Re: [PATCH next v2] softirq: enable MAX_SOFTIRQ_TIME tuning with sysctl, max_softirq_time_msecs

[Zhiqiang Liu]

Friendly ping ...

On 2019/6/25 11:13, Zhiqiang Liu wrote:
> From: Zhiqiang liu <liuzhiqiang26@xxxxxxxxxx>
> 
> In __do_softirq func, MAX_SOFTIRQ_TIME was set to 2ms via experimentation by
> commit c10d73671 ("softirq: reduce latencies") in 2013, which was designed
> to reduce latencies for various network workloads. The key reason is that the
> maximum number of microseconds in one NAPI polling cycle in net_rx_action func
> was set to 2 jiffies, so different HZ settting will lead to different latencies.
> 
> However, commit 7acf8a1e8 ("Replace 2 jiffies with sysctl netdev_budget_usecs
> to enable softirq tuning") adopts netdev_budget_usecs to tun maximum number of
> microseconds in one NAPI polling cycle. So the latencies of net_rx_action can be
> controlled by sysadmins to copy with hardware changes over time.
> 
> Correspondingly, the MAX_SOFTIRQ_TIME should be able to be tunned by sysadmins,
> who knows best about hardware performance, for excepted tradeoff between latence
> and fairness. Here, we add sysctl variable max_softirq_time_msecs to replace
> MAX_SOFTIRQ_TIME with 2ms default value.
> 
> Note: max_softirq_time_msecs will be coverted to jiffies, and any budget
> value will be rounded up to the next jiffies, which relates to CONFIG_HZ.
> The time accuracy of jiffies will result in a certain difference
> between the setting jiffies of max_softirq_time_msecs and the actual
> value, which is in one jiffies range.
> 
> Signed-off-by: Zhiqiang liu <liuzhiqiang26@xxxxxxxxxx>
> ---
>  Documentation/sysctl/kernel.txt | 17 +++++++++++++++++
>  kernel/softirq.c                |  8 +++++---
>  kernel/sysctl.c                 |  9 +++++++++
>  3 files changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index f0c86fbb3b48..23b36393f150 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -44,6 +44,7 @@ show up in /proc/sys/kernel:
>  - kexec_load_disabled
>  - kptr_restrict
>  - l2cr                        [ PPC only ]
> +- max_softirq_time_msecs
>  - modprobe                    ==> Documentation/debugging-modules.txt
>  - modules_disabled
>  - msg_next_id		      [ sysv ipc ]
> @@ -445,6 +446,22 @@ This flag controls the L2 cache of G3 processor boards. If
> 
>  ==============================================================
> 
> +max_softirq_time_msecs:
> +
> +Maximum number of milliseconds to break the loop of restarting softirq
> +processing for at most MAX_SOFTIRQ_RESTART times in __do_softirq().
> +max_softirq_time_msecs will be coverted to jiffies, and any budget
> +value will be rounded up to the next jiffies, which relates to CONFIG_HZ.
> +The time accuracy of jiffies will result in a certain difference
> +between the setting jiffies of max_softirq_time_msecs and the actual
> +value, which is in one jiffies range.
> +
> +max_softirq_time_msecs is a non-negative integer value, and setting
> +negative value is meaningless and will return error.
> +Default: 2
> +
> +==============================================================
> +
>  modules_disabled:
> 
>  A toggle value indicating if modules are allowed to be loaded
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index a6b81c6b6bff..1e456db70093 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -199,7 +199,8 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
> 
>  /*
>   * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
> - * but break the loop if need_resched() is set or after 2 ms.
> + * but break the loop if need_resched() is set or after
> + * max_softirq_time_msecs msecs.
>   * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
>   * certain cases, such as stop_machine(), jiffies may cease to
>   * increment and so we need the MAX_SOFTIRQ_RESTART limit as
> @@ -210,7 +211,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
>   * we want to handle softirqs as soon as possible, but they
>   * should not be able to lock up the box.
>   */
> -#define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)
> +unsigned int __read_mostly max_softirq_time_msecs = 2;
>  #define MAX_SOFTIRQ_RESTART 10
> 
>  #ifdef CONFIG_TRACE_IRQFLAGS
> @@ -248,7 +249,8 @@ static inline void lockdep_softirq_end(bool in_hardirq) { }
> 
>  asmlinkage __visible void __softirq_entry __do_softirq(void)
>  {
> -	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
> +	unsigned long end = jiffies +
> +		msecs_to_jiffies(max_softirq_time_msecs);
>  	unsigned long old_flags = current->flags;
>  	int max_restart = MAX_SOFTIRQ_RESTART;
>  	struct softirq_action *h;
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 1beca96fb625..96ff292ce7f6 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -118,6 +118,7 @@ extern unsigned int sysctl_nr_open_min, sysctl_nr_open_max;
>  #ifndef CONFIG_MMU
>  extern int sysctl_nr_trim_pages;
>  #endif
> +extern unsigned int max_softirq_time_msecs;
> 
>  /* Constants used for minimum and  maximum */
>  #ifdef CONFIG_LOCKUP_DETECTOR
> @@ -1276,6 +1277,14 @@ static struct ctl_table kern_table[] = {
>  		.extra2		= &one,
>  	},
>  #endif
> +	{
> +		.procname	= "max_softirq_time_msecs",
> +		.data		= &max_softirq_time_msecs,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler   = proc_dointvec_minmax,
> +		.extra1		= &zero,
> +	},
>  	{ }
>  };
>