[RESEND PATCH v7 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path
Yicong Yang
yangyicong at huawei.com
Wed May 24 01:05:29 PDT 2023
On 2023/5/23 21:44, Chen Yu wrote:
> On 2023-05-22 at 20:42:19 +0800, Yicong Yang wrote:
>> Hi Chen,
>>
>> On 2023/5/22 14:29, Chen Yu wrote:
>>> Hi Yicong,
>>> On 2022-09-15 at 15:34:23 +0800, Yicong Yang wrote:
>>>> From: Barry Song <song.bao.hua at hisilicon.com>
>>>>
> [snip...]
>>
>> Thanks for the further information. The result of netperf/tbench looks good as we
>> image, the cluster wakeup expects to gain more benefit when the system is under
>> loaded or well-loaded. May I know how many CPUs sharing cluster on Jacobsvilla?
>>
> There are 4 CPUs per cluster on Jacobsville.
> [snip...]
>>>> @@ -6550,7 +6574,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>>> /*
>>>> * If the previous CPU is cache affine and idle, don't be stupid:
>>>> */
>>>> - if (prev != target && cpus_share_cache(prev, target) &&
>>>> + if (prev != target && cpus_share_lowest_cache(prev, target) &&
>>> This change impacts hackbench in socket mode a bit. It seems that for hackbench even
>>> putting the wakee on its previous CPU in the same LLC is better than putting it on
>>> current cluster. But it seems to be hackbench specific.
>>>
>>
>> ...without this do you still see the same improvement at under-loaded case (threads less-equal the CPU
>> numbers) for tbench/netperf?
>> The idea here is to always try to wakeup in the same cluster of the
>> target to benefit from the cluster cache but the early test for the prev and recent used cpu may break
>> that. Keep it as is, at low load the prev cpu or recent used cpu get more chance to be idle so we take
>> less chance to benefit from the cluster and gain less performance improvement.
>>
> Right. Without above change I saw lower improvement at lightly load case for netperf/tbench.
>> In the hackbench case as you noticed, the utilization can reach 100% ideally so the SIS_UTIL
>> will regulate the scanning number to 4 or around. If the prev/recent used CPU is not in the same
>> cluster with target, we're about to scanning the cluster and when found no idle CPU and has
>> run out of the scanning number, we'll fallback to wakeup on the target. That maybe the reason
>> why observed more wakeups on target rather than previous CPU.
>>
> Looks reasonable. When the budget of scanning number is low, we can not find an idle target
> on local cluster and terminates scanning for an idle prev on remote cluster, although that
> prev could be a better choice than target cpu.
>> In this case I wondering choosing prev cpu or recent used cpu after scanning the cluster can help
>> the situation here, like the snippet below (kinds of messy though).
>>
> This change makes sense to me. I only modified it a little bit to only give prev a second
> chance. With your patch applied, the improvement of netperf/tbench remains while the
> hackbench big regress was gone.
>
Thanks for the test, it looks justified to have this. Will include this change in next version.
> hackbench
> =========
> case load baseline(std%) compare%( std%)
> process-pipe 1-groups 1.00 ( 2.35) -0.65 ( 1.81)
> process-pipe 2-groups 1.00 ( 0.42) -2.16 ( 1.12)
> process-pipe 4-groups 1.00 ( 1.84) +0.72 ( 1.34)
> process-pipe 8-groups 1.00 ( 2.81) +1.12 ( 3.88)
> process-sockets 1-groups 1.00 ( 1.88) -0.99 ( 4.84)
> process-sockets 2-groups 1.00 ( 5.49) -4.50 ( 4.09)
> process-sockets 4-groups 1.00 ( 3.54) +2.28 ( 3.13)
> process-sockets 8-groups 1.00 ( 0.79) -0.13 ( 1.28)
