Re: [PATCH v2] Documentation: sysctl: document net core sysctls

[Jay Vosburgh]

Simon Horman <horms@xxxxxxxxxx> wrote:

>On Thu, Apr 09, 2026 at 11:18:59PM +0530, Shubham Chakraborty wrote:

[...]

>>  netdev_budget_usecs
>>  ---------------------
>>  
>
>The lines above the following hunk are:
>
>netdev_budget_usecs
>---------------------
>
>Maximum number of microseconds in one NAPI polling cycle. Polling
>
>> @@ -297,12 +332,16 @@ Maximum number of microseconds in one NAPI polling cycle. Polling
>>  will exit when either netdev_budget_usecs have elapsed during the
>>  poll cycle or the number of packets processed reaches netdev_budget.
>>  
>> +Default: ``2 * USEC_PER_SEC / HZ`` (2000 when ``HZ`` is 1000)
>> +
>
>Well, that is awkward.
>
>Looking at git history, it seems that this sysctl was added by 7acf8a1e8a28
>("Replace 2 jiffies with sysctl netdev_budget_usecs to enable softirq
>tuning") in 2017. And at that time the unic was us, and the default was 2000 us.
>
>But that was changed by a fix for that commit, a4837980fd9f ("net: revert
>default NAPI poll timeout to 2 jiffies"), in 2020. As a side-effect of
>that commit, the default was changed to what you have documented above,
>and the unit changed to jiffies.
>
>So while what you have is correct it seems nonsensical to me for the unit
>to be jiffies. Because that's not a meaningful unit for users. And because
>the name of the sysctl ends in usecs.

	I don't think the units for netdev_budget_usecs are actually
jiffies, even after a4837980fd9f.  The default value, for example, is
2000 if HZ is 1000.  However, the granularity of the measurement is in
jiffies, via:

static __latent_entropy void net_rx_action(void)
{
        struct softnet_data *sd = this_cpu_ptr(&softnet_data);
        unsigned long time_limit = jiffies +
                usecs_to_jiffies(READ_ONCE(net_hotdata.netdev_budget_usecs));


	I'm not sure offhand if usecs_to_jiffies rounds up or down, but
the netdev_budget_usecs looks to be interpreted as usecs.

	-J

>But I'm unsure what to do about it. Since changing the unit this would
>represent (another) KABI break.
>
>* Add another knob that shadows this one (But what to call it?)
>* Simply remove this one (KAPI break)
>* Change the unit of this knob (KAPI break)
>
>If the code is left as is, then I think it should be documented that the
>unit is jiffies.
>
>...
>

---
	-Jay Vosburgh, jv@xxxxxxxxxxxxx