>>> >>> Currently rcu_cpu_stall_reset() set rcu_state.jiffies_stall to one check >>> period later, i.e. jiffies + rcu_jiffies_till_stall_check(). But jiffies >>> is only updated in the timer interrupt, so when kgdb_cpu_enter() begins >>> to run there may already be nearly one rcu check period after jiffies. >>> Since all interrupts are disabled during kgdb_cpu_enter(), jiffies will >>> not be updated. When kgdb_cpu_enter() returns, rcu_state.jiffies_stall >>> maybe already gets timeout. >>> >>> We can set rcu_state.jiffies_stall to two rcu check periods later, e.g. >>> jiffies + (rcu_jiffies_till_stall_check() * 2) in rcu_cpu_stall_reset() >>> to avoid this problem. But this isn't a complete solution because kgdb >>> may take a very long time in irq disabled context. >>> >>> Instead, update jiffies at the beginning of rcu_cpu_stall_reset() can >>> solve all kinds of problems. >> >> Would it make sense for there to be a kgdb_cpu_exit()? In that case, >> the stalls could simply be suppressed at the beginning of the debug >> session and re-enabled upon exit, as is currently done for sysrq output >> via rcu_sysrq_start() and rcu_sysrq_end(). > Thank you for your advice, but that doesn't help. Because > rcu_sysrq_start() and rcu_sysrq_end() try to suppress the warnings > during sysrq, but kgdb already has no warnings during kgdb_cpu_enter() > since it is executed in irq disabled context. Instead, this patch > wants to suppress the warnings *after* kgdb_cpu_enter() due to a very > old jiffies value. >
Hello, Huacai
Is it possible to set the rcu_cpu_stall_suppress is true in dbg_touch_watchdogs() and reset the rcu_cpu_stall_suppress at the beginning and end of the RCU grace period?
This is possible but not the best: 1, kgdb is not the only caller of rcu_cpu_stall_reset(); 2, it is difficult to find the "end" to reset rcu_cpu_stall_suppress.
You can replace rcu_state.jiffies_stall update by setting rcu_cpu_stall_suppress in rcu_cpu_stall_reset(), and reset rcu_cpu_stall_suppress in rcu_gp_init() and rcu_gp_cleanup().
What's the advantage compared with updating jiffies? Updating jiffies seems more straight forward.
In do_update_jiffies_64(), need to acquire jiffies_lock raw spinlock, like you said, kgdb is not the only caller of rcu_cpu_stall_reset(), the rcu_cpu_stall_reset() maybe invoke in NMI (arch/x86/platform/uv/uv_nmi.c)
Reset rcu_cpu_stall_suppress in rcu_gp_init()/rcu_gp_cleanup() is still not so good to me, because it does a useless operation in most cases. Moreover, the rcu core is refactored again and again, something may be changed in future.
If do_update_jiffies_64() cannot be used in NMI context, can we
What about updating jiffies in dbg_touch_watchdogs or adding a wrapper which updates both jiffies and jiffies_stall?
consider my old method [1]? https://lore.kernel.org/rcu/CAAhV-H7j9Y=VvRLm8thLw-EX1PGqBA9YfT4G1AN7ucYS=iP...
Of course we should set rcu_state.jiffies_stall large enough, so we can do like this:
void rcu_cpu_stall_reset(void) { WRITE_ONCE(rcu_state.jiffies_stall,
- jiffies + rcu_jiffies_till_stall_check());
- jiffies + 300 * HZ);
}
300s is the largest timeout value, and I think 300s is enough here in practice.
Huacai
Thanks Zqiang
Huacai
Thanks Zqiang
or set rcupdate.rcu_cpu_stall_suppress_at_boot=1 in bootargs can suppress RCU stall in booting.
This is also possible, but it suppresses all kinds of stall warnings, which is not what we want.
Huacai
Thanks Zqiang
> > Huacai > >> >> Thanx, Paul >> >>> Cc: stable@vger.kernel.org >>> Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()") >>> Reported-off-by: Binbin Zhou zhoubinbin@loongson.cn >>> Signed-off-by: Huacai Chen chenhuacai@loongson.cn >>> --- >>> kernel/rcu/tree_stall.h | 1 + >>> 1 file changed, 1 insertion(+) >>> >>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h >>> index b10b8349bb2a..1c7b540985bf 100644 >>> --- a/kernel/rcu/tree_stall.h >>> +++ b/kernel/rcu/tree_stall.h >>> @@ -153,6 +153,7 @@ static void panic_on_rcu_stall(void) >>> */ >>> void rcu_cpu_stall_reset(void) >>> { >>> + do_update_jiffies_64(ktime_get()); >>> WRITE_ONCE(rcu_state.jiffies_stall, >>> jiffies + rcu_jiffies_till_stall_check()); >>> } >>> -- >>> 2.39.3