From: Paul E. McKenney Date: Mon, 21 Dec 2020 23:40:47 +0000 (-0800) Subject: clocksource: Check per-CPU clock synchronization when marked unstable X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=20459f96321faae402c9abc92aacb3e8c05131d4;p=users%2Fjedix%2Flinux-maple.git clocksource: Check per-CPU clock synchronization when marked unstable Some sorts of per-CPU clock sources have a history of going out of synchronization with each other. However, this problem has purportedy been solved in the past ten years. Except that it is all too possible that the problem has instead simply been made less likely, which might mean that some of the occasional "Marking clocksource 'tsc' as unstable" messages might be due to desynchronization. How would anyone know? Therefore apply CPU-to-CPU synchronization checking to newly unstable clocksource that are marked with the new CLOCK_SOURCE_VERIFY_PERCPU flag. Lists of desynchronized CPUs are printed, with the caveat that if it is the reporting CPU that is itself desynchronized, it will appear that all the other clocks are wrong. Just like in real life. Link: https://lore.kernel.org/lkml/202104291438.PuHsxRkl-lkp@intel.com/ Link: https://lore.kernel.org/lkml/20210429140440.GT975577@paulmck-ThinkPad-P17-Gen-1 Link: https://lore.kernel.org/lkml/20210425224540.GA1312438@paulmck-ThinkPad-P17-Gen-1/ Link: https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/ Link: https://lore.kernel.org/lkml/20210106004013.GA11179@paulmck-ThinkPad-P72/ Link: https://lore.kernel.org/lkml/20210414043435.GA2812539@paulmck-ThinkPad-P17-Gen-1/ Link: https://lore.kernel.org/lkml/20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1/ Cc: John Stultz Cc: Thomas Gleixner Cc: Stephen Boyd Cc: Jonathan Corbet Cc: Mark Rutland Cc: Marc Zyngier Cc: Andi Kleen Cc: Feng Tang Cc: Xing Zhengjun Reported-by: Chris Mason [ paulmck: Add "static" to clocksource_verify_one_cpu() per kernel test robot feedback. ] [ paulmck: Apply Thomas Gleixner feedback. ] Signed-off-by: Paul E. McKenney --- diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index f70dffc2771f5..56289170753c5 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -1151,7 +1151,8 @@ static struct clocksource clocksource_tsc = { .mask = CLOCKSOURCE_MASK(64), .flags = CLOCK_SOURCE_IS_CONTINUOUS | CLOCK_SOURCE_VALID_FOR_HRES | - CLOCK_SOURCE_MUST_VERIFY, + CLOCK_SOURCE_MUST_VERIFY | + CLOCK_SOURCE_VERIFY_PERCPU, .vdso_clock_mode = VDSO_CLOCKMODE_TSC, .enable = tsc_cs_enable, .resume = tsc_resume, diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h index 86d143db65231..83a3ebff74560 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -131,7 +131,7 @@ struct clocksource { #define CLOCK_SOURCE_UNSTABLE 0x40 #define CLOCK_SOURCE_SUSPEND_NONSTOP 0x80 #define CLOCK_SOURCE_RESELECT 0x100 - +#define CLOCK_SOURCE_VERIFY_PERCPU 0x200 /* simplify initialization of mask field */ #define CLOCKSOURCE_MASK(bits) GENMASK_ULL((bits) - 1, 0) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 157530ae73ac5..5ba978a5f45d0 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -223,6 +223,60 @@ static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow) return false; } +static u64 csnow_mid; +static cpumask_t cpus_ahead; +static cpumask_t cpus_behind; + +static void clocksource_verify_one_cpu(void *csin) +{ + struct clocksource *cs = (struct clocksource *)csin; + + csnow_mid = cs->read(cs); +} + +static void clocksource_verify_percpu(struct clocksource *cs) +{ + int64_t cs_nsec, cs_nsec_max = 0, cs_nsec_min = LLONG_MAX; + u64 csnow_begin, csnow_end; + int cpu, testcpu; + s64 delta; + + cpumask_clear(&cpus_ahead); + cpumask_clear(&cpus_behind); + preempt_disable(); + testcpu = smp_processor_id(); + pr_warn("Checking clocksource %s synchronization from CPU %d.\n", cs->name, testcpu); + for_each_online_cpu(cpu) { + if (cpu == testcpu) + continue; + csnow_begin = cs->read(cs); + smp_call_function_single(cpu, clocksource_verify_one_cpu, cs, 1); + csnow_end = cs->read(cs); + delta = (s64)((csnow_mid - csnow_begin) & cs->mask); + if (delta < 0) + cpumask_set_cpu(cpu, &cpus_behind); + delta = (csnow_end - csnow_mid) & cs->mask; + if (delta < 0) + cpumask_set_cpu(cpu, &cpus_ahead); + delta = clocksource_delta(csnow_end, csnow_begin, cs->mask); + cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift); + if (cs_nsec > cs_nsec_max) + cs_nsec_max = cs_nsec; + if (cs_nsec < cs_nsec_min) + cs_nsec_min = cs_nsec; + } + preempt_enable(); + if (!cpumask_empty(&cpus_ahead)) + pr_warn(" CPUs %*pbl ahead of CPU %d for clocksource %s.\n", + cpumask_pr_args(&cpus_ahead), testcpu, cs->name); + if (!cpumask_empty(&cpus_behind)) + pr_warn(" CPUs %*pbl behind CPU %d for clocksource %s.\n", + cpumask_pr_args(&cpus_behind), testcpu, cs->name); + if (!cpumask_empty(&cpus_ahead) || !cpumask_empty(&cpus_behind)) + pr_warn(" CPU %d check durations %lldns - %lldns for clocksource %s.\n", + testcpu, cs_nsec_min, cs_nsec_max, cs->name); +} + static void clocksource_watchdog(struct timer_list *unused) { u64 csnow, wdnow, cslast, wdlast, delta; @@ -447,6 +501,12 @@ static int __clocksource_watchdog_kthread(void) unsigned long flags; int select = 0; + /* Do any required per-CPU skew verification. */ + if (curr_clocksource && + curr_clocksource->flags & CLOCK_SOURCE_UNSTABLE && + curr_clocksource->flags & CLOCK_SOURCE_VERIFY_PERCPU) + clocksource_verify_percpu(curr_clocksource); + spin_lock_irqsave(&watchdog_lock, flags); list_for_each_entry_safe(cs, tmp, &watchdog_list, wd_list) { if (cs->flags & CLOCK_SOURCE_UNSTABLE) {