Re: [patch 20/48] x86/apic: Enable TSC coupled programming mode

[Thomas Gleixner]

On Tue, Mar 03 2026 at 15:37, Thomas Gleixner wrote:
> On Mon, Mar 02 2026 at 18:29, Nathan Chancellor wrote:
>>
>> After this change landed in -next as commit f246ec3478cf ("x86/apic:
>> Enable TSC coupled programming mode"), two of my Intel-based test
>> machines fail to boot. Unfortunately, I do not think I have any serial
>> access on these, so I have little introspective ability. Is there any
>> information I can provide or patches I can test to try and help figure
>> out what is going on here? I have attached the output of lscpu of both
>> machines, in case there is some common thread there.
>
> Grmbl. I stared at it for a while and I have a suspicion. Can you try
> the patch below and also provide from one of the machines the output of
>
>   dmesg | grep -i tsc
>
> In case that does not work, I'll send a debug patch in reply to this
> mail.

Here you go. It fails the coupled mode, but emits all relevant data via
trace_printk. Just collect it right after booting and please tell me
where the boot stops in the boot process so I can narrow down the search.

Thanks,

        tglx
---
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -424,9 +424,10 @@ static int lapic_next_deadline(unsigned
 	 * There is no weak_wrmsr_fence() required here as all of this is purely
 	 * CPU local. Avoid the [ml]fence overhead.
 	 */
-	u64 tsc = rdtsc();
+	u64 dl = rdtsc() + (((u64) delta) * TSC_DIVISOR);
 
-	native_wrmsrq(MSR_IA32_TSC_DEADLINE, tsc + (((u64) delta) * TSC_DIVISOR));
+	native_wrmsrq(MSR_IA32_TSC_DEADLINE, dl);
+	trace_printk("APIC    deadline: %16llu\n", dl);
 	return 0;
 }
 
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -310,11 +310,9 @@ static inline bool clockevent_set_next_c
 	if (unlikely(!ktime_expiry_to_cycles(dev->cs_id, expires, &cycles)))
 		return false;
 
-	if (IS_ENABLED(CONFIG_GENERIC_CLOCKEVENTS_COUPLED_INLINE))
-		arch_inlined_clockevent_set_next_coupled(cycles, dev);
-	else
-		dev->set_next_coupled(cycles, dev);
-	return true;
+	trace_printk("Coupled deadline: %16llu Exp: %16lld\n", cycles, expires);
+
+	return false;
 }
 
 #else
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -404,6 +404,7 @@ static void tk_setup_internals(struct ti
 		 */
 		clocks_calc_mult_shift(&tk->cs_ns_to_cyc_mult, &tk->cs_ns_to_cyc_shift,
 				       NSEC_PER_MSEC, clock->freq_khz, 3600 * 1000);
+		tk->cs_ns_to_cyc_maxns = div_u64(clock->mask, tk->cs_ns_to_cyc_mult);
 	}
 }
 
@@ -762,6 +763,11 @@ static inline void tk_update_ns_to_cyc(s
 	shift = tkrs->shift + tks->cs_ns_to_cyc_shift;
 	tks->cs_ns_to_cyc_mult = (u32)div_u64(1ULL << shift, tkrs->mult);
 	tks->cs_ns_to_cyc_maxns = div_u64(tkrs->clock->mask, tks->cs_ns_to_cyc_mult);
+	trace_printk("CSM: %8u CSS: %8u CEM: %8u CES: %8u MNS: %16llu BNS: %16lld BCY: %16llu\n",
+		     tkrs->shift, tkrs->mult, tks->cs_ns_to_cyc_mult,
+		     tks->cs_ns_to_cyc_shift, tks->cs_ns_to_cyc_maxns,
+		     tkrs->base + (tkrs->xtime_nsec >> tkrs->shift),
+		     tkrs->cycle_last);
 }
 
 /*