On Tue, May 05, 2020 at 08:34:02PM +0200, SeongJae Park wrote: > On Tue, 5 May 2020 11:17:07 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > On Tue, May 05, 2020 at 07:56:05PM +0200, SeongJae Park wrote: > > > On Tue, 5 May 2020 10:30:36 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> 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.dumazet@xxxxxxxxx> 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 <edumazet@xxxxxxxxxx> wrote: > > > > > > >>> > > > > > > >>>> On Tue, May 5, 2020 at 8:47 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote: > > > > > > >>>>> > > > > > > >>>>> On Tue, 5 May 2020 08:20:50 -0700 Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote: > > > > > > >>>>> > > > > > > >>>>>> > > > > > > >>>>>> > > > > > > >>>>>> On 5/5/20 8:07 AM, SeongJae Park wrote: > > > > > > >>>>>>> On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet <edumazet@xxxxxxxxxx> 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. > > > > OK, another thing to check is the duration of the low-memory episode. > > Does this duration exceed the RCU CPU stall warning time? (21 seconds > > in mainline, 60 in many distros, but check rcupdate.rcu_cpu_stall_timeout > > to be sure.) > > The benchmark takes about 36 seconds for 10,000 repeats of the test. > > The value on the test machine is 60. > > So the duration would not exceeded the warning time and therefore I haven't > seen the warning message. > > As told in other mail, I will also adjust this value to shorter one. Sounds good, thank you! > > Also, any chance of a .config? Or at least the RCU portions? I am > > guessing CONFIG_PREEMPT=n, for example. > > I guess this would be ok. > > # CONFIG_PREEMPT is not set > > # > # RCU Subsystem > # > CONFIG_TREE_RCU=y > CONFIG_RCU_EXPERT=y > CONFIG_SRCU=y > CONFIG_TREE_SRCU=y > CONFIG_RCU_STALL_COMMON=y > CONFIG_RCU_NEED_SEGCBLIST=y > CONFIG_RCU_FANOUT=64 > CONFIG_RCU_FANOUT_LEAF=16 > # CONFIG_RCU_FAST_NO_HZ is not set > CONFIG_RCU_NOCB_CPU=y > # end of RCU Subsystem And thank you again! Thanx, Paul
