linux/arch/tile
Chris Metcalf e658a6f14d 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 4cecf6d401 ("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>
2016-11-23 15:28:54 -05:00
..
configs Merge branch 'akpm' (patches from Andrew) 2016-05-23 19:42:28 -07:00
gxio Fix typo 2016-05-23 12:17:22 -04:00
include tile: handle __ro_after_init like parisc does 2016-11-14 16:46:41 -05:00
kernel tile: avoid using clocksource_cyc2ns with absolute cycle count 2016-11-23 15:28:54 -05:00
kvm rcu: Make SRCU optional by using CONFIG_SRCU 2015-01-06 11:04:29 -08:00
lib Merge git://git.kernel.org/pub/scm/linux/kernel/git/cmetcalf/linux-tile 2016-07-27 10:54:11 -07:00
mm tile: use simpler API for random address requests 2016-10-11 15:06:32 -07:00
Kbuild
Kconfig atomic64: no need for CONFIG_ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE 2016-10-07 18:46:30 -07:00
Kconfig.debug tile: remove DEBUG_EXTRA_FLAGS kernel config option 2013-09-03 14:52:17 -04:00
Makefile tile: remove DEBUG_EXTRA_FLAGS kernel config option 2013-09-03 14:52:17 -04:00