Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <2c0c61a1c4c54d06905279a9a724a9390d9ee5c3.camel@linux.intel.com>
Subject: Re: [PATCH] sched/fair: no sync wakeup from interrupt context
From:   Tim Chen <tim.c.chen@linux.intel.com>
To:     Libo Chen <libo.chen@oracle.com>, peterz@infradead.org,
        vincent.guittot@linaro.org, mgorman@suse.de, 21cnbao@gmail.com,
        dietmar.eggemann@arm.com
Cc:     linux-kernel@vger.kernel.org, tglx@linutronix.de
Date:   Wed, 13 Jul 2022 09:40:51 -0700
In-Reply-To: <20220711224704.1672831-1-libo.chen@oracle.com>
References: <20220711224704.1672831-1-libo.chen@oracle.com>
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.34.4 (3.34.4-1.fc31) 
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

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@oracle.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
>