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Message-ID: <2c0c61a1c4c54d06905279a9a724a9390d9ee5c3.camel@linux.intel.com>
Date: Wed, 13 Jul 2022 09:40:51 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Libo Chen <libo.chen@...cle.com>, peterz@...radead.org,
vincent.guittot@...aro.org, mgorman@...e.de, 21cnbao@...il.com,
dietmar.eggemann@....com
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH] sched/fair: no sync wakeup from interrupt context
On Mon, 2022-07-11 at 15:47 -0700, Libo Chen wrote:
> Barry Song first pointed out that replacing sync wakeup with regular wakeup
> seems to reduce overeager wakeup pulling and shows noticeable performance
> improvement.[1]
>
> This patch argues that allowing sync for wakeups from interrupt context
> is a bug and fixing it can improve performance even when irq/softirq is
> evenly spread out.
>
> For wakeups from ISR, the waking CPU is just the CPU of ISR and the so-called
> waker can be any random task that happens to be running on that CPU when the
> interrupt comes in. This is completely different from other wakups where the
> task running on the waking CPU is the actual waker. For example, two tasks
> communicate through a pipe or mutiple tasks access the same critical section,
> etc. This difference is important because with sync we assume the waker will
> get off the runqueue and go to sleep immedately after the wakeup. The
> assumption is built into wake_affine() where it discounts the waker's presence
> from the runqueue when sync is true. The random waker from interrupts bears no
> relation to the wakee and don't usually go to sleep immediately afterwards
> unless wakeup granularity is reached. Plus the scheduler no longer enforces the
> preepmtion of waker for sync wakeup as it used to before
> patch f2e74eeac03ffb7 ("sched: Remove WAKEUP_SYNC feature"). Enforcing sync
> wakeup preemption for wakeups from interrupt contexts doesn't seem to be
> appropriate too but at least sync wakeup will do what it's supposed to do.
Will there be scenarios where you do want the task being woken up be pulled
to the CPU where the interrupt happened, as the data that needs to be accessed is
on local CPU/NUMA that interrupt happened? For example, interrupt associated with network
packets received. Sync still seems desirable, at least if there is no task currently
running on the CPU where interrupt happened. So maybe we should have some consideration
of the load on the CPU/NUMA before deciding whether we should do sync wake for such
interrupt.
>
> Add a check to make sure that sync can only be set when in_task() is true. If
> a wakeup is from interrupt context, sync flag will be 0 because in_task()
> returns 0.
>
> Tested in two scenarios: wakeups from 1) task contexts, expected to see no
> performance changes; 2) interrupt contexts, expected to see better performance
> under low/medium load and no regression under heavy load.
>
> Use hackbench for scenario 1 and pgbench for scenarios 2 both from mmtests on
> a 2-socket Xeon E5-2699v3 box with 256G memory in total. Running 5.18 kernel
> with SELinux disabled. Scheduler/MM tunables are all default. Irqbalance
> daemon is active.
>
> Hackbench (config-scheduler-unbound)
> =========
> process-pipes:
> Baseline Patched
> Amean 1 0.4300 ( 0.00%) 0.4420 ( -2.79%)
> Amean 4 0.8757 ( 0.00%) 0.8774 ( -0.20%)
> Amean 7 1.3712 ( 0.00%) 1.3789 ( -0.56%)
> Amean 12 2.3541 ( 0.00%) 2.3714 ( -0.73%)
> Amean 21 4.2229 ( 0.00%) 4.2439 ( -0.50%)
> Amean 30 5.9113 ( 0.00%) 5.9451 ( -0.57%)
> Amean 48 9.3873 ( 0.00%) 9.4898 ( -1.09%)
> Amean 79 15.9281 ( 0.00%) 16.1385 ( -1.32%)
> Amean 110 22.0961 ( 0.00%) 22.3433 ( -1.12%)
> Amean 141 28.2973 ( 0.00%) 28.6209 ( -1.14%)
> Amean 172 34.4709 ( 0.00%) 34.9347 ( -1.35%)
> Amean 203 40.7621 ( 0.00%) 41.2497 ( -1.20%)
> Amean 234 47.0416 ( 0.00%) 47.6470 ( -1.29%)
> Amean 265 53.3048 ( 0.00%) 54.1625 ( -1.61%)
> Amean 288 58.0595 ( 0.00%) 58.8096 ( -1.29%)
>
> process-sockets:
> Baseline Patched
> Amean 1 0.6776 ( 0.00%) 0.6611 ( 2.43%)
> Amean 4 2.6183 ( 0.00%) 2.5769 ( 1.58%)
