Re: [PATCH] rcu/tree: Force quiescent state on callback overload

[Neeraj Upadhyay]

Hi Paul,

On 6/22/2020 8:43 AM, Paul E. McKenney wrote:
On Mon, Jun 22, 2020 at 01:30:31AM +0530, Neeraj Upadhyay wrote:
Hi Paul,
On 6/22/2020 1:20 AM, Paul E. McKenney wrote:
On Mon, Jun 22, 2020 at 12:07:27AM +0530, Neeraj Upadhyay wrote:
On callback overload, we want to force quiescent state immediately,
for the first and second fqs. Enforce the same, by including
RCU_GP_FLAG_OVLD flag, in fqsstart check.

Signed-off-by: Neeraj Upadhyay <neer...@codeaurora.org>

Good catch!

But what did you do to verify that this change does the right thing?

                                                Thanx, Paul


I haven't done a runtime verification of this code path; I posted this,
based on review of this code.

My concern is that under overload, the FQS scans would happen continuously
rather than accelerating only the first such scan in a given grace period.
This would of course result in a CPU-bound grace-period kthread, which
users might not be all that happy with.

Or am I missing something subtle that prevents this?

Looks like under overload, only the first and second scans are accelerated?

    gf = 0;
    if (first_gp_fqs) {
         first_gp_fqs = false;
          gf = rcu_state.cbovld ? RCU_GP_FLAG_OVLD : 0;
    }


Thanks
Neeraj


But yes, it does look like the current mainline code fails to do the
first scan immediately, so again, good catch!

                                                        Thanx, Paul


Thanks
Neeraj

---
   kernel/rcu/tree.c | 2 +-
   1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d0988a1..6226bfb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1865,7 +1865,7 @@ static void rcu_gp_fqs_loop(void)
                        break;
                /* If time for quiescent-state forcing, do it. */
                if (!time_after(rcu_state.jiffies_force_qs, jiffies) ||
-                   (gf & RCU_GP_FLAG_FQS)) {
+                   (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) {
                        trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
                                               TPS("fqsstart"));
                        rcu_gp_fqs(first_gp_fqs);
--
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QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation