[patch v3 12/36] [patch V2 12/38] x86/smpboot: Make TSC synchronization function call based

Thomas Gleixner
2023-05-08T19:44:33Z

From: Thomas Gleixner <tglx@linutronix.de>

Spin-waiting on the control CPU until the AP reaches the TSC
synchronization is just a waste especially in the case that there is no
synchronization required.

As the synchronization has to run with interrupts disabled the control CPU
part can just be done from a SMP function call. The upcoming AP issues that
call async only in the case that synchronization is required.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>

---
arch/x86/include/asm/tsc.h | 2 --
arch/x86/kernel/smpboot.c | 20 +++-----------------
arch/x86/kernel/tsc_sync.c | 36 +++++++++++-------------------------
3 files changed, 14 insertions(+), 44 deletions(-)
---

--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -55,12 +55,10 @@ extern bool tsc_async_resets;
#ifdef CONFIG_X86_TSC
extern bool tsc_store_and_check_tsc_adjust(bool bootcpu);
extern void tsc_verify_tsc_adjust(bool resume);
-extern void check_tsc_sync_source(int cpu);
extern void check_tsc_sync_target(void);
#else
static inline bool tsc_store_and_check_tsc_adjust(bool bootcpu) { return false; }
static inline void tsc_verify_tsc_adjust(bool resume) { }
-static inline void check_tsc_sync_source(int cpu) { }
static inline void check_tsc_sync_target(void) { }
#endif

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -278,11 +278,7 @@ static void notrace start_secondary(void
/* Otherwise gcc will move up smp_processor_id() before cpu_init() */
barrier();

- /*
- * Check TSC synchronization with the control CPU, which will do
- * its part of this from wait_cpu_online(), making it an implicit
- * synchronization point.
- */
+ /* Check TSC synchronization with the control CPU. */
check_tsc_sync_target();

/*
@@ -1144,21 +1140,11 @@ static void wait_cpu_callin(unsigned int
}

/*
- * Bringup step four: Synchronize the TSC and wait for the target AP
- * to reach set_cpu_online() in start_secondary().
+ * Bringup step four: Wait for the target AP to reach set_cpu_online() in
+ * start_secondary().
*/
static void wait_cpu_online(unsigned int cpu)
{
- unsigned long flags;
-
- /*
- * Check TSC synchronization with the AP (keep irqs disabled
- * while doing so):
- */
- local_irq_save(flags);
- check_tsc_sync_source(cpu);
- local_irq_restore(flags);
-
/*
* Wait for the AP to mark itself online, so the core caller
* can drop sparse_irq_lock.
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -245,7 +245,6 @@ bool tsc_store_and_check_tsc_adjust(bool
*/
static atomic_t start_count;
static atomic_t stop_count;
-static atomic_t skip_test;
static atomic_t test_runs;

/*
@@ -344,21 +343,14 @@ static inline unsigned int loop_timeout(
}

/*
- * Source CPU calls into this - it waits for the freshly booted
- * target CPU to arrive and then starts the measurement:
+ * The freshly booted CPU initiates this via an async SMP function call.
*/
-void check_tsc_sync_source(int cpu)
+static void check_tsc_sync_source(void *__cpu)
{
+ unsigned int cpu = (unsigned long)__cpu;
int cpus = 2;

/*
- * No need to check if we already know that the TSC is not
- * synchronized or if we have no TSC.
- */
- if (unsynchronized_tsc())
- return;
-
- /*
* Set the maximum number of test runs to
* 1 if the CPU does not provide the TSC_ADJUST MSR
* 3 if the MSR is available, so the target can try to adjust
@@ -368,16 +360,9 @@ void check_tsc_sync_source(int cpu)
else
atomic_set(&test_runs, 3);
retry:
- /*
- * Wait for the target to start or to skip the test:
- */
- while (atomic_read(&start_count) != cpus - 1) {
- if (atomic_read(&skip_test) > 0) {
- atomic_set(&skip_test, 0);
- return;
- }
+ /* Wait for the target to start. */
+ while (atomic_read(&start_count) != cpus - 1)
cpu_relax();
- }

/*
* Trigger the target to continue into the measurement too:
@@ -397,14 +382,14 @@ void check_tsc_sync_source(int cpu)
if (!nr_warps) {
atomic_set(&test_runs, 0);

- pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
+ pr_debug("TSC synchronization [CPU#%d -> CPU#%u]: passed\n",
smp_processor_id(), cpu);

} else if (atomic_dec_and_test(&test_runs) || random_warps) {
/* Force it to 0 if random warps brought us here */
atomic_set(&test_runs, 0);

- pr_warn("TSC synchronization [CPU#%d -> CPU#%d]:\n",
+ pr_warn("TSC synchronization [CPU#%d -> CPU#%u]:\n",
smp_processor_id(), cpu);
pr_warn("Measured %Ld cycles TSC warp between CPUs, "
"turning off TSC clock.\n", max_warp);
@@ -457,11 +442,12 @@ void check_tsc_sync_target(void)
* SoCs the TSC is frequency synchronized, but still the TSC ADJUST
* register might have been wreckaged by the BIOS..
*/
- if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable) {
- atomic_inc(&skip_test);
+ if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable)
return;
- }

+ /* Kick the control CPU into the TSC synchronization function */
+ smp_call_function_single(cpumask_first(cpu_online_mask), check_tsc_sync_source,
+ (unsigned long *)(unsigned long)cpu, 0);
retry:
/*
* Register this CPU's participation and wait for the