Softirq preemption

Softirq preemption

[Lee Revell]

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Ingo,

Is there any chance that the softirq preemption feature from the -rt
kernel can be pushed upstream?

Currently I am testing 2.6.14-rt21 with PREEMPT_DESKTOP and hard/softirq
preemption disabled in order to simulate the behavior of 2.6.14
mainline.  The network TX softirq is the longest non-preemptible code
path, it can run for 5-6ms.  However I can see that the preempt_count
drops to zero on many occasions, which IIRC means that no locks are held
and we could easily preempt at that point.

Without softirq	preemption I don't see how we have any chance of meeting
the 1ms goal for desktop responsiveness that was decided on at OLS 2004.

Lee

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Re: Softirq preemption

[Lee Revell]

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On Thu, 2005-12-01 at 14:08 -0500, Lee Revell wrote:
> Ingo,
> 
> Is there any chance that the softirq preemption feature from the -rt
> kernel can be pushed upstream?
> 

FWIW, here is the longest latency I see with 2.6.14-rt22, with the same
preemption settings as 2.6.14 + PREEMPT.  AFAICT a longish network
softirq gets interrupted by several disk and network interrupts which
adds up to ~1ms.  Is there any easy way to break up this code path that
could be acceptable for mainline, or does this seem like a reasonable
lower limit on achievable latency (until/unless PREEMPT_RT gets merged
etc)?

Lee

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Re: Softirq preemption

[Ingo Molnar]

* Lee Revell <rlrevell@joe-job.com> wrote:

> Is there any chance that the softirq preemption feature from the -rt 
> kernel can be pushed upstream?

yeah, will do it, once the current hrtimer push has been completed. The 
softirq/hardirq preemption feature is dependent on the genirq patch too.

	Ingo

SoftIRQ preemption

[Meha Mehta]

Hi All,

I have a question regarding SoftIRQ in Linux RT patch. I see that
preemption is disabled during softirq execution. Shouldn't they be
preemptible? If not, what are the problems in Linux with SoftIRQ
preemption?

Thank you in advance for your help.

Thanks,
Meha