Different priorities for different softirqs

Different priorities for different softirqs

[Christian Meier]

Hi,

On https://rt.wiki.kernel.org/articles/r/t/_/RT_PREEMPT_HOWTO_6bc9.html I
noticed that there are different softirq kthreads for every softirq. So they
could be prioritized differently.
In my rt-patched kernels 3.2.11-rt20 and 3.0.24-rt42 I can only see one
ksoftirqd instance per cpu.

I did not find any hints in kernel/softirq.c where each of those kthreads is
generated. I only see ksoftirqd.

>From RT_PREEMPT_HOWTO_6bc9.html:
...
2 ?        S      0:00 [softirq-high/0] 
3 ?        S      0:00 [softirq-timer/0] 
4 ?        S      0:00 [softirq-net-tx/] 
5 ?        S      0:00 [softirq-net-rx/] 
6 ?        S      0:00 [softirq-block/0] 
7 ?        S      0:00 [softirq-tasklet] 
8 ?        S      0:00 [softirq-hrtreal] 
9 ?        S      0:00 [softirq-hrtmono]
...

How can I get these softirq different threads? Did I miss a configuration
option?

Thanks
Christian

Re: Different priorities for different softirqs

[Luis Claudio R. Goncalves]

On Wed, Mar 21, 2012 at 02:04:10PM +0100, Christian Meier wrote:
| Hi,
| 
| On https://rt.wiki.kernel.org/articles/r/t/_/RT_PREEMPT_HOWTO_6bc9.html I
| noticed that there are different softirq kthreads for every softirq. So they
| could be prioritized differently.
| In my rt-patched kernels 3.2.11-rt20 and 3.0.24-rt42 I can only see one
| ksoftirqd instance per cpu.
| 
| I did not find any hints in kernel/softirq.c where each of those kthreads is
| generated. I only see ksoftirqd.
| 
| >From RT_PREEMPT_HOWTO_6bc9.html:
| ...
| 2 ?        S      0:00 [softirq-high/0] 
| 3 ?        S      0:00 [softirq-timer/0] 
| 4 ?        S      0:00 [softirq-net-tx/] 
| 5 ?        S      0:00 [softirq-net-rx/] 
| 6 ?        S      0:00 [softirq-block/0] 
| 7 ?        S      0:00 [softirq-tasklet] 
| 8 ?        S      0:00 [softirq-hrtreal] 
| 9 ?        S      0:00 [softirq-hrtmono]
| ...
| 
| How can I get these softirq different threads? Did I miss a configuration
| option?

The examples there are from older kernels (probably 2.6.18 and 2.6.23).
Several changes happened since then, to the kernels and even to the general
model of the RT patch. Among these changes, more than once softirqs have
been folded inside ksoftirqd and later unfolded. As of now, they are folded
and it seems to have helped to minimize a performance loss someone (tglx?)
observed in the early 3.0.x-rt days.

Luis
-- 
[ Luis Claudio R. Goncalves                    Bass - Gospel - RT ]
[ Fingerprint: 4FDD B8C4 3C59 34BD 8BE9  2696 7203 D980 A448 C8F8 ]

Re: Different priorities for different softirqs

[Steven Rostedt]

On Wed, 2012-03-21 at 10:08 -0300, Luis Claudio R. Goncalves wrote:
> he examples there are from older kernels (probably 2.6.18 and 2.6.23).
> Several changes happened since then, to the kernels and even to the general
> model of the RT patch. Among these changes, more than once softirqs have
> been folded inside ksoftirqd and later unfolded. As of now, they are folded
> and it seems to have helped to minimize a performance loss someone (tglx?)
> observed in the early 3.0.x-rt days.

I discussed this in Chemnitz, and will be working to at least pull the
timer softirq out into its own thread.

We need to look at this again and see how we can fix it in a clean
fashion.

-- Steve

Re: Different priorities for different softirqs

[John Kacur]

On Wed, Mar 21, 2012 at 2:04 PM, Christian Meier <chmeie@gmx.com> wrote:
> Hi,
>
> On https://rt.wiki.kernel.org/articles/r/t/_/RT_PREEMPT_HOWTO_6bc9.html I
> noticed that there are different softirq kthreads for every softirq. So they
> could be prioritized differently.
> In my rt-patched kernels 3.2.11-rt20 and 3.0.24-rt42 I can only see one
> ksoftirqd instance per cpu.
>
> I did not find any hints in kernel/softirq.c where each of those kthreads is
> generated. I only see ksoftirqd.
>
> From RT_PREEMPT_HOWTO_6bc9.html:
> ...
> 2 ?        S      0:00 [softirq-high/0]
> 3 ?        S      0:00 [softirq-timer/0]
> 4 ?        S      0:00 [softirq-net-tx/]
> 5 ?        S      0:00 [softirq-net-rx/]
> 6 ?        S      0:00 [softirq-block/0]
> 7 ?        S      0:00 [softirq-tasklet]
> 8 ?        S      0:00 [softirq-hrtreal]
> 9 ?        S      0:00 [softirq-hrtmono]
> ...
>

You can read the section on Software interrupts here too to learn a
bit about the change.
https://lwn.net/Articles/464180/

Thanks
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