On 06/23/2018 12:56 PM, Thomas Gleixner wrote:
> On Thu, 21 Jun 2018, Pavel Tatashin wrote:
>> /*
>> * Scheduler clock - returns current time in nanosec units.
>> */
>> @@ -1354,6 +1364,7 @@ void __init tsc_early_delay_calibrate(void)
>> lpj = tsc_khz * 1000;
>> do_div(lpj, HZ);
>> loops_per_jiffy = lpj;
>> + sched_clock_early_init(tsc_khz);
>> }
>>
>> void __init tsc_init(void)
>> @@ -1382,6 +1393,7 @@ void __init tsc_init(void)
>> if (!tsc_khz) {
>> mark_tsc_unstable("could not calculate TSC khz");
>> setup_clear_cpu_cap(X86_FEATURE_TSC_DEADLINE_TIMER);
>> + static_branch_disable(&__use_tsc);
>
> This wants a proper comment.
Yes, I will add a comment. Basically, this change is for a rare scenario where early in boot we were able to get TSC frequency but failed to do so later in boot when we tried to calculate it more precisely. Though, I am not sure if this is actually possible. However, if this is indeed possible, we will get into the same continuity problem as the one that is solved in "x86/tsc: prepare for early sched_clock". So, I guess we would have to modify either "jiffies_64" or __sched_clock_offset if we want to take care of this corner case.
Thank you,
Pavel
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