softirqd

softirqd

[kernwek jalsl]

Hi All;

On my embedded board I see that the preemption
patch(Love etal.) is not of much use because of the
following reaons:

1) calling the scheduler after the hardirq returns is
not of much use because the actual work of puting the
task in the run queue is done in the bottom half
executed by ksoftirqd
2) ksoftirqd is not a real time thread

Will the preemption patch work better if the ksoftirqd
is made a real time thread?

Another related question : has anyone thought of
introducing prioirty among tasklets? Right now
ksoftirqd treats them in a FIFO manner. Adding
priority among the various tasklets and making sure
that ksoftirqd looks ahead in the queue would ensure a
real time treatment to a real time interrupt...

I am sorry in case this has been discussed earlier; in
case it has been I would like someone to post me the
URl for the discussion.

Regards

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Re: softirqd

[Richard B. Johnson]

On Mon, 17 Nov 2003, kernwek jalsl wrote:

> Hi All;
>
> On my embedded board I see that the preemption
> patch(Love etal.) is not of much use because of the
> following reaons:
>
> 1) calling the scheduler after the hardirq returns is
> not of much use because the actual work of puting the
> task in the run queue is done in the bottom half
> executed by ksoftirqd
> 2) ksoftirqd is not a real time thread
>
> Will the preemption patch work better if the ksoftirqd
> is made a real time thread?
>
> Another related question : has anyone thought of
> introducing prioirty among tasklets? Right now
> ksoftirqd treats them in a FIFO manner. Adding
> priority among the various tasklets and making sure
> that ksoftirqd looks ahead in the queue would ensure a
> real time treatment to a real time interrupt...
>

What is the problem that you are attempting to fix?
Stating that x is not y does not mean anything useful.
'Real-time' related to an 'interrupt' has no meaning.

An interrupt occurs as soon as hardware demands it.
The software response to that interrupt, i.e., interrupt-
latency is dependent upon the ISR setup code in the
kernel plus the execution time of any higher-priority
interrupts that may be occurring plus the execution time
of any prior code sharing the same interrupt. By the time
your ISR code gets control, the time is bounded by some
worse-case execution times and real-time has no meaning.

Then your ISR code must handle the immediate requirements
of the hardware as fast and efficiently as possible. Things
that can be deferred may be handed over to the soft-IRQ
stuff. If you have things that can't be deferred, you must
not do it in the soft-IRQ (used to be called bottom-half)
handler(s).

Specifically, the things that can be handled in a soft-IRQ
are hardware configuration changes and such where here are
no deadlines. If you have deadlines, i.e., you must get
the data out of a device now before it gets overwritten by
new data, then it must be done in the top-half handler.

Also, If you ever enable interrupts in your ISR, plan that
the CPU __will__ get taken away. It may be hundreds of
milliseconds before your ISR ever gets the CPU again. It
all depends upon the behavior of other drivers and some
network drivers even loop inside their ISR. Once they get
control, you may not get the CPU until a break in network
traffic!

In the context of a multi-user time-share operating system,
"real-time" means "fast enough". If your complete system can
keep up with the demands of these external events without
ever losing data, then it is "real-time". However, you can't
use these kinds of systems (fast-enough systems) in closed
servo-loops because the time from an interrupting event to
the completion of the necessary response to that event is
not known. For those kinds of "real-time" systems, you need
a "real-time kernel". Real-time kernels define the poles in
the system transfer-function so that the delay is known and
bounded within strict limits. Such systems are not "better".
In fact, they might be much slower in response to the usual
requirements for interactivity.

Realtime kernels do this by scheduling on strict time-boundaries.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.

Re: softirqd

[kernwek jalsl]

Hi;

Sorry in case I was not very clear with my
requirements.   With real time interrupt I meant a
real time task waiting for IO from this interrupt.
Assume that I have a high priority interrupt and a
real time task waiting for it. Well followimg are the
various latencies involved:
L1- interrupt latency
L2- hard and soft IRQ completion
L3 - scheduler latency
L4 - scheduler completion

L1 is pretty acceptable on Linux. For L3 we have the
preemption and low latency patch. And for L4 the O(1)
scheduler solves the problem. So I see L2 as the
bootleneck especially with soft IRQ since the softIRQs
get scheduled in a non real time thread and there is
no wayI can tell the softIRQd that I want highest
priority for the interrupt that will wake up my real
time task. I was seeking a solution to this. 

