Re: [PATCH v2 1/2] xen: credit2: fix two s_time_t handling issues in load balancing

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

On Tue, Jul 19, 2016 at 4:33 PM, Dario Faggioli
<dario.faggioli@citrix.com> wrote:
> both introduced in d205f8a7f48e2ec ("xen: credit2: rework
> load tracking logic").
>
> First, in __update_runq_load(), the ASSERT() was actually
> useless. Let's instead check that the computed value of
> the load has not overflowed (and hence gone negative).
>
> While there, do that in __update_svc_load() as well.
>
> Second, in balance_load(), cpus_max needs being extended
> in order to be correctly shifted, and the result compared
> with an s_time_t value, without risking loosing info.
>
> Spotted by Coverity.
>
> Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
> Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>

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

And queued.

> ---
> Cc: George Dunlap <george.dunlap@citrix.com>
> Cc: Anshul Makkar <anshul.makkar@citrix.com>
> ---
> Changed from v1:
>  * fixed a '> 0' which wanted to be '>= 0' in the ASSERT()-s;
>  * cite Coverity in the changelog.
> ---
>  xen/common/sched_credit2.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
> index b33ba7a..a55240f 100644
> --- a/xen/common/sched_credit2.c
> +++ b/xen/common/sched_credit2.c
> @@ -656,7 +656,8 @@ __update_runq_load(const struct scheduler *ops,
>      rqd->load += change;
>      rqd->load_last_update = now;
>
> -    ASSERT(rqd->avgload <= STIME_MAX && rqd->b_avgload <= STIME_MAX);
> +    /* Overflow, capable of making the load look negative, must not occur. */
> +    ASSERT(rqd->avgload >= 0 && rqd->b_avgload >= 0);
>
>      if ( unlikely(tb_init_done) )
>      {
> @@ -714,6 +715,9 @@ __update_svc_load(const struct scheduler *ops,
>      }
>      svc->load_last_update = now;
>
> +    /* Overflow, capable of making the load look negative, must not occur. */
> +    ASSERT(svc->avgload >= 0);
> +
>      if ( unlikely(tb_init_done) )
>      {
>          struct {
> @@ -1742,7 +1746,7 @@ retry:
>           * If we're under 100% capacaty, only shift if load difference
>           * is > 1.  otherwise, shift if under 12.5%
>           */
> -        if ( load_max < (cpus_max << prv->load_precision_shift) )
> +        if ( load_max < ((s_time_t)cpus_max << prv->load_precision_shift) )
>          {
>              if ( st.load_delta < (1ULL << (prv->load_precision_shift +
>                                             opt_underload_balance_tolerance)) )
>
>
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