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Message-ID: <0a5b196b-6ddf-5eb7-248f-da587f615559@redhat.com>
Date: Mon, 18 Jan 2021 13:51:28 +0100
From: Daniel Bristot de Oliveira <bristot@...hat.com>
To: Dietmar Eggemann <dietmar.eggemann@....com>,
linux-kernel@...r.kernel.org
Cc: Marco Perronet <perronet@...-sws.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Valentin Schneider <valentin.schneider@....com>,
cgroups@...r.kernel.org
Subject: Re: [PATCH 3/6] sched/deadline: Allow DL tasks on empty (cgroup v2)
cpusets
On 1/14/21 1:12 PM, Dietmar Eggemann wrote:
> 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
Ops, I "forgot" to add this step because Fedora/systemd does it by default.
I will add this to the log.
>> # 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.
Correct, I will change the "non active" to "an invalid CPU (>= > nr_cpu_ids)"
>> 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
Good, thanks!
-- Daniel
>> Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Juri Lelli <juri.lelli@...hat.com>
>> Cc: Vincent Guittot <vincent.guittot@...aro.org>
>> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
>> Cc: Steven Rostedt <rostedt@...dmis.org>
>> Cc: Ben Segall <bsegall@...gle.com>
>> Cc: Mel Gorman <mgorman@...e.de>
>> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
>> Cc: Li Zefan <lizefan@...wei.com>
>> Cc: Tejun Heo <tj@...nel.org>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Valentin Schneider <valentin.schneider@....com>
>> Cc: linux-kernel@...r.kernel.org
>> Cc: cgroups@...r.kernel.org
>> ---
>> 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.
>>
>
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