On Tue, 5 May 2020 10:30:36 -0700 "Paul E. McKenney" <paul...@kernel.org> wrote:
> On Tue, May 05, 2020 at 07:05:53PM +0200, SeongJae Park wrote: > > On Tue, 5 May 2020 09:37:42 -0700 Eric Dumazet <eric.duma...@gmail.com> > > wrote: > > > > > > > > > > > On 5/5/20 9:31 AM, Eric Dumazet wrote: > > > > > > > > > > > > On 5/5/20 9:25 AM, Eric Dumazet wrote: > > > >> > > > >> > > > >> On 5/5/20 9:13 AM, SeongJae Park wrote: > > > >>> On Tue, 5 May 2020 09:00:44 -0700 Eric Dumazet <eduma...@google.com> > > > >>> wrote: > > > >>> > > > >>>> On Tue, May 5, 2020 at 8:47 AM SeongJae Park <sjp...@amazon.com> > > > >>>> wrote: > > > >>>>> > > > >>>>> On Tue, 5 May 2020 08:20:50 -0700 Eric Dumazet > > > >>>>> <eric.duma...@gmail.com> wrote: > > > >>>>> > > > >>>>>> > > > >>>>>> > > > >>>>>> On 5/5/20 8:07 AM, SeongJae Park wrote: > > > >>>>>>> On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet > > > >>>>>>> <eduma...@google.com> wrote: > > > >>>>>>> > > > >>>>>> > > [...] > > > >> > > > >> I would ask Paul opinion on this issue, because we have many objects > > > >> being freed after RCU grace periods. > > > >> > > > >> If RCU subsystem can not keep-up, I guess other workloads will also > > > >> suffer. > > > >> > > > >> Sure, we can revert patches there and there trying to work around the > > > >> issue, > > > >> but for objects allocated from process context, we should not have > > > >> these problems. > > > >> > > > > > > > > I wonder if simply adjusting rcu_divisor to 6 or 5 would help > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index > > > > d9a49cd6065a20936edbda1b334136ab597cde52..fde833bac0f9f81e8536211b4dad6e7575c1219a > > > > 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -427,7 +427,7 @@ module_param(qovld, long, 0444); > > > > static ulong jiffies_till_first_fqs = ULONG_MAX; > > > > static ulong jiffies_till_next_fqs = ULONG_MAX; > > > > static bool rcu_kick_kthreads; > > > > -static int rcu_divisor = 7; > > > > +static int rcu_divisor = 6; > > > > module_param(rcu_divisor, int, 0644); > > > > > > > > /* Force an exit from rcu_do_batch() after 3 milliseconds. */ > > > > > > > > > > To be clear, you can adjust the value without building a new kernel. > > > > > > echo 6 >/sys/module/rcutree/parameters/rcu_divisor > > > > I tried value 6, 5, and 4, but none of those removed the problem. > > Thank you for checking this! > > Was your earlier discussion on long RCU readers speculation, or do you > have measurements? It was just a guess without any measurement or dedicated investigation. Thanks, SeongJae Park > > Thanx, Paul
