Messages in this thread | | | Subject | Re: [PATCH] rcu/tree: Force quiescent state on callback overload | From | Neeraj Upadhyay <> | Date | Tue, 23 Jun 2020 11:49:38 +0530 |
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Hi Paul,
On 6/23/2020 4:23 AM, Paul E. McKenney wrote: > On Mon, Jun 22, 2020 at 09:16:24AM +0530, Neeraj Upadhyay wrote: >> 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 <neeraju@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; >> } > > Very good, it does sound like you understand this, and it matches my > analysis and passes light testing, so I queued this one. I did improve > the commit log, please check below. The added detail is helpful to people > (including ourselves, by the way) who might need to look at this commit > some time in the future. >
Thanks; patch looks good; I will try to put more efforts on commit log for future patches.
> If you have an x86 system lying around, running rcutorture is quite > straightforward. Non-x86 systems can also run rcutorture, if nothing > else by using modprobe and rmmod as described here: > > https://paulmck.livejournal.com/57769.html > > The scripting described in the latter part of this document has worked > on ARMv8 and PowerPC, and might still work for all I know.
I will set it up at my end; the livejournal is pretty detailed! Thanks for sharing this!
Thanks Neeraj
> > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 9482524d7dd0aea5d32a6efa2979223eea07c029 > Author: Neeraj Upadhyay <neeraju@codeaurora.org> > Date: Mon Jun 22 00:07:27 2020 +0530 > > rcu/tree: Force quiescent state on callback overload > > On callback overload, it is necessary to quickly detect idle CPUs, > and rcu_gp_fqs_check_wake() checks for this condition. Unfortunately, > the code following the call to this function does not repeat this check, > which means that in reality no actual quiescent-state forcing, instead > only a couple of quick and pointless wakeups at the beginning of the > grace period. > > This commit therefore adds a check for the RCU_GP_FLAG_OVLD flag in > the post-wakeup "if" statement in rcu_gp_fqs_loop(). > > Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > > 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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