Re: [PATCH] powerpc/64: Use tick accounting by default

[Christophe Leroy]

Le 22/05/2017 à 07:13, Anton Blanchard a écrit :

Hi Michael,


ppc64 is the only architecture that turns on
VIRT_CPU_ACCOUNTING_NATIVE by default. The overhead of this option
is extremely high - a context switch microbenchmark using
sched_yield() is almost 20% slower.


Running on what? It should all be nop'ed out unless you're on a
platform that needs it (SPLPAR).


POWERNV native. We don't nop out all the vtime_account_* gunk do we? It
is all those functions that are a large part of the problem.


To get finer grained user/hardirq/softirq statitics, the
IRQ_TIME_ACCOUNTING option can be used instead, which has much lower
overhead.


Can it? We don't select HAVE_IRQ_TIME_ACCOUNTING, so AFAICS it can't
be enabled.


I have a separate patch to enable it.


Doesn't dropping this mean we never count stolen time?


Perhaps. Do we have any applications left that care?



This patch has been superseded by Nick's patch 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220525081346.871535-1-npig...@gmail.com/


Christophe

Re: [PATCH] powerpc/64: Use tick accounting by default

[Christophe LEROY]

Le 19/05/2017 à 16:41, Anton Blanchard a écrit :

From: Anton Blanchard 

ppc64 is the only architecture that turns on VIRT_CPU_ACCOUNTING_NATIVE
by default. The overhead of this option is extremely high - a context
switch microbenchmark using sched_yield() is almost 20% slower.

To get finer grained user/hardirq/softirq statitics, the
IRQ_TIME_ACCOUNTING option can be used instead, which has much lower
overhead.


Does IRQ_TIME_ACCOUNTING allow the same granularity as 
VIRT_CPU_ACCOUNTING_NATIVE ?
I've quickly looked into the source, it seems that this option still 
accounts based on time ticks.
On a hardware doing mainly VoIP handling, the processes run a few ms 
every 20 ms. With TICK_CPU_ACCOUNTING and a 10ms tick, the processes 
either appear with 50% CPU or almost 0%.

Would it be more precise with IRQ_TIME_ACCOUNTING ?

Christophe



As such, disable this option by default. If a user really wants it,
they can still enable it manually.

Signed-off-by: Anton Blanchard 
---
 init/Kconfig | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 1d3475fc9496..a5c30acc1ede 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -342,8 +342,7 @@ config VIRT_CPU_ACCOUNTING

 choice
prompt "Cputime accounting"
-   default TICK_CPU_ACCOUNTING if !PPC64
-   default VIRT_CPU_ACCOUNTING_NATIVE if PPC64
+   default TICK_CPU_ACCOUNTING

 # Kind of a stub config for the pure tick based cputime accounting
 config TICK_CPU_ACCOUNTING

Re: [PATCH] powerpc/64: Use tick accounting by default

[Anton Blanchard]

Hi Michael,

> > ppc64 is the only architecture that turns on
> > VIRT_CPU_ACCOUNTING_NATIVE by default. The overhead of this option
> > is extremely high - a context switch microbenchmark using
> > sched_yield() is almost 20% slower.  
> 
> Running on what? It should all be nop'ed out unless you're on a
> platform that needs it (SPLPAR).

POWERNV native. We don't nop out all the vtime_account_* gunk do we? It
is all those functions that are a large part of the problem.

> > To get finer grained user/hardirq/softirq statitics, the
> > IRQ_TIME_ACCOUNTING option can be used instead, which has much lower
> > overhead.  
> 
> Can it? We don't select HAVE_IRQ_TIME_ACCOUNTING, so AFAICS it can't
> be enabled.

I have a separate patch to enable it.

> Doesn't dropping this mean we never count stolen time?

Perhaps. Do we have any applications left that care?

Anton

Re: [PATCH] powerpc/64: Use tick accounting by default

[Nicholas Piggin]

On Mon, 22 May 2017 14:15:57 +1000
Michael Ellerman  wrote:

> Anton Blanchard  writes:
> 
> > From: Anton Blanchard 
> >
> > ppc64 is the only architecture that turns on VIRT_CPU_ACCOUNTING_NATIVE
> > by default. The overhead of this option is extremely high - a context
> > switch microbenchmark using sched_yield() is almost 20% slower.  
> 
> Running on what? It should all be nop'ed out unless you're on a platform
> that needs it (SPLPAR).

Not ACCOUNT_CPU_USER_ENTRY/EXIT, which adds a fair cost to kernel
entry/exit (my notes say ~70 cycles on getppid() out of 3-4 hundred).

> 
> > To get finer grained user/hardirq/softirq statitics, the
> > IRQ_TIME_ACCOUNTING option can be used instead, which has much lower
> > overhead.  
> 
> Can it? We don't select HAVE_IRQ_TIME_ACCOUNTING, so AFAICS it can't be
> enabled.
> 
> Doesn't dropping this mean we never count stolen time?
> 
> cheers

Re: [PATCH] powerpc/64: Use tick accounting by default

[Michael Ellerman]

Anton Blanchard  writes:

> From: Anton Blanchard 
>
> ppc64 is the only architecture that turns on VIRT_CPU_ACCOUNTING_NATIVE
> by default. The overhead of this option is extremely high - a context
> switch microbenchmark using sched_yield() is almost 20% slower.

Running on what? It should all be nop'ed out unless you're on a platform
that needs it (SPLPAR).

> To get finer grained user/hardirq/softirq statitics, the
> IRQ_TIME_ACCOUNTING option can be used instead, which has much lower
> overhead.

Can it? We don't select HAVE_IRQ_TIME_ACCOUNTING, so AFAICS it can't be
enabled.

Doesn't dropping this mean we never count stolen time?

cheers

[PATCH] powerpc/64: Use tick accounting by default

[Anton Blanchard]

From: Anton Blanchard 

ppc64 is the only architecture that turns on VIRT_CPU_ACCOUNTING_NATIVE
by default. The overhead of this option is extremely high - a context
switch microbenchmark using sched_yield() is almost 20% slower.

To get finer grained user/hardirq/softirq statitics, the
IRQ_TIME_ACCOUNTING option can be used instead, which has much lower
overhead.

As such, disable this option by default. If a user really wants it,
they can still enable it manually.

Signed-off-by: Anton Blanchard 
---
 init/Kconfig | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 1d3475fc9496..a5c30acc1ede 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -342,8 +342,7 @@ config VIRT_CPU_ACCOUNTING
 
 choice
prompt "Cputime accounting"
-   default TICK_CPU_ACCOUNTING if !PPC64
-   default VIRT_CPU_ACCOUNTING_NATIVE if PPC64
+   default TICK_CPU_ACCOUNTING
 
 # Kind of a stub config for the pure tick based cputime accounting
 config TICK_CPU_ACCOUNTING
-- 
2.11.0