Re: [PATCH v2 1/2] credit: Limit load balancing to once per millisecond

[Juergen Gross <jgross@xxxxxxxx>]

On 21.09.23 14:23, George Dunlap wrote:
> The credit scheduler tries as hard as it can to ensure that it always
> runs scheduling units with positive credit (PRI_TS_UNDER) before
> running those with negative credit (PRI_TS_OVER).  If the next
> runnable scheduling unit is of priority OVER, it will always run the
> load balancer, which will scour the system looking for another
> scheduling unit of the UNDER priority.
>
> Unfortunately, as the number of cores on a system has grown, the cost
> of the work-stealing algorithm has dramatically increased; a recent
> trace on a system with 128 cores showed this taking over 50
> microseconds.
>
> Add a parameter, load_balance_ratelimit, to limit the frequency of
> load balance operations on a given pcpu.  Default this to 1
> millisecond.
>
> Invert the load balancing conditional to make it more clear, and line
> up more closely with the comment above it.
>
> Overall it might be cleaner to have the last_load_balance checking
> happen inside csched_load_balance(), but that would require either
> passing both now and spc into the function, or looking them up again;
> both of which seemed to be worse than simply checking and setting the
> values before calling it.
>
> On a system with a vcpu:pcpu ratio of 2:1, running Windows guests
> (which will end up calling YIELD during spinlock contention), this
> patch increased performance significantly.
>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxx>
> ---
> Changes since v1:
> - Fix editing mistake in commit message
> - Improve documentation
> - global var is __ro_after_init
> - Remove sysctl, as it's not used.  Define max value in credit.c.
> - Fix some style issues
> - Move comment tweak to the right patch
> - In the event that the commandline-parameter value is too high, clip
>    to the maximum value rather than setting to the default.
>
> CC: Dario Faggioli <dfaggioli@xxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: George Dunlap <george.dunlap@xxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Julien Grall <julien@xxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> ---
>   docs/misc/xen-command-line.pandoc |  8 ++++++
>   xen/common/sched/credit.c         | 47 +++++++++++++++++++++++++------
>   2 files changed, 46 insertions(+), 9 deletions(-)
>
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index f88e6a70ae..9c3c72a7f9 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -1884,6 +1884,14 @@ By default, Xen will use the INVPCID instruction for TLB 
> management if
>   it is available.  This option can be used to cause Xen to fall back to
>   older mechanisms, which are generally slower.
>   
> +### load-balance-ratelimit
> +> `= <integer>`
> +
> +The minimum interval between load balancing events on a given pcpu, in
> +microseconds.  A value of '0' will disable rate limiting.  Maximum
> +value 1 second. At the moment only credit honors this parameter.
> +Default 1ms.
> +
>   ### noirqbalance (x86)
>   > `= <boolean>`
>   
> diff --git a/xen/common/sched/credit.c b/xen/common/sched/credit.c
> index f2cd3d9da3..5c06f596d2 100644
> --- a/xen/common/sched/credit.c
> +++ b/xen/common/sched/credit.c
> @@ -50,6 +50,10 @@
>   #define CSCHED_TICKS_PER_TSLICE     3
>   /* Default timeslice: 30ms */
>   #define CSCHED_DEFAULT_TSLICE_MS    30
> +/* Default load balancing ratelimit: 1ms */
> +#define CSCHED_DEFAULT_LOAD_BALANCE_RATELIMIT_US 1000
> +/* Max load balancing ratelimit: 1s */
> +#define CSCHED_MAX_LOAD_BALANCE_RATELIMIT_US     1000000
>   #define CSCHED_CREDITS_PER_MSEC     10
>   /* Never set a timer shorter than this value. */
>   #define CSCHED_MIN_TIMER            XEN_SYSCTL_SCHED_RATELIMIT_MIN
> @@ -153,6 +157,7 @@ struct csched_pcpu {
>   
>       unsigned int idle_bias;
>       unsigned int nr_runnable;
> +    s_time_t last_load_balance;
>   
>       unsigned int tick;
>       struct timer ticker;
> @@ -218,7 +223,7 @@ struct csched_private {
>   
>       /* Period of master and tick in milliseconds */
>       unsigned int tick_period_us, ticks_per_tslice;
> -    s_time_t ratelimit, tslice, unit_migr_delay;
> +    s_time_t ratelimit, tslice, unit_migr_delay, load_balance_ratelimit;
>   
>       struct list_head active_sdom;
>       uint32_t weight;
> @@ -612,6 +617,8 @@ init_pdata(struct csched_private *prv, struct csched_pcpu 
> *spc, int cpu)
>       BUG_ON(!is_idle_unit(curr_on_cpu(cpu)));
>       cpumask_set_cpu(cpu, prv->idlers);
>       spc->nr_runnable = 0;
> +
> +    spc->last_load_balance = NOW();
>   }
>   
>   static void cf_check
> @@ -1676,9 +1683,17 @@ csched_runq_steal(int peer_cpu, int cpu, int pri, int 
> balance_step)
>       return NULL;
>   }
>   
> +/*
> + * Minimum delay, in microseconds, between load balance operations.
> + * This prevents spending too much time doing load balancing, particularly
> + * when the system has a high number of YIELDs due to spinlock priority 
> inversion.
> + */
> +static unsigned int __ro_after_init load_balance_ratelimit_us = 
> CSCHED_DEFAULT_LOAD_BALANCE_RATELIMIT_US;
> +integer_param("load-balance-ratelimit", load_balance_ratelimit_us);
> +
>   static struct csched_unit *
>   csched_load_balance(struct csched_private *prv, int cpu,
> -    struct csched_unit *snext, bool *stolen)
> +                    struct csched_unit *snext, bool *stolen)
>   {
>       const struct cpupool *c = get_sched_res(cpu)->cpupool;
>       struct csched_unit *speer;
> @@ -1958,15 +1973,19 @@ static void cf_check csched_schedule(
>           /*
>            * SMP Load balance:
>            *
> -         * If the next highest priority local runnable UNIT has already eaten
> -         * through its credits, look on other PCPUs to see if we have more
> -         * urgent work... If not, csched_load_balance() will return snext, but
> -         * already removed from the runq.
> +         * If the next highest priority local runnable UNIT has
> +         * already eaten through its credits (and we're below the
> +         * balancing ratelimit), look on other PCPUs to see if we have
> +         * more urgent work... If we don't, csched_load_balance() will
> +         * return snext, but already removed from the runq.
>            */
> -        if ( snext->pri > CSCHED_PRI_TS_OVER )
> -            __runq_remove(snext);
> -        else
> +        if ( snext->pri <= CSCHED_PRI_TS_OVER
> +             && now - spc->last_load_balance > prv->load_balance_ratelimit) {
^ Just found a style issue (after Andrew pointing out the ones in patch 2).

My R-b stands with that fixed.


Juergen

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