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SubjectRe: [RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp
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Hi Feng,

Using a global variable for this is not going to work, because you are adding a conditional branch and a load to a very hot path for the live of the system, not only for the duration of the boot.

Pavel

>
> +int tsc_inited;
> /*
> * TSC can be unstable due to cpufreq or due to unsynced TSCs
> */
> @@ -192,7 +193,7 @@ static void set_cyc2ns_scale(unsigned long khz, int cpu, unsigned long long tsc_
> */
> u64 native_sched_clock(void)
> {
> - if (static_branch_likely(&__use_tsc)) {
> + if (static_branch_likely(&__use_tsc) || tsc_inited) {
> u64 tsc_now = rdtsc();
>
> /* return the value in ns */
> @@ -1387,30 +1391,16 @@ static int __init init_tsc_clocksource(void)
> */
> device_initcall(init_tsc_clocksource);
>
> -void __init tsc_early_delay_calibrate(void)
> -{
> - unsigned long lpj;
> -
> - if (!boot_cpu_has(X86_FEATURE_TSC))
> - return;
> -
> - cpu_khz = x86_platform.calibrate_cpu();
> - tsc_khz = x86_platform.calibrate_tsc();
> -
> - tsc_khz = tsc_khz ? : cpu_khz;
> - if (!tsc_khz)
> - return;
> -
> - lpj = tsc_khz * 1000;
> - do_div(lpj, HZ);
> - loops_per_jiffy = lpj;
> -}
> -
> void __init tsc_init(void)
> {
> u64 lpj, cyc;
> int cpu;
>
> + if (tsc_inited)
> + return;
> +
> + tsc_inited = 1;
> +
> if (!boot_cpu_has(X86_FEATURE_TSC)) {
> setup_clear_cpu_cap(X86_FEATURE_TSC_DEADLINE_TIMER);
> return;
> @@ -1474,11 +1464,15 @@ void __init tsc_init(void)
> lpj = ((u64)tsc_khz * 1000);
> do_div(lpj, HZ);
> lpj_fine = lpj;
> + loops_per_jiffy = lpj;
>
> use_tsc_delay();
>
> check_system_tsc_reliable();
>
> + extern void early_set_sched_clock_stable(u64 sched_clock_offset);
> + early_set_sched_clock_stable(div64_u64(rdtsc() * 1000, tsc_khz));
> +
> if (unsynchronized_tsc()) {
> mark_tsc_unstable("TSCs unsynchronized");
> return;
> diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> index 10c83e7..6c5c22d 100644
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -119,6 +119,13 @@ static void __scd_stamp(struct sched_clock_data *scd)
> scd->tick_raw = sched_clock();
> }
>
> +
> +void early_set_sched_clock_stable(u64 sched_clock_offset)
> +{
> + __sched_clock_offset = sched_clock_offset;
> + static_branch_enable(&__sched_clock_stable);
> +}
> +
> static void __set_sched_clock_stable(void)
> {
> struct sched_clock_data *scd;
> @@ -342,12 +349,14 @@ static u64 sched_clock_remote(struct sched_clock_data *scd)
> *
> * See cpu_clock().
> */
> +
> +extern int tsc_inited;
> u64 sched_clock_cpu(int cpu)
> {
> struct sched_clock_data *scd;
> u64 clock;
>
> - if (sched_clock_stable())
> + if (sched_clock_stable() || tsc_inited)
> return sched_clock() + __sched_clock_offset;
>
> if (unlikely(!sched_clock_running))
>
>
>
>
>>>
>>> If you have a dodgy part (sorry SKX), you'll just have to live with
>>> sched_clock starting late(r).
>>>
>>> Do not cobble things on the side, try and get the normal things running
>>> earlier.

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