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Message-ID: <PH0PR11MB58802F8D66E4FBA455E15CC8DAE89@PH0PR11MB5880.namprd11.prod.outlook.com>
Date: Thu, 22 Dec 2022 09:42:19 +0000
From: "Zhang, Qiang1" <qiang1.zhang@...el.com>
To: "paulmck@...nel.org" <paulmck@...nel.org>
CC: "frederic@...nel.org" <frederic@...nel.org>,
"quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
"joel@...lfernandes.org" <joel@...lfernandes.org>,
"rcu@...r.kernel.org" <rcu@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu: Remove duplicate settings for rcu boost kthreads
affinity
On Sat, Dec 17, 2022 at 01:02:03PM +0000, Zhang, Qiang1 wrote:
> On Thu, Dec 15, 2022 at 05:00:29PM +0800, Zqiang wrote:
> > Currently, when CPUs going offline, the rcu boost kthreads CPU
> > affinity has been reset in rcutree_offline_cpu(), therefore this
> > commit remove duplicate settings in rcutree_dead_cpu().
> >
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> >
> >One question...
> >
> >Given that rcutree_offline_cpu() causes rcu_boost_kthread_setaffinity()
> >to be invoked with the number of the outgoing CPU, but rcutree_dead_cpu()
> >instead passes in -1, are these two invocations really redundant?
> >
> >Although -1 is passed in, but at this time, the outgoing CPU has been removed from
> >rcu_rnp_online_cpus(rnp), in other words, we will eliminate the same CPU, which is
> >also we passed in cpu in the rcutree_offline_cpu() before.
> >
>
> I also used " qemu -smp 4 "for a simple test.
>
> 1) rcutree_offline_cpu()
>
> (gdb) bt
> #0 set_cpus_allowed_ptr (p=p@...ry=0xffff888100bd9cc0, new_mask=new_mask@...ry=0xffff888100ce7cd0)
> at kernel/sched/core.c:2939
> #1 0xffffffff811be169 in rcu_boost_kthread_setaffinity (rnp=0xffffffff837a4c00 <rcu_state>,
> outgoingcpu=outgoingcpu@...ry=3) at kernel/rcu/tree_plugin.h:1250
> #2 0xffffffff811cfa9f in rcutree_affinity_setting (outgoing=3, cpu=3) at kernel/rcu/tree.c:4297
> #3 rcutree_offline_cpu (cpu=3) at kernel/rcu/tree.c:4341
> #4 0xffffffff810c4e6f in cpuhp_invoke_callback (cpu=cpu@...ry=3, state=state@...ry=CPUHP_AP_RCUTREE_ONLINE,
> bringup=bringup@...ry=false, node=<optimized out>, lastp=lastp@...ry=0xffff88815b027568) at kernel/cpu.c:192
> #5 0xffffffff810c6e24 in cpuhp_thread_fun (cpu=<optimized out>) at kernel/cpu.c:785
> #6 0xffffffff8111784e in smpboot_thread_fn (data=0xffff88810014c3c0) at kernel/smpboot.c:164
> #7 0xffffffff8110c88e in kthread (_create=0xffff8881009b8b00) at kernel/kthread.c:376
> #8 0xffffffff81002902 in ret_from_fork () at arch/x86/entry/entry_64.S:306
> #9 0x0000000000000000 in ?? ()
>
> (gdb) p/x *0xffff888100ce7cd0
> $1 = 0x7
>
> 2) rcutree_dead_cpu()
>
> #0 set_cpus_allowed_ptr (p=p@...ry=0xffff888100bd9cc0, new_mask=new_mask@...ry=0xffff888109fc7948)
> at kernel/sched/core.c:2939
> #1 0xffffffff811be169 in rcu_boost_kthread_setaffinity (rnp=0xffffffff837a4c00 <rcu_state>,
> outgoingcpu=outgoingcpu@...ry=-1) at kernel/rcu/tree_plugin.h:1250
> #2 0xffffffff811cf4a1 in rcutree_dead_cpu (cpu=3) at kernel/rcu/tree.c:4176
> #3 0xffffffff810c4e6f in cpuhp_invoke_callback (cpu=cpu@...ry=3, state=CPUHP_RCUTREE_PREP, bringup=bringup@...ry=false,
> node=node@...ry=0x0 <fixed_percpu_data>, lastp=lastp@...ry=0x0 <fixed_percpu_data>) at kernel/cpu.c:192
> #4 0xffffffff810c58f5 in cpuhp_invoke_callback_range (bringup=bringup@...ry=false, cpu=cpu@...ry=3,
> st=st@...ry=0xffff88815b027540, target=target@...ry=CPUHP_OFFLINE) at kernel/cpu.c:675
> #5 0xffffffff827a5f6a in cpuhp_down_callbacks (target=CPUHP_OFFLINE, st=0xffff88815b027540, cpu=3) at kernel/cpu.c:1170
> #6 _cpu_down (cpu=cpu@...ry=3, tasks_frozen=tasks_frozen@...ry=0, target=target@...ry=CPUHP_OFFLINE) at kernel/cpu.c:1170
> #7 0xffffffff810c6ff8 in cpu_down_maps_locked (target=CPUHP_OFFLINE, cpu=3) at kernel/cpu.c:1202
> #8 cpu_down (cpu=3, target=target@...ry=CPUHP_OFFLINE) at kernel/cpu.c:1210
> #9 0xffffffff810c7906 in cpu_device_down (dev=dev@...ry=0xffff88815b026988) at kernel/cpu.c:1227
>
> (gdb) p/x *0xffff888109fc7948
> $2 = 0x7
>
> As can be seen from the above, we have set the same cpumask twice for the same boost kthreads.
>
>Very good, thank you for checking!
>
>I have queued this as shown below, so please let me know of any issues.
I have checked and thank you for the clear description.
Thanks
Zqiang
>
> Thanx, Paul
>
>------------------------------------------------------------------------
commit 5815245b56229198d5f5d0c48c26d3c0c53ee222
Author: Zqiang <qiang1.zhang@...el.com>
Date: Wed Dec 21 11:15:43 2022 -0800
rcu: Remove redundant call to rcu_boost_kthread_setaffinity()
The rcu_boost_kthread_setaffinity() function is invoked at
rcutree_online_cpu() and rcutree_offline_cpu() time, early in the online
timeline and late in the offline timeline, respectively. It is also
invoked from rcutree_dead_cpu(), however, in the absence of userspace
manipulations (for which userspace must take responsibility), this call
is redundant with that from rcutree_offline_cpu(). This redundancy can
be demonstrated by printing out the relevant cpumasks
This commit therefore removes the call to rcu_boost_kthread_setaffinity()
from rcutree_dead_cpu().
Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c3255d27286c4..a6181b76fd5ab 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4165,15 +4165,10 @@ static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
*/
int rcutree_dead_cpu(unsigned int cpu)
{
- struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
- struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */
-
if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
return 0;
WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
- /* Adjust any no-longer-needed kthreads. */
- rcu_boost_kthread_setaffinity(rnp, -1);
// Stop-machine done, so allow nohz_full to disable tick.
tick_dep_clear(TICK_DEP_BIT_RCU);
return 0;
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