I know that TimeSYS has as patch for making the
softIRQ a real time thread as well as giving
priorities for both the top and bottom halves. Is
there any place where I can get some performance
figures for this patch?

Regards


--- "Richard B. Johnson" <root@chaos.analogic.com>
wrote:
> On Mon, 17 Nov 2003, kernwek jalsl wrote:
> 
> > Hi All;
> >
> > On my embedded board I see that the preemption
> > patch(Love etal.) is not of much use because of
> the
> > following reaons:
> >
> > 1) calling the scheduler after the hardirq returns
> is
> > not of much use because the actual work of puting
> the
> > task in the run queue is done in the bottom half
> > executed by ksoftirqd
> > 2) ksoftirqd is not a real time thread
> >
> > Will the preemption patch work better if the
> ksoftirqd
> > is made a real time thread?
> >
> > Another related question : has anyone thought of
> > introducing prioirty among tasklets? Right now
> > ksoftirqd treats them in a FIFO manner. Adding
> > priority among the various tasklets and making
> sure
> > that ksoftirqd looks ahead in the queue would
> ensure a
> > real time treatment to a real time interrupt...
> >
> 
> What is the problem that you are attempting to fix?
> Stating that x is not y does not mean anything
> useful.
> 'Real-time' related to an 'interrupt' has no
> meaning.
> 
> An interrupt occurs as soon as hardware demands it.
> The software response to that interrupt, i.e.,
> interrupt-
> latency is dependent upon the ISR setup code in the
> kernel plus the execution time of any
> higher-priority
> interrupts that may be occurring plus the execution
> time
> of any prior code sharing the same interrupt. By the
> time
> your ISR code gets control, the time is bounded by
> some
> worse-case execution times and real-time has no
> meaning.
> 
> Then your ISR code must handle the immediate
> requirements
> of the hardware as fast and efficiently as possible.
> Things
> that can be deferred may be handed over to the
> soft-IRQ
> stuff. If you have things that can't be deferred,
> you must
> not do it in the soft-IRQ (used to be called
> bottom-half)
> handler(s).
> 
> Specifically, the things that can be handled in a
> soft-IRQ
> are hardware configuration changes and such where
> here are
> no deadlines. If you have deadlines, i.e., you must
> get
> the data out of a device now before it gets
> overwritten by
> new data, then it must be done in the top-half
> handler.
> 
> Also, If you ever enable interrupts in your ISR,
> plan that
> the CPU __will__ get taken away. It may be hundreds
> of
> milliseconds before your ISR ever gets the CPU
> again. It
> all depends upon the behavior of other drivers and
> some
> network drivers even loop inside their ISR. Once
> they get
> control, you may not get the CPU until a break in
> network
> traffic!
> 
> In the context of a multi-user time-share operating
> system,
> "real-time" means "fast enough". If your complete
> system can
> keep up with the demands of these external events
> without
> ever losing data, then it is "real-time". However,
> you can't
> use these kinds of systems (fast-enough systems) in
> closed
> servo-loops because the time from an interrupting
> event to
> the completion of the necessary response to that
> event is
> not known. For those kinds of "real-time" systems,
> you need
> a "real-time kernel". Real-time kernels define the
> poles in
> the system transfer-function so that the delay is
> known and
> bounded within strict limits. Such systems are not
> "better".
> In fact, they might be much slower in response to
> the usual
> requirements for interactivity.
> 
> Realtime kernels do this by scheduling on strict
> time-boundaries.
> 
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.4.22 on an i686 machine
> (797.90 BogoMips).
>             Note 96.31% of all statistics are
> fiction.
> 
> 


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Re: softirqd

[Richard B. Johnson]

On Mon, 17 Nov 2003, kernwek jalsl wrote:

>
> Hi;
>
> Sorry in case I was not very clear with my
> requirements.   With real time interrupt I meant a
> real time task waiting for IO from this interrupt.
> Assume that I have a high priority interrupt and a
> real time task waiting for it. Well followimg are the
> various latencies involved:
> L1- interrupt latency
> L2- hard and soft IRQ completion
> L3 - scheduler latency
> L4 - scheduler completion
>
> L1 is pretty acceptable on Linux. For L3 we have the
> preemption and low latency patch. And for L4 the O(1)
> scheduler solves the problem. So I see L2 as the
> bootleneck especially with soft IRQ since the softIRQs
> get scheduled in a non real time thread and there is
> no wayI can tell the softIRQd that I want highest
> priority for the interrupt that will wake up my real
> time task. I was seeking a solution to this.

