Messages in this thread | | | Date | Wed, 3 Aug 2022 19:13:01 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] sched/debug: avoid executing show_state and causing rcu stall warning |
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* Steven Rostedt <rostedt@goodmis.org> wrote:
> > [ Adding Paul ] > > On Wed, 3 Aug 2022 09:18:45 +0800 > Liu Song <liusong@linux.alibaba.com> wrote: > > > From: Liu Song <liusong@linux.alibaba.com> > > > > If the number of CPUs is large, "sysrq_sched_debug_show" will execute for > > a long time. Every time I execute "echo t > /proc/sysrq-trigger" on my > > 128-core machine, the rcu stall warning will be triggered. Moreover, > > sysrq_sched_debug_show does not need to be protected by rcu_read_lock, > > and no rcu stall warning will appear after adjustment. > > > > Signed-off-by: Liu Song <liusong@linux.alibaba.com> > > --- > > kernel/sched/core.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 5555e49..82c117e 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -8879,11 +8879,11 @@ void show_state_filter(unsigned int state_filter) > > sched_show_task(p); > > } > > > > + rcu_read_unlock(); > > #ifdef CONFIG_SCHED_DEBUG > > if (!state_filter) > > sysrq_sched_debug_show(); > > If this is just because sysrq_sched_debug_show() is very slow, does RCU > have a way to "touch" it? Like the watchdogs have? That is, to tell RCU > "Yes I know I'm taking a long time, but I'm still making forward progress, > don't complain about me". Then the sysrq_sched_debug_show() could have: > > for_each_online_cpu(cpu) { > /* > * Need to reset softlockup watchdogs on all CPUs, because > * another CPU might be blocked waiting for us to process > * an IPI or stop_machine. > */ > touch_nmi_watchdog(); > touch_all_softlockup_watchdogs(); > + touch_rcu(); > print_cpu(NULL, cpu); > }
I'd much rather we use the specific exclusion primitive suitable for that sequence - in that case it should be cpus_read_lock()/unlock() I suspect.
But the entire code sequence should be reviewed - do we anywhere walk task lists that need RCU protection?
My main complaint was that we cannot just randomly drop the RCU lock with no inspection of the underlying code.
Ingo
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