Messages in this thread | | | Date | Mon, 23 Mar 2020 11:21:26 -0400 | From | Joel Fernandes <> | Subject | Re: [PATCH] sched: Use RCU-sched in core-scheduling balancing logic |
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On Mon, Mar 23, 2020 at 02:58:18PM +0800, Li, Aubrey wrote: > On 2020/3/14 8:30, Paul E. McKenney wrote: > > On Fri, Mar 13, 2020 at 07:29:18PM -0400, Joel Fernandes (Google) wrote: > >> rcu_read_unlock() can incur an infrequent deadlock in > >> sched_core_balance(). Fix this by using the RCU-sched flavor instead. > >> > >> This fixes the following spinlock recursion observed when testing the > >> core scheduling patches on PREEMPT=y kernel on ChromeOS: > >> > >> [ 14.998590] watchdog: BUG: soft lockup - CPU#0 stuck for 11s! [kworker/0:10:965] > >> > > > > The original could indeed deadlock, and this would avoid that deadlock. > > (The commit to solve this deadlock is sadly not yet in mainline.) > > > > Acked-by: Paul E. McKenney <paulmck@kernel.org> > > I saw this in dmesg with this patch, is it expected? > > [ 117.000905] ============================= > [ 117.000907] WARNING: suspicious RCU usage > [ 117.000911] 5.5.7+ #160 Not tainted > [ 117.000913] ----------------------------- > [ 117.000916] kernel/sched/core.c:4747 suspicious rcu_dereference_check() usage! > [ 117.000918] > other info that might help us debug this:
Sigh, this is because for_each_domain() expects rcu_read_lock(). From an RCU PoV, the code is correct (warning doesn't cause any issue).
To silence warning, we could replace the rcu_read_lock_sched() in my patch with: preempt_disable(); rcu_read_lock();
and replace the unlock with:
rcu_read_unlock(); preempt_enable();
That should both take care of both the warning and the scheduler-related deadlock. Thoughts?
Does that fix the warning for you?
thanks,
- Joel
> > [ 117.000921] > rcu_scheduler_active = 2, debug_locks = 1 > [ 117.000923] 1 lock held by swapper/52/0: > [ 117.000925] #0: ffffffff82670960 (rcu_read_lock_sched){....}, at: sched_core_balance+0x5/0x700 > [ 117.000937] > stack backtrace: > [ 117.000940] CPU: 52 PID: 0 Comm: swapper/52 Kdump: loaded Not tainted 5.5.7+ #160 > [ 117.000943] Hardware name: Intel Corporation S2600WFD/S2600WFD, BIOS SE5C620.86B.01.00.0412.020920172159 02/09/2017 > [ 117.000945] Call Trace: > [ 117.000955] dump_stack+0x86/0xcb > [ 117.000962] sched_core_balance+0x634/0x700 > [ 117.000982] __balance_callback+0x49/0xa0 > [ 117.000990] __schedule+0x1416/0x1620 > [ 117.001000] ? lockdep_hardirqs_off+0xa0/0xe0 > [ 117.001005] ? _raw_spin_unlock_irqrestore+0x41/0x70 > [ 117.001024] schedule_idle+0x28/0x40 > [ 117.001030] do_idle+0x17e/0x2a0 > [ 117.001041] cpu_startup_entry+0x19/0x20 > [ 117.001048] start_secondary+0x16c/0x1c0 > [ 117.001055] secondary_startup_64+0xa4/0xb0 > > > > >> --- > >> kernel/sched/core.c | 4 ++-- > >> 1 file changed, 2 insertions(+), 2 deletions(-) > >> > >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c > >> index 3045bd50e249..037e8f2e2686 100644 > >> --- a/kernel/sched/core.c > >> +++ b/kernel/sched/core.c > >> @@ -4735,7 +4735,7 @@ static void sched_core_balance(struct rq *rq) > >> struct sched_domain *sd; > >> int cpu = cpu_of(rq); > >> > >> - rcu_read_lock(); > >> + rcu_read_lock_sched(); > >> raw_spin_unlock_irq(rq_lockp(rq)); > >> for_each_domain(cpu, sd) { > >> if (!(sd->flags & SD_LOAD_BALANCE)) > >> @@ -4748,7 +4748,7 @@ static void sched_core_balance(struct rq *rq) > >> break; > >> } > >> raw_spin_lock_irq(rq_lockp(rq)); > >> - rcu_read_unlock(); > >> + rcu_read_unlock_sched(); > >> } > >> > >> static DEFINE_PER_CPU(struct callback_head, core_balance_head); > >> -- > >> 2.25.1.481.gfbce0eb801-goog > >> >
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