On 12/01/2021 16:53, Daniel Bristot de Oliveira wrote: > cgroups v2 allows the cpuset controller to be enabled/disabled on > demand. On Fedora 32, cpuset is disabled by default. To enable it, > a user needs to: > > # cd /sys/fs/cgroup/ > # echo +cpuset > cgroup.subtree_control > > Existing cgroups will expose the cpuset interface (e.g., cpuset.cpus > file). By default, cpuset.cpus has no CPU assigned, which means that > existing tasks will move to a cpuset without cpus. > > With that in mind, look what happens if a SCHED_DEADLINE task exists > on any cgroup (user.slice by default on Fedora): > > ----- %< ----- > # chrt -d --sched-period 1000000000 --sched-runtime 100000000 0 sleep 100 & Like you mentioned above, to see the issue the DL task has to be moved into the cgroup (e.g. user.slice) here: echo $PID > /sys/fs/cgroup/user.slice/cgroup.procs > # cd /sys/fs/cgroup/ > # echo '+cpuset' > cgroup.subtree_control > [ 65.384041] BUG: unable to handle page fault for address: ffffffffb720f7e0 > [ 65.384551] #PF: supervisor read access in kernel mode > [ 65.384923] #PF: error_code(0x0000) - not-present page > [ 65.385298] PGD 61a15067 P4D 61a15067 PUD 61a16063 PMD 800fffff9ddff062 > [ 65.385781] Oops: 0000 [#1] SMP PTI > [ 65.386042] CPU: 0 PID: 799 Comm: sh Not tainted 5.10.0-rc3 #1 > [ 65.386461] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014 > [ 65.387077] RIP: 0010:dl_task_can_attach+0x40/0x250 > [ 65.387429] Code: 54 55 53 48 83 ec 18 48 89 3c 24 bf ff ff ff ff e8 05 a2 52 00 > 4c 63 f0 48 c7 c5 00 9e 02 00 4a 8b 04 f5 00 09 47 b6 48 89 ea > <4c> 8b a4 10 e0 09 00 00 49 8d 44 24 40 48 89 c7 48 89 44 24 > 08 e8 > [ 65.388768] RSP: 0018:ffffaee8c056fcd8 EFLAGS: 00010283 > [ 65.389148] RAX: ffffffffb71e5000 RBX: ffffaee8c056fdd0 RCX: 0000000000000040 > [ 65.389661] RDX: 0000000000029e00 RSI: ffff9db202534e48 RDI: ffffffffb6d3a3e0 > [ 65.390174] RBP: 0000000000029e00 R08: 0000000000000000 R09: 0000000000000004 > [ 65.390686] R10: 0000000000000001 R11: 00000000ffa6fbff R12: ffffaee8c056fbf0 > [ 65.391196] R13: ffff9db2024e1400 R14: 0000000000000004 R15: ffff9db20ebb31e0 > [ 65.391710] FS: 00007f6df41b1740(0000) GS:ffff9db377c00000(0000) knlGS:0000000000000000 > [ 65.392289] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 65.392705] CR2: ffffffffb720f7e0 CR3: 000000010680a003 CR4: 0000000000370ef0 > [ 65.393220] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > [ 65.393732] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > [ 65.394244] Call Trace: > [ 65.394437] cpuset_can_attach+0x8b/0x110 > [ 65.394732] cgroup_migrate_execute+0x70/0x430 > [ 65.395057] cgroup_update_dfl_csses+0x222/0x230 > [ 65.395392] cgroup_subtree_control_write+0x2c6/0x3c0 > [ 65.395759] kernfs_fop_write+0xce/0x1b0 > [ 65.396048] vfs_write+0xc2/0x230 > [ 65.396291] ksys_write+0x4f/0xc0 > [ 65.396533] do_syscall_64+0x33/0x40 > [ 65.396797] entry_SYSCALL_64_after_hwframe+0x44/0xa9 > [ 65.397166] RIP: 0033:0x7f6df42a6537 > [ 65.397428] Code: 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f > 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 > <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 > 74 24 > [ 65.398766] RSP: 002b:00007ffee4128018 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 > [ 65.399838] RAX: ffffffffffffffda RBX: 0000000000000008 RCX: 00007f6df42a6537 > [ 65.400923] RDX: 0000000000000008 RSI: 000055b3f7e549e0 RDI: 0000000000000001 > [ 65.402003] RBP: 000055b3f7e549e0 R08: 000000000000000a R09: 0000000000000007 > [ 65.403082] R10: 0000000000000004 R11: 0000000000000246 R12: 0000000000000008 > [ 65.404156] R13: 00007f6df4378500 R14: 0000000000000008 R15: 00007f6df4378700 > [ 65.405218] Modules linked in: <lots of modules> > [ 65.414172] CR2: ffffffffb720f7e0 > [ 65.415117] ---[ end trace 2dbff1a688549e65 ]--- > ----- >% ----- > > That happens because on dl_task_can_attach(): > dest_cpu = cpumask_any_and(cpu_active_mask, cs_cpus_allowed); > > returns a non active cpu. Since cs_cpus_allowed is empty dest_cpu should be an invalid CPU (>= nr_cpu_ids) here. > Initially, I thought about returning an error and blocking the > operation. However, that is indeed not needed. The cpuset without > CPUs assigned will be a non-root cpuset, hence its cpu mask will > be the same as the root one. So, the bandwidth was already accounted, > and the task can proceed. LGTM. After the '/sys/fs/cgroup# echo '+cpuset' > cgroup.subtree_control': root's cpuset.cpus.effective == user.slice's cpuset.cpus.effective > Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Juri Lelli <juri.lelli@xxxxxxxxxx> > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx> > Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > Cc: Ben Segall <bsegall@xxxxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxx> > Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> > Cc: Li Zefan <lizefan@xxxxxxxxxx> > Cc: Tejun Heo <tj@xxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Valentin Schneider <valentin.schneider@xxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx > Cc: cgroups@xxxxxxxxxxxxxxx > --- > kernel/sched/deadline.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index 943aa32cc1bc..788a391657a5 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -2871,6 +2871,13 @@ int dl_task_can_attach(struct task_struct *p, > bool overflow; > int ret; > > + /* > + * The cpuset has no cpus assigned, so the thread will not > + * change its affinity. > + */ > + if (cpumask_empty(cs_cpus_allowed)) > + return 0; > + > /* > * The task is not moving to another root domain, so it is > * already accounted. >
