Re: [Xen-devel] [PATCH v2 2/7] xen: credit: (micro) optimize csched_runq_steal().

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 06/04/17 09:16, Dario Faggioli wrote:
> Chacking whether or not a vCPU can be 'stolen'
> from a peer pCPU's runqueue is relatively cheap.
> 
> Therefore, let's do that  as early as possible,
> avoiding potentially useless complex checks, and
> cpumask manipulations.
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
> ---
> Changes from v1:
> * fixed a typo in a comment.
> ---
>  xen/common/sched_credit.c |   17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> index 63a8675..5a3f13f 100644
> --- a/xen/common/sched_credit.c
> +++ b/xen/common/sched_credit.c
> @@ -708,12 +708,10 @@ static inline int
>  __csched_vcpu_is_migrateable(struct vcpu *vc, int dest_cpu, cpumask_t *mask)
>  {
>      /*
> -     * Don't pick up work that's in the peer's scheduling tail or hot on
> -     * peer PCPU. Only pick up work that prefers and/or is allowed to run
> -     * on our CPU.
> +     * Don't pick up work that's hot on peer PCPU, or that can't (or
> +     * would prefer not to) run on cpu.
>       */
> -    return !vc->is_running &&
> -           !__csched_vcpu_is_cache_hot(vc) &&
> +    return !__csched_vcpu_is_cache_hot(vc) &&
>             cpumask_test_cpu(dest_cpu, mask);

+1 to moving the check earlier; but there's a risk that this function
will be used somewhere else and that the is_running check will be missed.

Should we maybe add an ASSERT() here (perhaps with a comment that the
check can be added back in if necessary)?

 -George


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