This is a note to let you know that I've just added the patch titled
tile: avoid using clocksource_cyc2ns with absolute cycle count
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tile-avoid-using-clocksource_cyc2ns-with-absolute-cycle-count.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From e658a6f14d7c0243205f035979d0ecf6c12a036f Mon Sep 17 00:00:00 2001
From: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
Date: Wed, 16 Nov 2016 11:18:05 -0500
Subject: tile: avoid using clocksource_cyc2ns with absolute cycle count
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: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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
@@ -218,8 +218,8 @@ 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)
Patches currently in stable-queue which might be from cmetcalf@xxxxxxxxxxxx are
queue-4.4/tile-avoid-using-clocksource_cyc2ns-with-absolute-cycle-count.patch
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