Re: [Xen-devel] [PATCH 01/19] xen: sched: leave CPUs doing tasklet work alone.

[anshul makkar <anshul.makkar@xxxxxxxxxx>]

On 18/06/16 00:11, 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 bas, 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@xxxxxxxxxx>
Reviewed-by: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
> Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>   xen/common/sched_credit.c  |   12 ++++++------
>   xen/common/sched_credit2.c |   14 ++++++++++----
>   2 files changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> index a38a63d..a6645a2 100644
> --- a/xen/common/sched_credit.c
> +++ b/xen/common/sched_credit.c
> @@ -1819,24 +1819,24 @@ csched_schedule(
>       else
>           snext = csched_load_balance(prv, cpu, snext, &ret.migrated);
>
> + out:
>       /*
>        * 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);
> +        if ( cpumask_test_cpu(cpu, prv->idlers) )
> +            cpumask_clear_cpu(cpu, prv->idlers);
>       }
> -    else if ( cpumask_test_cpu(cpu, prv->idlers) )
> +    else if ( !cpumask_test_cpu(cpu, prv->idlers) )
>       {
> -        cpumask_clear_cpu(cpu, prv->idlers);
> +        cpumask_set_cpu(cpu, prv->idlers);
>       }
>
>       if ( !is_idle_vcpu(snext->vcpu) )
>           snext->start_time += now;
>
> -out:
>       /*
>        * Return task to run next...
>        */
> diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
> index 1933ff1..cf8455c 100644
> --- a/xen/common/sched_credit2.c
> +++ b/xen/common/sched_credit2.c
> @@ -1910,8 +1910,16 @@ 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 */
> @@ -2291,8 +2299,6 @@ 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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