On Wed, Apr 17, 2013 at 06:24:47PM +0200, Frederic Weisbecker wrote: > The timekeeping job must be able to run early on boot > because there may be some pre-SMP (and thus pre-initcalls ) > components that rely on it. The IO-APIC is one such users > as it tests the timer health by watching jiffies progression. > > Given that it happens before we know the initial online > set, we can't rely on it to select a timekeeper. We need > one before SMP time otherwise we simply crash on boot. > > To fix this and keep things simple for now, force the boot CPU > outside of the full dynticks range in any case and do this early > on kernel parameter parsing time. > > We might want a trickier solution later, expecially for aSMP > architectures that need to assign housekeeping tasks to arbitrary > low power CPUs. > > But it's still first pass KISS time for now. > > Signed-off-by: Frederic Weisbecker <[email protected]> > Cc: Andrew Morton <[email protected]> > Cc: Chris Metcalf <[email protected]> > Cc: Christoph Lameter <[email protected]> > Cc: Geoff Levand <[email protected]> > Cc: Gilad Ben Yossef <[email protected]> > Cc: Hakan Akkan <[email protected]> > Cc: Ingo Molnar <[email protected]> > Cc: Kevin Hilman <[email protected]> > Cc: Li Zhong <[email protected]> > Cc: Paul E. McKenney <[email protected]> > Cc: Paul Gortmaker <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Steven Rostedt <[email protected]> > Cc: Thomas Gleixner <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]> (Plus updated the documentation accordingly.) > --- > Documentation/kernel-parameters.txt | 4 +- > kernel/time/tick-sched.c | 54 +++++++++------------------------- > 2 files changed, 17 insertions(+), 41 deletions(-) > > diff --git a/Documentation/kernel-parameters.txt > b/Documentation/kernel-parameters.txt > index 82365dd..887b297 100644 > --- a/Documentation/kernel-parameters.txt > +++ b/Documentation/kernel-parameters.txt > @@ -1916,8 +1916,8 @@ bytes respectively. Such letter suffixes can also be > entirely omitted. > nohz_full= [KNL,BOOT] > In kernels built with CONFIG_NO_HZ_FULL=y, set > the specified list of CPUs whose tick will be stopped > - whenever possible. You need to keep at least one online > - CPU outside the range to maintain the timekeeping. > + whenever possible. The boot CPU will be forced outside > + the range to maintain the timekeeping. > > noiotrap [SH] Disables trapped I/O port accesses. > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index 369b576..2bac5ea 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -158,11 +158,21 @@ int tick_nohz_full_cpu(int cpu) > /* Parse the boot-time nohz CPU list from the kernel parameters. */ > static int __init tick_nohz_full_setup(char *str) > { > + int cpu; > + > alloc_bootmem_cpumask_var(&nohz_full_mask); > - if (cpulist_parse(str, nohz_full_mask) < 0) > + if (cpulist_parse(str, nohz_full_mask) < 0) { > pr_warning("NOHZ: Incorrect nohz_full cpumask\n"); > - else > - have_nohz_full_mask = true; > + return 1; > + } > + > + cpu = smp_processor_id(); > + if (cpumask_test_cpu(cpu, nohz_full_mask)) { > + pr_warning("NO_HZ: Clearing %d from nohz_full range for > timekeeping\n", cpu); > + cpumask_clear_cpu(cpu, nohz_full_mask); > + } > + have_nohz_full_mask = true; > + > return 1; > } > __setup("nohz_full=", tick_nohz_full_setup); > @@ -195,42 +205,8 @@ static char __initdata nohz_full_buf[NR_CPUS + 1]; > > static int __init init_tick_nohz_full(void) > { > - cpumask_var_t online_nohz; > - int cpu; > - > - if (!have_nohz_full_mask) > - return 0; > - > - cpu_notifier(tick_nohz_cpu_down_callback, 0); > - > - if (!zalloc_cpumask_var(&online_nohz, GFP_KERNEL)) { > - pr_warning("NO_HZ: Not enough memory to check full nohz > mask\n"); > - return -ENOMEM; > - } > - > - /* > - * CPUs can probably not be concurrently offlined on initcall time. > - * But we are paranoid, aren't we? > - */ > - get_online_cpus(); > - > - /* Ensure we keep a CPU outside the dynticks range for timekeeping */ > - cpumask_and(online_nohz, cpu_online_mask, nohz_full_mask); > - if (cpumask_equal(online_nohz, cpu_online_mask)) { > - pr_warning("NO_HZ: Must keep at least one online CPU " > - "out of nohz_full range\n"); > - /* > - * We know the current CPU doesn't have its tick stopped. > - * Let's use it for the timekeeping duty. > - */ > - preempt_disable(); > - cpu = smp_processor_id(); > - pr_warning("NO_HZ: Clearing %d from nohz_full range\n", cpu); > - cpumask_clear_cpu(cpu, nohz_full_mask); > - preempt_enable(); > - } > - put_online_cpus(); > - free_cpumask_var(online_nohz); > + if (have_nohz_full_mask) > + cpu_notifier(tick_nohz_cpu_down_callback, 0); > > cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), nohz_full_mask); > pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf); > -- > 1.7.5.4 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
