Re: [Xen-devel] [PATCH v2] xen/sched: fix cpu offlining with core scheduling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.03.2020 18:39, Juergen Gross wrote:
> --- a/xen/common/sched/core.c
> +++ b/xen/common/sched/core.c
> @@ -2299,6 +2299,10 @@ void sched_context_switched(struct vcpu *vprev, struct 
> vcpu *vnext)
>      rcu_read_unlock(&sched_res_rculock);
>  }
>  
> +/*
> + * Switch to a new context or keep the current one running.
> + * On x86 it won't return, so it will drop the still held sched_res_rculock.
> + */
>  static void sched_context_switch(struct vcpu *vprev, struct vcpu *vnext,
>                                   bool reset_idle_unit, s_time_t now)
>  {

I don't follow the comment: There's

        return continue_running(vprev);

in the function which afaict can happen on all architectures.
The lock gets dropped there too. I see no path through this
function where the lock would not get dropped.

> @@ -2408,6 +2412,9 @@ static struct vcpu *sched_force_context_switch(struct 
> vcpu *vprev,
>   * zero do_schedule() is called and the rendezvous counter for leaving
>   * context_switch() is set. All other members will wait until the counter is
>   * becoming zero, dropping the schedule lock in between.
> + * Either returns the new unit to run, or NULL if no context switch is
> + * required or (on ARM) has already been performed. If NULL is returned
> + * sched_res_rculock has been dropped.

I guess official Arm folks would like Arm to not be spelled all
upper case anymore.

> @@ -2482,6 +2490,21 @@ static struct sched_unit 
> *sched_wait_rendezvous_in(struct sched_unit *prev,
>              atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
>              prev->rendezvous_in_cnt = 0;
>          }
> +
> +        /*
> +         * Check for scheduling resourced switched. This happens when we are
> +         * moved away from our cpupool and cpus are subject of the idle
> +         * scheduler now.
> +         */

The 'd' on both "resourced" and "switched" are odd to read at
least to me, and hence make me uncertain whether I actually
correctly understand what is meant here.

Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel