Re: [PATCH 4/8] cputime: Generic on-demand virtual cputimeaccounting

[Stephen Rothwell]

Hi Frederic,

On Mon, 28 Jan 2013 20:04:01 +0100 Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
>
> If we want to stop the tick further idle, we need to be
> able to account the cputime without using the tick.
> 
> Virtual based cputime accounting solves that problem by
> hooking into kernel/user boundaries.
> 
> However implementing CONFIG_VIRT_CPU_ACCOUNTING require
> low level hooks and involves more overhead. But we already
> have a generic context tracking subsystem that is required
> for RCU needs by archs which plan to shut down the tick
> outside idle.
> 
> This patch implements a generic virtual based cputime
> accounting that relies on these generic kernel/user hooks.
> 
> There are some upsides of doing this:
> 
> - This requires no arch code to implement CONFIG_VIRT_CPU_ACCOUNTING
> if context tracking is already built (already necessary for RCU in full
> tickless mode).
> 
> - We can rely on the generic context tracking subsystem to dynamically
> (de)activate the hooks, so that we can switch anytime between virtual
> and tick based accounting. This way we don't have the overhead
> of the virtual accounting when the tick is running periodically.
> 
> And one downside:
> 
> - There is probably more overhead than a native virtual based cputime
> accounting. But this relies on hooks that are already set anyway.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Li Zhong <zhong@xxxxxxxxxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung.kim@xxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

This patch has the side effect of changing the default configurations:
(This is PowerPC pseries_defconfig before/after this patch)

@@ -119,8 +120,8 @@
 #
 # CPU/Task time and stats accounting
 #
-# CONFIG_TICK_CPU_ACCOUNTING is not set
-CONFIG_VIRT_CPU_ACCOUNTING=y
+CONFIG_TICK_CPU_ACCOUNTING=y
+# CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is not set
 # CONFIG_BSD_PROCESS_ACCT is not set
 CONFIG_TASKSTATS=y
 CONFIG_TASK_DELAY_ACCT=y

I don't know if that was deliberate, but it was suprising.  I noticed
when this patch entered next-20130207.

-- 
Cheers,
Stephen Rothwell                    sfr@xxxxxxxxxxxxxxxx

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