cputime: Generic on-demand virtual cputime accounting

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@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/asm-generic/cputime_nsecs.h b/include/asm-generic/cputime_nsecs.h
index c73d182f4751..b6485cafb7bd 100644
--- a/include/asm-generic/cputime_nsecs.h
+++ b/include/asm-generic/cputime_nsecs.h
@@ -26,6 +26,7 @@ typedef u64 __nocast cputime64_t;
  */
 #define cputime_to_jiffies(__ct)        \
         ((__force u64)(__ct) / (NSEC_PER_SEC / HZ))
+#define cputime_to_scaled(__ct)         (__ct)
 #define jiffies_to_cputime(__jif)       \
         (__force cputime_t)((__jif) * (NSEC_PER_SEC / HZ))
 #define cputime64_to_jiffies64(__ct)    \
@@ -33,6 +34,13 @@ typedef u64 __nocast cputime64_t;
 #define jiffies64_to_cputime64(__jif)   \
         (__force cputime64_t)((__jif) * (NSEC_PER_SEC / HZ))
 
+
+/*
+ * Convert cputime <-> nanoseconds
+ */
+#define nsecs_to_cputime(__nsecs)       ((__force u64)(__nsecs))
+
+
 /*
  * Convert cputime <-> microseconds
  */