> Amean 7 4.5662 ( 0.00%) 4.4801 ( 1.89%)
> Amean 12 7.7638 ( 0.00%) 7.6201 ( 1.85%)
> Amean 21 13.5335 ( 0.00%) 13.2915 ( 1.79%)
> Amean 30 19.3369 ( 0.00%) 18.9811 ( 1.84%)
> Amean 48 31.0724 ( 0.00%) 30.6015 ( 1.52%)
> Amean 79 51.1881 ( 0.00%) 50.4251 ( 1.49%)
> Amean 110 71.3399 ( 0.00%) 70.4578 ( 1.24%)
> Amean 141 91.4675 ( 0.00%) 90.3769 ( 1.19%)
> Amean 172 111.7463 ( 0.00%) 110.3947 ( 1.21%)
> Amean 203 131.6927 ( 0.00%) 130.3270 ( 1.04%)
> Amean 234 151.7459 ( 0.00%) 150.1320 ( 1.06%)
> Amean 265 171.9101 ( 0.00%) 169.9751 ( 1.13%)
> Amean 288 186.9231 ( 0.00%) 184.7706 ( 1.15%)
>
> thread-pipes:
> Baseline Patched
> Amean 1 0.4523 ( 0.00%) 0.4535 ( -0.28%)
> Amean 4 0.9041 ( 0.00%) 0.9085 ( -0.48%)
> Amean 7 1.4111 ( 0.00%) 1.4146 ( -0.25%)
> Amean 12 2.3532 ( 0.00%) 2.3688 ( -0.66%)
> Amean 21 4.1550 ( 0.00%) 4.1701 ( -0.36%)
> Amean 30 6.1043 ( 0.00%) 6.2391 ( -2.21%)
> Amean 48 10.2077 ( 0.00%) 10.3511 ( -1.40%)
> Amean 79 16.7922 ( 0.00%) 17.0427 ( -1.49%)
> Amean 110 23.3350 ( 0.00%) 23.6522 ( -1.36%)
> Amean 141 29.6458 ( 0.00%) 30.2617 ( -2.08%)
> Amean 172 35.8649 ( 0.00%) 36.4225 ( -1.55%)
> Amean 203 42.4477 ( 0.00%) 42.8332 ( -0.91%)
> Amean 234 48.7117 ( 0.00%) 49.4042 ( -1.42%)
> Amean 265 54.9171 ( 0.00%) 55.6551 ( -1.34%)
> Amean 288 59.5282 ( 0.00%) 60.2903 ( -1.28%)
>
> thread-sockets:
> Baseline Patched
> Amean 1 0.6917 ( 0.00%) 0.6892 ( 0.37%)
> Amean 4 2.6651 ( 0.00%) 2.6017 ( 2.38%)
> Amean 7 4.6734 ( 0.00%) 4.5637 ( 2.35%)
> Amean 12 8.0156 ( 0.00%) 7.8079 ( 2.59%)
> Amean 21 14.0451 ( 0.00%) 13.6679 ( 2.69%)
> Amean 30 20.0963 ( 0.00%) 19.5657 ( 2.64%)
> Amean 48 32.4115 ( 0.00%) 31.6001 ( 2.50%)
> Amean 79 53.1104 ( 0.00%) 51.8395 ( 2.39%)
> Amean 110 74.0929 ( 0.00%) 72.4391 ( 2.23%)
> Amean 141 95.1506 ( 0.00%) 93.0992 ( 2.16%)
> Amean 172 116.1969 ( 0.00%) 113.8307 ( 2.04%)
> Amean 203 137.4413 ( 0.00%) 134.5247 ( 2.12%)
> Amean 234 158.5395 ( 0.00%) 155.2793 ( 2.06%)
> Amean 265 179.7729 ( 0.00%) 176.1099 ( 2.04%)
> Amean 288 195.5573 ( 0.00%) 191.3977 ( 2.13%)
>
> Pgbench (config-db-pgbench-timed-ro-small)
> =======
> Baseline Patched
> Hmean 1 68.54 ( 0.00%) 69.72 ( 1.71%)
> Hmean 6 27725.78 ( 0.00%) 34119.11 ( 23.06%)
> Hmean 12 55724.26 ( 0.00%) 63158.22 ( 13.34%)
> Hmean 22 72806.26 ( 0.00%) 73389.98 ( 0.80%)
> Hmean 30 79000.45 ( 0.00%) 75197.02 ( -4.81%)
> Hmean 48 78180.16 ( 0.00%) 75074.09 ( -3.97%)
> Hmean 80 75001.93 ( 0.00%) 70590.72 ( -5.88%)
> Hmean 110 74812.25 ( 0.00%) 74128.57 ( -0.91%)
> Hmean 142 74261.05 ( 0.00%) 72910.48 ( -1.82%)
> Hmean 144 75375.35 ( 0.00%) 71295.72 ( -5.41%)
>
> For hackbench, +-3% fluctuation is normal on this two-socket box, it's safe to
> conclude that there are no performance differences for wakeups from task context.
> For pgbench, after many runs, 10~30% gains are very consistent at lower
> client counts (< #cores per socket).
Can you provide some further insights on why pgebench is helped at low load
case? Is it because the woken tasks tend to stay put and not get moved around with interrupts
and maintain cache warmth?
Tim
> For higher client counts, both kernels are
> very close, +-5% swings are quite common. Also NET_TX|RX softirq load
> does spread out across both NUMA nodes in this test so there is no need to do
> any explicit RPS/RFS.
>
> [1]: https://lkml.org/lkml/2021/11/5/234
>
> Signed-off-by: Libo Chen <libo.chen@...cle.com>
> ---
> kernel/sched/fair.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..59b210d2cdb5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6704,7 +6704,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> static int
> select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> {
> - int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> + /* Don't set sync for wakeup from irq/soft ctx */
> + int sync = in_task() && (wake_flags & WF_SYNC)
> + && !(current->flags & PF_EXITING);
> struct sched_domain *tmp, *sd = NULL;
> int cpu = smp_processor_id();
> int new_cpu = prev_cpu;
> --
> 2.31.1
>
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