> threads-pipe 1-groups 1.00 ( 1.73) -2.39 ( 0.40)
> threads-pipe 2-groups 1.00 ( 0.73) +2.88 ( 1.94)
> threads-pipe 4-groups 1.00 ( 0.64) +1.12 ( 1.82)
> threads-pipe 8-groups 1.00 ( 1.55) -1.59 ( 1.20)
> threads-sockets 1-groups 1.00 ( 3.76) +3.21 ( 3.56)
> threads-sockets 2-groups 1.00 ( 1.20) -5.56 ( 2.64)
> threads-sockets 4-groups 1.00 ( 2.65) +1.48 ( 4.91)
> threads-sockets 8-groups 1.00 ( 0.08) +0.18 ( 0.15)
>
> netperf
> =======
> case load baseline(std%) compare%( std%)
> TCP_RR 6-threads 1.00 ( 0.91) +2.87 ( 0.83)
> TCP_RR 12-threads 1.00 ( 0.22) +3.48 ( 0.31)
> TCP_RR 18-threads 1.00 ( 0.41) +7.81 ( 0.48)
> TCP_RR 24-threads 1.00 ( 1.02) -0.32 ( 1.25)
> TCP_RR 30-threads 1.00 ( 4.67) -0.04 ( 5.14)
> TCP_RR 36-threads 1.00 ( 4.53) -0.13 ( 4.37)
> TCP_RR 42-threads 1.00 ( 3.92) -0.15 ( 3.07)
> TCP_RR 48-threads 1.00 ( 2.07) -0.17 ( 1.52)
> UDP_RR 6-threads 1.00 ( 0.98) +4.50 ( 2.38)
> UDP_RR 12-threads 1.00 ( 0.26) +3.64 ( 0.25)
> UDP_RR 18-threads 1.00 ( 0.27) +9.93 ( 0.55)
> UDP_RR 24-threads 1.00 ( 1.22) +0.13 ( 1.33)
> UDP_RR 30-threads 1.00 ( 3.86) -0.03 ( 5.05)
> UDP_RR 36-threads 1.00 ( 2.81) +0.10 ( 3.37)
> UDP_RR 42-threads 1.00 ( 3.51) -0.26 ( 2.94)
> UDP_RR 48-threads 1.00 ( 12.54) +0.74 ( 9.44)
>
> tbench
> ======
> case load baseline(std%) compare%( std%)
> loopback 6-threads 1.00 ( 0.04) +2.94 ( 0.26)
> loopback 12-threads 1.00 ( 0.30) +4.58 ( 0.12)
> loopback 18-threads 1.00 ( 0.37) +12.38 ( 0.10)
> loopback 24-threads 1.00 ( 0.56) -0.27 ( 0.50)
> loopback 30-threads 1.00 ( 0.17) -0.18 ( 0.06)
> loopback 36-threads 1.00 ( 0.25) -0.73 ( 0.44)
> loopback 42-threads 1.00 ( 0.10) -0.22 ( 0.18)
> loopback 48-threads 1.00 ( 0.29) -0.48 ( 0.19)
>
> schbench
> ========
> case load baseline(std%) compare%( std%)
> normal 1-mthreads 1.00 ( 0.00) +0.00 ( 0.00)
> normal 2-mthreads 1.00 ( 0.00) +0.00 ( 0.00)
> normal 4-mthreads 1.00 ( 6.80) +2.78 ( 8.08)
> normal 8-mthreads 1.00 ( 3.65) -0.23 ( 4.30)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0989116b0796..07495b44c68f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7127,7 +7127,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> bool has_idle_core = false;
> struct sched_domain *sd;
> unsigned long task_util, util_min, util_max;
> - int i, recent_used_cpu;
> + int i, recent_used_cpu, prev_aff = -1;
>
> /*
> * On asymmetric system, update task utilization because we will check
> @@ -7152,10 +7152,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> /*
> * If the previous CPU is cache affine and idle, don't be stupid:
> */
> - if (prev != target && cpus_share_lowest_cache(prev, target) &&
> + if (prev != target && cpus_share_cache(prev, target) &&
> (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
> - asym_fits_cpu(task_util, util_min, util_max, prev))
> - return prev;
> + asym_fits_cpu(task_util, util_min, util_max, prev)) {
> + if (cpus_share_lowest_cache(prev, target))
> + return prev;
> + prev_aff = prev;
> + }
>
> /*
> * Allow a per-cpu kthread to stack with the wakee if the
> @@ -7223,6 +7226,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> if ((unsigned)i < nr_cpumask_bits)
> return i;
>
> + /*
> + * Give prev another chance, in case prev has not been
> + * scanned in select_idle_cpu() due to nr constrain.
> + */
> + if (prev_aff != -1)
> + return prev_aff;
> +
It looks neater. We should also give recent_used_cpu a chance based on the current implementation
if it does no harm.
Thanks,
Yicong
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