5.10-stable review patch. If anyone has any objections, please let me know.
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From: Paul E. McKenney paulmck@kernel.org
[ Upstream commit fa218f1cce6ba40069c8daab8821de7e6be1cdd0 ]
Currently, if skew is detected on a clock marked CLOCK_SOURCE_VERIFY_PERCPU, that clock is checked on all CPUs. This is thorough, but might not be what you want on a system with a few tens of CPUs, let alone a few hundred of them.
Therefore, by default check only up to eight randomly chosen CPUs. Also provide a new clocksource.verify_n_cpus kernel boot parameter. A value of -1 says to check all of the CPUs, and a non-negative value says to randomly select that number of CPUs, without concern about selecting the same CPU multiple times. However, make use of a cpumask so that a given CPU will be checked at most once.
Suggested-by: Thomas Gleixner tglx@linutronix.de # For verify_n_cpus=1. Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Acked-by: Feng Tang feng.tang@intel.com Link: https://lore.kernel.org/r/20210527190124.440372-3-paulmck@kernel.org Stable-dep-of: 6bb05a33337b ("clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context") Signed-off-by: Sasha Levin sashal@kernel.org --- .../admin-guide/kernel-parameters.txt | 10 +++ kernel/time/clocksource.c | 74 ++++++++++++++++++- 2 files changed, 82 insertions(+), 2 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 097ef49b3d3a0..88110e74b3f7a 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -583,6 +583,16 @@ unstable. Defaults to three retries, that is, four attempts to read the clock under test.
+ clocksource.verify_n_cpus= [KNL] + Limit the number of CPUs checked for clocksources + marked with CLOCK_SOURCE_VERIFY_PERCPU that + are marked unstable due to excessive skew. + A negative value says to check all CPUs, while + zero says not to check any. Values larger than + nr_cpu_ids are silently truncated to nr_cpu_ids. + The actual CPUs are chosen randomly, with + no replacement if the same CPU is chosen twice. + clearcpuid=BITNUM[,BITNUM...] [X86] Disable CPUID feature X for the kernel. See arch/x86/include/asm/cpufeatures.h for the valid bit diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 754e93edb2f79..00cf99cb74496 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -14,6 +14,8 @@ #include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */ #include <linux/tick.h> #include <linux/kthread.h> +#include <linux/prandom.h> +#include <linux/cpu.h>
#include "tick-internal.h" #include "timekeeping_internal.h" @@ -201,6 +203,8 @@ void clocksource_mark_unstable(struct clocksource *cs)
static ulong max_cswd_read_retries = 3; module_param(max_cswd_read_retries, ulong, 0644); +static int verify_n_cpus = 8; +module_param(verify_n_cpus, int, 0644);
enum wd_read_status { WD_READ_SUCCESS, @@ -263,6 +267,55 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, static u64 csnow_mid; static cpumask_t cpus_ahead; static cpumask_t cpus_behind; +static cpumask_t cpus_chosen; + +static void clocksource_verify_choose_cpus(void) +{ + int cpu, i, n = verify_n_cpus; + + if (n < 0) { + /* Check all of the CPUs. */ + cpumask_copy(&cpus_chosen, cpu_online_mask); + cpumask_clear_cpu(smp_processor_id(), &cpus_chosen); + return; + } + + /* If no checking desired, or no other CPU to check, leave. */ + cpumask_clear(&cpus_chosen); + if (n == 0 || num_online_cpus() <= 1) + return; + + /* Make sure to select at least one CPU other than the current CPU. */ + cpu = cpumask_next(-1, cpu_online_mask); + if (cpu == smp_processor_id()) + cpu = cpumask_next(cpu, cpu_online_mask); + if (WARN_ON_ONCE(cpu >= nr_cpu_ids)) + return; + cpumask_set_cpu(cpu, &cpus_chosen); + + /* Force a sane value for the boot parameter. */ + if (n > nr_cpu_ids) + n = nr_cpu_ids; + + /* + * Randomly select the specified number of CPUs. If the same + * CPU is selected multiple times, that CPU is checked only once, + * and no replacement CPU is selected. This gracefully handles + * situations where verify_n_cpus is greater than the number of + * CPUs that are currently online. + */ + for (i = 1; i < n; i++) { + cpu = prandom_u32() % nr_cpu_ids; + cpu = cpumask_next(cpu - 1, cpu_online_mask); + if (cpu >= nr_cpu_ids) + cpu = cpumask_next(-1, cpu_online_mask); + if (!WARN_ON_ONCE(cpu >= nr_cpu_ids)) + cpumask_set_cpu(cpu, &cpus_chosen); + } + + /* Don't verify ourselves. */ + cpumask_clear_cpu(smp_processor_id(), &cpus_chosen); +}
static void clocksource_verify_one_cpu(void *csin) { @@ -278,12 +331,22 @@ static void clocksource_verify_percpu(struct clocksource *cs) int cpu, testcpu; s64 delta;
+ if (verify_n_cpus == 0) + return; cpumask_clear(&cpus_ahead); cpumask_clear(&cpus_behind); + get_online_cpus(); preempt_disable(); + clocksource_verify_choose_cpus(); + if (cpumask_weight(&cpus_chosen) == 0) { + preempt_enable(); + put_online_cpus(); + pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name); + return; + } testcpu = smp_processor_id(); - pr_warn("Checking clocksource %s synchronization from CPU %d.\n", cs->name, testcpu); - for_each_online_cpu(cpu) { + pr_warn("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n", cs->name, testcpu, cpumask_pr_args(&cpus_chosen)); + for_each_cpu(cpu, &cpus_chosen) { if (cpu == testcpu) continue; csnow_begin = cs->read(cs); @@ -303,6 +366,7 @@ static void clocksource_verify_percpu(struct clocksource *cs) cs_nsec_min = cs_nsec; } preempt_enable(); + put_online_cpus(); 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); @@ -427,6 +491,12 @@ static void clocksource_watchdog(struct timer_list *unused) watchdog->name, wdnow, wdlast, watchdog->mask); pr_warn(" '%s' cs_now: %llx cs_last: %llx mask: %llx\n", cs->name, csnow, cslast, cs->mask); + if (curr_clocksource == cs) + pr_warn(" '%s' is current clocksource.\n", cs->name); + else if (curr_clocksource) + pr_warn(" '%s' (not '%s') is current clocksource.\n", curr_clocksource->name, cs->name); + else + pr_warn(" No current clocksource.\n"); __clocksource_unstable(cs); continue; }