remove of ktimersoftd in linux-rt-devel 5.0

remove of ktimersoftd in linux-rt-devel 5.0

[Alison Chaiken]

About a year ago the following commit greatly simplified softirqs:

commit 6f6ba7715a91877cf5fd2b357db3799baa331d9b
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Wed May 29 18:47:32 2019 +0200
    softirq: rework

It also removed ktimersoftd, without comment as far as I can see.  Is
it obvious that performance improvements due to lockless
synchronization render useless the earlier split of ktimersoftd from
ksoftirqd?   Is it now less likely that a large number of other
softirqs that are run immediately when hard IRQs terminate will use up
ksoftirqd's time slice and prevent the timer softirqs from getting a
chance to run?

Thanks,
Alison Chaiken
achaiken@aurora.tech
Aurora Innovation

Re: remove of ktimersoftd in linux-rt-devel 5.0

[Sebastian Andrzej Siewior]

On 2020-05-15 16:46:48 [-0700], Alison Chaiken wrote:
> About a year ago the following commit greatly simplified softirqs:
> 
> commit 6f6ba7715a91877cf5fd2b357db3799baa331d9b
> Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Date:   Wed May 29 18:47:32 2019 +0200
>     softirq: rework
> 
> It also removed ktimersoftd, without comment as far as I can see.  Is
> it obvious that performance improvements due to lockless
> synchronization render useless the earlier split of ktimersoftd from
> ksoftirqd?   Is it now less likely that a large number of other
> softirqs that are run immediately when hard IRQs terminate will use up
> ksoftirqd's time slice and prevent the timer softirqs from getting a
> chance to run?

When hard IRQs terminate and raise the softirq then the ksoftirqd is
woken up. The softirq may be processed by the thread _or_ by anything
else that is running happens to process softirqs. That can be any
force-threaded interrupt handler.
In general nothing raises softirqs from hardirq context. One exception
here are POSIX timer. The plan was to rework POSIX-timer code.

Sebastian