Re: [PATCH v2 01/11] xen: sched: leave CPUs doing tasklet work alone.

[George Dunlap <george.dunlap@citrix.com>]

On 15/07/16 15:49, Dario Faggioli wrote:
> In both Credit1 and Credit2, stop considering a pCPU idle,
> if the reason why the idle vCPU is being selected, is to
> do tasklet work.
> 
> Not doing so means that the tickling and load balancing
> logic, seeing the pCPU as idle, considers it a candidate
> for picking up vCPUs. But the pCPU won't actually pick
> up or schedule any vCPU, which would then remain in the
> runqueue, which is bad, especially if there were other,
> truly idle pCPUs, that could execute it.
> 
> The only drawback is that we can't assume that a pCPU is
> in always marked as idle when being removed from an
> instance of the Credit2 scheduler (csched2_deinit_pdata).
> In fact, if we are in stop-machine (i.e., during suspend
> or shutdown), the pCPUs are running the stopmachine_tasklet
> and hence are actually marked as busy. On the other hand,
> when removing a pCPU from a Credit2 pool, it will indeed
> be idle. The only thing we can do, therefore, is to
> remove the BUG_ON() check.
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>

Reviewed-by: George Dunlap <george.dunlap@citrix.com>

> ---
> Cc: George Dunlap <george.dunlap@citrix.com>
> Cc: Jan Beulich <JBeulich@suse.com>
> Cc: Anshul Makkar <anshul.makkar@citrix.com>
> Cc: David Vrabel <david.vrabel@citrix.com>
> ---
> Changes in v1:
>  * reorg. the Credit1 code for minimal churn, as requested during review;
>  * don't move out: label up in Credit1, as requested during review.
> ---
> As code changed, I did not apply Anshul's R-b tag. Anshul, re-issue it if you
> want (if you feel like it, of course).
> ---
>  xen/common/sched_credit.c  |    2 +-
>  xen/common/sched_credit2.c |   14 ++++++++++----
>  2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> index ac22746..d547716 100644
> --- a/xen/common/sched_credit.c
> +++ b/xen/common/sched_credit.c
> @@ -1827,7 +1827,7 @@ csched_schedule(
>       * Update idlers mask if necessary. When we're idling, other CPUs
>       * will tickle us when they get extra work.
>       */
> -    if ( snext->pri == CSCHED_PRI_IDLE )
> +    if ( !tasklet_work_scheduled && snext->pri == CSCHED_PRI_IDLE )
>      {
>          if ( !cpumask_test_cpu(cpu, prv->idlers) )
>              cpumask_set_cpu(cpu, prv->idlers);
> diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
> index 8b95a47..7e572bf 100644
> --- a/xen/common/sched_credit2.c
> +++ b/xen/common/sched_credit2.c
> @@ -1923,8 +1923,16 @@ csched2_schedule(
>      }
>      else
>      {
> -        /* Update the idle mask if necessary */
> -        if ( !cpumask_test_cpu(cpu, &rqd->idle) )
> +        /*
> +         * Update the idle mask if necessary. Note that, if we're scheduling
> +         * idle in order to carry on some tasklet work, we want to play busy!
> +         */
> +        if ( tasklet_work_scheduled )
> +        {
> +            if ( cpumask_test_cpu(cpu, &rqd->idle) )
> +                cpumask_clear_cpu(cpu, &rqd->idle);
> +        }
> +        else if ( !cpumask_test_cpu(cpu, &rqd->idle) )
>              cpumask_set_cpu(cpu, &rqd->idle);
>          /* Make sure avgload gets updated periodically even
>           * if there's no activity */
> @@ -2304,8 +2312,6 @@ csched2_deinit_pdata(const struct scheduler *ops, void *pcpu, int cpu)
>      /* No need to save IRQs here, they're already disabled */
>      spin_lock(&rqd->lock);
>  
> -    BUG_ON(!cpumask_test_cpu(cpu, &rqd->idle));
> -
>      printk("Removing cpu %d from runqueue %d\n", cpu, rqi);
>  
>      cpumask_clear_cpu(cpu, &rqd->idle);
> 


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