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SubjectRe: [PATCH] rcu: Fix the (t=0 jiffies) false positive
Hi Paul,

Sorry for the inconvenience.
I created my patch based on mainline' commit 776e49e8ddb5169e6477fd33a396e9c7b2eb7400

Thank you for the professional description and hand applying.
I will send future rcu related patches based on your "dev" branch of linux-rcu tree.

Regards,
Zhaolong


At 2020-03-06 07:05:15, "Paul E. McKenney" <paulmck@kernel.org> wrote:
>On Thu, Mar 05, 2020 at 03:45:57PM +0800, Zhaolong Zhang wrote:
>> Calculate 't' with the previously recorded 'gps' instead of 'gp_start'.
>>
>> Signed-off-by: Zhaolong Zhang <zhangzl2013@126.com>
>
>Good catch, thank you!
>
>I had to apply this by hand. My guess is that you developed against
>mainline rather than the "dev" branch of the -rcu tree:
>
>git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>
>Or perhaps your email client corrupted the patch?
>
>Regardless, please check the version below and let me know if I messed
>anything up.
>
> Thanx, Paul
>
>------------------------------------------------------------------------
>
>commit 7dd581bbbcc00246fec35eda60add4b32200b0a0
>Author: Zhaolong Zhang <zhangzl2013@126.com>
>Date: Thu Mar 5 14:56:11 2020 -0800
>
> rcu: Fix the (t=0 jiffies) false positive
>
> It is possible that an over-long grace period will end while the RCU
> CPU stall warning message is printing. In this case, the estimate of
> the offending grace period's duration can be erroneous due to refetching
> of rcu_state.gp_start, which will now be the time of the newly started
> grace period. Computation of this duration clearly needs to use the
> start time for the old over-long grace period, not the fresh new one.
> This commit avoids such errors by causing both print_other_cpu_stall() and
> print_cpu_stall() to reuse the value previously fetched by their caller.
>
> Signed-off-by: Zhaolong Zhang <zhangzl2013@126.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
>diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>index b17cd9b..502b4dd 100644
>--- a/kernel/rcu/tree_stall.h
>+++ b/kernel/rcu/tree_stall.h
>@@ -371,7 +371,7 @@ static void rcu_check_gp_kthread_starvation(void)
> }
> }
>
>-static void print_other_cpu_stall(unsigned long gp_seq)
>+static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
> {
> int cpu;
> unsigned long flags;
>@@ -408,7 +408,7 @@ static void print_other_cpu_stall(unsigned long gp_seq)
> for_each_possible_cpu(cpu)
> totqlen += rcu_get_n_cbs_cpu(cpu);
> pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
>- smp_processor_id(), (long)(jiffies - rcu_state.gp_start),
>+ smp_processor_id(), (long)(jiffies - gps),
> (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
> if (ndetected) {
> rcu_dump_cpu_stacks();
>@@ -442,7 +442,7 @@ static void print_other_cpu_stall(unsigned long gp_seq)
> rcu_force_quiescent_state(); /* Kick them all. */
> }
>
>-static void print_cpu_stall(void)
>+static void print_cpu_stall(unsigned long gps)
> {
> int cpu;
> unsigned long flags;
>@@ -467,7 +467,7 @@ static void print_cpu_stall(void)
> for_each_possible_cpu(cpu)
> totqlen += rcu_get_n_cbs_cpu(cpu);
> pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n",
>- jiffies - rcu_state.gp_start,
>+ jiffies - gps,
> (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
>
> rcu_check_gp_kthread_starvation();
>@@ -546,7 +546,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
> cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
>
> /* We haven't checked in, so go dump stack. */
>- print_cpu_stall();
>+ print_cpu_stall(gps);
> if (rcu_cpu_stall_ftrace_dump)
> rcu_ftrace_dump(DUMP_ALL);
>
>@@ -555,7 +555,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
> cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
>
> /* They had a few time units to dump stack, so complain. */
>- print_other_cpu_stall(gs2);
>+ print_other_cpu_stall(gs2, gps);
> if (rcu_cpu_stall_ftrace_dump)
> rcu_ftrace_dump(DUMP_ALL);
> }
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