[PATCH 3.16 257/306] tile: avoid using clocksource_cyc2ns with absolute cycle count

[Ben Hutchings]

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>

commit e658a6f14d7c0243205f035979d0ecf6c12a036f upstream.

For large values of "mult" and long uptimes, the intermediate
result of "cycles * mult" can overflow 64 bits.  For example,
the tile platform calls clocksource_cyc2ns with a 1.2 GHz clock;
we have mult = 853, and after 208.5 days, we overflow 64 bits.

Since clocksource_cyc2ns() is intended to be used for relative
cycle counts, not absolute cycle counts, performance is more
importance than accepting a wider range of cycle values.  So,
just use mult_frac() directly in tile's sched_clock().

Commit 4cecf6d401a0 ("sched, x86: Avoid unnecessary overflow
in sched_clock") by Salman Qazi results in essentially the same
generated code for x86 as this change does for tile.  In fact,
a follow-on change by Salman introduced mult_frac() and switched
to using it, so the C code was largely identical at that point too.

Peter Zijlstra then added mul_u64_u32_shr() and switched x86
to use it.  This is, in principle, better; by optimizing the
64x64->64 multiplies to be 32x32->64 multiplies we can potentially
save some time.  However, the compiler piplines the 64x64->64
multiplies pretty well, and the conditional branch in the generic
mul_u64_u32_shr() causes some bubbles in execution, with the
result that it's pretty much a wash.  If tilegx provided its own
implementation of mul_u64_u32_shr() without the conditional branch,
we could potentially save 3 cycles, but that seems like small gain
for a fair amount of additional build scaffolding; no other platform
currently provides a mul_u64_u32_shr() override, and tile doesn't
currently have an <asm/div64.h> header to put the override in.

Additionally, gcc currently has an optimization bug that prevents
it from recognizing the opportunity to use a 32x32->64 multiply,
and so the result would be no better than the existing mult_frac()
until such time as the compiler is fixed.

For now, just using mult_frac() seems like the right answer.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/tile/kernel/time.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/tile/kernel/time.c
+++ b/arch/tile/kernel/time.c
@@ -216,8 +216,8 @@ void do_timer_interrupt(struct pt_regs *
  */
 unsigned long long sched_clock(void)
 {
-	return clocksource_cyc2ns(get_cycles(),
-				  sched_clock_mult, SCHED_CLOCK_SHIFT);
+	return mult_frac(get_cycles(),
+			 sched_clock_mult, 1ULL << SCHED_CLOCK_SHIFT);
 }
 
 int setup_profiling_timer(unsigned int multiplier)