Date:   Wed, 12 Jun 2019 12:31:02 -0400
From:   Waiman Long <longman@...hat.com>
To:     Joel Savitz <jsavitz@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Li Zefan <lizefan@...wei.com>, Phil Auld <pauld@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        Michal Koutný <mkoutny@...e.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, cgroups@...r.kernel.org
Subject: Re: [RESEND PATCH v3] cpuset: restore sanity to
 cpuset_cpus_allowed_fallback()

On 6/12/19 11:50 AM, Joel Savitz wrote:
> In the case that a process is constrained by taskset(1) (i.e.
> sched_setaffinity(2)) to a subset of available cpus, and all of those are
> subsequently offlined, the scheduler will set tsk->cpus_allowed to
> the current value of task_cs(tsk)->effective_cpus.
>
> This is done via a call to do_set_cpus_allowed() in the context of 
> cpuset_cpus_allowed_fallback() made by the scheduler when this case is
> detected. This is the only call made to cpuset_cpus_allowed_fallback()
> in the latest mainline kernel.
>
> However, this is not sane behavior.
>
> I will demonstrate this on a system running the latest upstream kernel
> with the following initial configuration:
>
> 	# grep -i cpu /proc/$$/status
> 	Cpus_allowed:	ffffffff,fffffff
> 	Cpus_allowed_list:	0-63
>
> (Where cpus 32-63 are provided via smt.)
>
> If we limit our current shell process to cpu2 only and then offline it
> and reonline it:
>
> 	# taskset -p 4 $$
> 	pid 2272's current affinity mask: ffffffffffffffff
> 	pid 2272's new affinity mask: 4
>
> 	# echo off > /sys/devices/system/cpu/cpu2/online
> 	# dmesg | tail -3
> 	[ 2195.866089] process 2272 (bash) no longer affine to cpu2
> 	[ 2195.872700] IRQ 114: no longer affine to CPU2
> 	[ 2195.879128] smpboot: CPU 2 is now offline
>
> 	# echo on > /sys/devices/system/cpu/cpu2/online
> 	# dmesg | tail -1
> 	[ 2617.043572] smpboot: Booting Node 0 Processor 2 APIC 0x4
>
>
> We see that our current process now has an affinity mask containing
> every cpu available on the system _except_ the one we originally
> constrained it to:
>
> 	# grep -i cpu /proc/$$/status
> 	Cpus_allowed:   ffffffff,fffffffb
> 	Cpus_allowed_list:      0-1,3-63 
>
> This is not sane behavior, as the scheduler can now not only place the
> process on previously forbidden cpus, it can't even schedule it on
> the cpu it was originally constrained to!
>
> Other cases result in even more exotic affinity masks. Take for instance
> a process with an affinity mask containing only cpus provided by smt at
> the moment that smt is toggled, in a configuration such as the following:
>
> 	# taskset -p f000000000 $$
> 	# grep -i cpu /proc/$$/status
> 	Cpus_allowed:	000000f0,00000000
> 	Cpus_allowed_list:	36-39
>
> A double toggle of smt results in the following behavior:
>
> 	# echo off > /sys/devices/system/cpu/smt/control
> 	# echo on > /sys/devices/system/cpu/smt/control
> 	# grep -i cpus /proc/$$/status
> 	Cpus_allowed:	ffffff00,ffffffff
> 	Cpus_allowed_list:	0-31,40-63
>
> This is even less sane than the previous case, as the new affinity mask
> excludes all smt-provided cpus with ids less than those that were
> previously in the affinity mask, as well as those that were actually in
> the mask.
>
> With this patch applied, both of these cases end in the following state:
>
> 	# grep -i cpu /proc/$$/status
> 	Cpus_allowed:	ffffffff,ffffffff
> 	Cpus_allowed_list:	0-63
>
> The original policy is discarded. Though not ideal, it is the simplest way
> to restore sanity to this fallback case without reinventing the cpuset
> wheel that rolls down the kernel just fine in cgroup v2. A user who wishes
> for the previous affinity mask to be restored in this fallback case can use
> that mechanism instead.
>
> This patch modifies scheduler behavior by instead resetting the mask to
> task_cs(tsk)->cpus_allowed by default, and cpu_possible mask in legacy
> mode. I tested the cases above on both modes.
>
> Note that the scheduler uses this fallback mechanism if and only if
> _every_ other valid avenue has been traveled, and it is the last resort
> before calling BUG().
>
> Suggested-by: Waiman Long <longman@...hat.com>
> Suggested-by: Phil Auld <pauld@...hat.com>
> Signed-off-by: Joel Savitz <jsavitz@...hat.com>
> ---
>  kernel/cgroup/cpuset.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6a1942ed781c..515525ff1cfd 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3254,10 +3254,23 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>  	spin_unlock_irqrestore(&callback_lock, flags);
>  }
>  
> +/**
> + * cpuset_cpus_allowed_fallback - final fallback before complete catastrophe.
> + * @tsk: pointer to task_struct with which the scheduler is struggling
> + *
> + * Description: In the case that the scheduler cannot find an allowed cpu in
> + * tsk->cpus_allowed, we fall back to task_cs(tsk)->cpus_allowed. In legacy
> + * mode however, this value is the same as task_cs(tsk)->effective_cpus,
> + * which will not contain a sane cpumask during cases such as cpu hotplugging.
> + * This is the absolute last resort for the scheduler and it is only used if
> + * _every_ other avenue has been traveled.
> + **/
> +
>  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
>  {
>  	rcu_read_lock();
> -	do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus);
> +	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> +		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
>  	rcu_read_unlock();
>  
>  	/*

Acked-by: Waiman Long <longman@...hat.com>

Good job!

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