Re: [Xen-devel] [PATCH v2 02/10] xen: credit1: don't rate limit context switches in case of yields

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 30/09/16 03:53, Dario Faggioli wrote:
> Rate limiting has been primarily introduced to avoid too
> heavy context switch rate due to interrupts, and, in
> general, asynchronous events.
> 
> If a vcpu "voluntarily" yields, we really should let it
> give up the cpu for a while.
> 
> In fact, it may be that it is yielding because it's about
> to start spinning, and there's few point in forcing a vcpu
> to spin for (potentially) the entire rate-limiting period.
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>

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

> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
> ---
> Changes from v1:
>  * move this patch up in the series, and remove the Credit2 bits, as suggested
>    during review;
> ---
>  xen/common/sched_credit.c |   13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> index 4d84b5f..5700763 100644
> --- a/xen/common/sched_credit.c
> +++ b/xen/common/sched_credit.c
> @@ -1802,9 +1802,16 @@ csched_schedule(
>       *   cpu and steal it.
>       */
>  
> -    /* If we have schedule rate limiting enabled, check to see
> -     * how long we've run for. */
> -    if ( !tasklet_work_scheduled
> +    /*
> +     * If we have schedule rate limiting enabled, check to see
> +     * how long we've run for.
> +     *
> +     * If scurr is yielding, however, we don't let rate limiting kick in.
> +     * In fact, it may be the case that scurr is about to spin, and there's
> +     * no point forcing it to do so until rate limiting expires.
> +     */
> +    if ( !test_bit(CSCHED_FLAG_VCPU_YIELD, &scurr->flags)
> +         && !tasklet_work_scheduled
>           && prv->ratelimit_us
>           && vcpu_runnable(current)
>           && !is_idle_vcpu(current)
> 


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