Messages in this thread | | | Date | Tue, 1 Aug 2023 14:58:11 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH] watchdog/hardlockup: Avoid large stack frames in watchdog_hardlockup_check() |
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On Mon 31-07-23 09:17:59, Douglas Anderson wrote: > After commit 77c12fc95980 ("watchdog/hardlockup: add a "cpu" param to > watchdog_hardlockup_check()") we started storing a `struct cpumask` on > the stack in watchdog_hardlockup_check(). On systems with > CONFIG_NR_CPUS set to 8192 this takes up 1K on the stack. That > triggers warnings with `CONFIG_FRAME_WARN` set to 1024. > > Instead of putting this `struct cpumask` on the stack, let's declare > it as `static`. This has the downside of taking up 1K of memory all > the time on systems with `CONFIG_NR_CPUS` to 8192, but on systems with > smaller `CONFIG_NR_CPUS` it's not much emory (with 128 CPUs it's only > 16 bytes of memory). Presumably anyone building a system with > `CONFIG_NR_CPUS=8192` can afford the extra 1K of memory. > > NOTE: as part of this change, we no longer check the return value of > trigger_single_cpu_backtrace(). While we could do this and only call > cpumask_clear_cpu() if trigger_single_cpu_backtrace() didn't fail, > that's probably not worth it. There's no reason to believe that > trigger_cpumask_backtrace() will succeed at backtracing the CPU when > trigger_single_cpu_backtrace() failed. > > Alternatives considered: > - Use kmalloc with GFP_ATOMIC to allocate. I decided against this > since relying on kmalloc when the system is hard locked up seems > like a bad idea. > - Change the arch_trigger_cpumask_backtrace() across all architectures > to take an extra parameter to get the needed behavior. This seems > like a lot of churn for a small savings. > > Fixes: 77c12fc95980 ("watchdog/hardlockup: add a "cpu" param to watchdog_hardlockup_check()") > Reported-by: kernel test robot <lkp@intel.com> > Closes: https://lore.kernel.org/r/202307310955.pLZDhpnl-lkp@intel.com > Signed-off-by: Douglas Anderson <dianders@chromium.org> > --- > > kernel/watchdog.c | 14 +++++++------- > 1 file changed, 7 insertions(+), 7 deletions(-) > > diff --git a/kernel/watchdog.c b/kernel/watchdog.c > index be38276a365f..19db2357969a 100644 > --- a/kernel/watchdog.c > +++ b/kernel/watchdog.c > @@ -151,9 +151,6 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs) > */ > if (is_hardlockup(cpu)) { > unsigned int this_cpu = smp_processor_id(); > - struct cpumask backtrace_mask; > - > - cpumask_copy(&backtrace_mask, cpu_online_mask); > > /* Only print hardlockups once. */ > if (per_cpu(watchdog_hardlockup_warned, cpu)) > @@ -167,10 +164,8 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs) > show_regs(regs); > else > dump_stack(); > - cpumask_clear_cpu(cpu, &backtrace_mask); > } else { > - if (trigger_single_cpu_backtrace(cpu)) > - cpumask_clear_cpu(cpu, &backtrace_mask); > + trigger_single_cpu_backtrace(cpu); > } > > /* > @@ -178,8 +173,13 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs) > * hardlockups generating interleaving traces > */ > if (sysctl_hardlockup_all_cpu_backtrace && > - !test_and_set_bit(0, &watchdog_hardlockup_all_cpu_dumped)) > + !test_and_set_bit(0, &watchdog_hardlockup_all_cpu_dumped)) { > + static struct cpumask backtrace_mask; > + > + cpumask_copy(&backtrace_mask, cpu_online_mask); > + cpumask_clear_cpu(cpu, &backtrace_mask); > trigger_cpumask_backtrace(&backtrace_mask);
This looks rather wasteful to just copy the cpumask over to backtrace_mask in nmi_trigger_cpumask_backtrace (which all but sparc arches do AFAICS).
Would it be possible to use arch_trigger_cpumask_backtrace(cpu_online_mask, false) and special case cpu != this_cpu && sysctl_hardlockup_all_cpu_backtrace?
> + } > > if (hardlockup_panic) > nmi_panic(regs, "Hard LOCKUP"); > -- > 2.41.0.487.g6d72f3e995-goog >
-- Michal Hocko SUSE Labs
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