a point where TSC seems to be halfway reliable and less abused by BIOS tinkerers. TSC_ADJUST was really key as we can now detect even small modifications reliably and the important point is that we can cure them as well (not pretty but better than all other options)."
As feature #3 X86_FEATURE_NONSTOP_TSC_S3 only exists on several generations of Atom processor, and is always coupled with X86_FEATURE_CONSTANT_TSC and X86_FEATURE_NONSTOP_TSC, skip checking it,. and also be more defensive to use maxim of 2 sockets.
The check is done inside tsc_init() before registering 'tsc-early' and 'tsc' clocksources, as there were cases that both of them had been wrongly judged as unreliable.
Another idea is to not lift the check here, but inside the tsc_sync_check_timer, stop the clocksource watchdog timer if there is no TSC_ADJUST overridden found in 10 minutes after boot.
[1]. https://lore.kernel.org/lkml/87eekfk8bd.fsf@nanos.tec.linutronix.de/ Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Feng Tang <feng.tang@intel.com> --- arch/x86/kernel/tsc.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index f70dffc..3a451e3 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -1193,6 +1193,17 @@ static void __init check_system_tsc_reliable(void) #endif if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) tsc_clocksource_reliable = 1; + + /* + * Ideally the socket number should be checked, but this is called + * by tsc_init() which is in early boot phase and the socket numbers + * may not be available. Use 'nr_online_nodes' as a fallback solution + */ + if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) && + boot_cpu_has(X86_FEATURE_TSC_ADJUST) && + nr_online_nodes <= 2) + tsc_clocksource_reliable = 1; } /* -- 2.7.4
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