The fastest would not use softIRQ at all. As previously
taught, softIRQ is used for things that can be deferred.
You should handle everything in the primary ISR and
issue wake_up_interruptible() from the ISR. You should
be waking anybody sleeping in select() and anybody sleeping
in read().

>
> I know that TimeSYS has as patch for making the
> softIRQ a real time thread as well as giving
> priorities for both the top and bottom halves. Is
> there any place where I can get some performance
> figures for this patch?
>
> Regards
>

Search RTLinux.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.

Re: softirqd

[Peter Chubb]

Kernwek Jalsl said:

Kernwek> Sorry in case I was not very clear with my
Kernwek> requirements.   With real time interrupt I meant a
Kernwek> real time task waiting for IO from this interrupt.
Kernwek> Assume that I have a high priority interrupt and a
Kernwek> real time task waiting for it. Well followimg are the
Kernwek> various latencies involved:
Kernwek> L1- interrupt latency
Kernwek> L2- hard and soft IRQ completion
Kernwek> L3 - scheduler latency
Kernwek> L4 - scheduler completion

Kernwek> L1 is pretty acceptable on Linux. 

I've been trying to measure this.  On IA64 I'm measuring around
2.5microseconds (on a 900MHz machine).  I personally think that this
is too big, and could be reduced.

One thing I think we need to do early in 2.7 is to merge all those
architecture-dependent arch/XXX/kernel/irq.c files, and try to reduce
the amount of duplicated work done in the new merged file and the
lower level architecture-specific files.

--
Dr Peter Chubb  http://www.gelato.unsw.edu.au  peterc AT gelato.unsw.edu.au
The technical we do immediately,  the political takes *forever*

Re: softirqd

[Richard B. Johnson]

On Wed, 19 Nov 2003, Peter Chubb wrote:

>
>
>
> Kernwek Jalsl said:
>
> Kernwek> Sorry in case I was not very clear with my
> Kernwek> requirements.   With real time interrupt I meant a
> Kernwek> real time task waiting for IO from this interrupt.
> Kernwek> Assume that I have a high priority interrupt and a
> Kernwek> real time task waiting for it. Well followimg are the
> Kernwek> various latencies involved:
> Kernwek> L1- interrupt latency
> Kernwek> L2- hard and soft IRQ completion
> Kernwek> L3 - scheduler latency
> Kernwek> L4 - scheduler completion
>
> Kernwek> L1 is pretty acceptable on Linux.
>
> I've been trying to measure this.  On IA64 I'm measuring around
> 2.5microseconds (on a 900MHz machine).  I personally think that this
> is too big, and could be reduced.


I have a driver which, upon getting an IRQ7 (printer-port interrupt),
just reads the 'paper-out' bit and writes the result to the data-port.
This allows me to toggle the bit and measure the time using a 'scope
and a function generator. The complete time for everything is about
900 nanoseconds on this machine:

processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 5
model name	: Pentium II (Deschutes)
stepping	: 1
cpu MHz		: 399.568
cache size	: 512 KB
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 2
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 mmx fxsr
bogomips	: 797.90

processor	: 1
vendor_id	: GenuineIntel
cpu family	: 6
model		: 5
model name	: Pentium II (Deschutes)
stepping	: 1
cpu MHz		: 399.568
cache size	: 512 KB
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 2
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 mmx fxsr
bogomips	: 797.90


I have to disconnect the network before making the test because
the network driver will hog too much CPU time handling broadcast
messages to make any useful measurments.

Considering that it takes 150 to 200 ns to read/write the printer-
port, I don't think that's too bad.


[SNIPPED...]



Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.