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authorChris Metcalf <cmetcalf@mellanox.com>2016-11-16 11:18:05 -0500
committerChris Metcalf <cmetcalf@mellanox.com>2016-11-23 15:28:54 -0500
commite658a6f14d7c0243205f035979d0ecf6c12a036f (patch)
tree6531618af4a41adfc421fed8131d8263e59f1b8d /arch
parentded9b5dd205ef04aa095c3b731c635b201191a59 (diff)
tile: avoid using clocksource_cyc2ns with absolute cycle count
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. Cc: stable@kernel.org [v3.4+] Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Diffstat (limited to 'arch')
-rw-r--r--arch/tile/kernel/time.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c
index 178989e6d3e3..ea960d660917 100644
--- a/arch/tile/kernel/time.c
+++ b/arch/tile/kernel/time.c
@@ -218,8 +218,8 @@ void do_timer_interrupt(struct pt_regs *regs, int fault_num)
*/
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)