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This patch fixes 3 issues:
a) it removes the dependency on jiffies, because jiffies are incremented
by a single CPU, and the tick is not synchronized between CPUs. Therefore
relying on it to calculate a window to clip whacky TSC values doesn't work
as it can drift around.
So instead use [GTOD, GTOD+TICK_NSEC) as the window.
b) __update_sched_clock() did (roughly speaking):
delta = sched_clock() - scd->tick_raw;
clock += delta;
Which gives exponential growth, instead of linear.
c) allows the sched_clock_cpu() value to warp the u64 without breaking.
the results are more reliable sched_clock() deltas:
before after sched_clock
cpu_clock: 15750 51312 51488
cpu_clock: 59719 51052 50947
cpu_clock: 15879 51249 51061
cpu_clock: 1 50933 51198
cpu_clock: 1 50931 51039
cpu_clock: 1 51093 50981
cpu_clock: 1 51043 51040
cpu_clock: 1 50959 50938
cpu_clock: 1 50981 51011
cpu_clock: 1 51364 51212
cpu_clock: 1 51219 51273
cpu_clock: 1 51389 51048
cpu_clock: 1 51285 51611
cpu_clock: 1 50964 51137
cpu_clock: 1 50973 50968
cpu_clock: 1 50967 50972
cpu_clock: 1 58910 58485
cpu_clock: 1 51082 51025
cpu_clock: 1 50957 50958
cpu_clock: 1 50958 50957
cpu_clock: 1006128 51128 50971
cpu_clock: 1 51107 51155
cpu_clock: 1 51371 51081
cpu_clock: 1 51104 51365
cpu_clock: 1 51363 51309
cpu_clock: 1 51107 51160
cpu_clock: 1 51139 51100
cpu_clock: 1 51216 51136
cpu_clock: 1 51207 51215
cpu_clock: 1 51087 51263
cpu_clock: 1 51249 51177
cpu_clock: 1 51519 51412
cpu_clock: 1 51416 51255
cpu_clock: 1 51591 51594
cpu_clock: 1 50966 51374
cpu_clock: 1 50966 50966
cpu_clock: 1 51291 50948
cpu_clock: 1 50973 50867
cpu_clock: 1 50970 50970
cpu_clock: 998306 50970 50971
cpu_clock: 1 50971 50970
cpu_clock: 1 50970 50970
cpu_clock: 1 50971 50971
cpu_clock: 1 50970 50970
cpu_clock: 1 51351 50970
cpu_clock: 1 50970 51352
cpu_clock: 1 50971 50970
cpu_clock: 1 50970 50970
cpu_clock: 1 51321 50971
cpu_clock: 1 50974 51324
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Some arch's can't handle sched_clock() being called too early - delay
this until sched_clock_init() has been called.
Reported-by: Bill Gatliff <bgat@billgatliff.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: Nishanth Aravamudan <nacc@us.ibm.com>
CC: Russell King - ARM Linux <linux@arm.linux.org.uk>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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When taking the time of a remote CPU, use the opportunity to
couple (sync) the clocks to each other. (in a monotonic way)
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>
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- return the current clock instead of letting callers
fetch it from scd->clock
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>
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eliminate prev_raw and use tick_raw instead.
It's enough to base the current time on the scheduler tick timestamp
alone - the monotonicity and maximum checks will prevent any damage.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>
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- simplify the remote clock rebasing
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>
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Found an interactivity problem on a quad core test-system - simple
CPU loops would occasionally delay the system un an unacceptable way.
After much debugging with Peter Zijlstra it turned out that the problem
is caused by the string of sched_clock() changes - they caused the CPU
clock to jump backwards a bit - which confuses the scheduler arithmetics.
(which is unsigned for performance reasons)
So revert:
# c300ba2: sched_clock: and multiplier for TSC to gtod drift
# c0c8773: sched_clock: only update deltas with local reads.
# af52a90: sched_clock: stop maximum check on NO HZ
# f7cce27: sched_clock: widen the max and min time
This solves the interactivity problems.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>
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Move sched_clock() up to stop warning: weak declaration of `sched_clock'
after first use results in unspecified behavior (if -fno-unit-at-a-time).
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Mike Travis <travis@sgi.com>
Cc: Ben Herrenschmidt <benh@kernel.crashing.org>
Cc: Linuxppc-dev@ozlabs.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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The sched_clock code currently tries to keep all CPU clocks of all CPUS
somewhat in sync. At every clock tick it records the gtod clock and
uses that and jiffies and the TSC to calculate a CPU clock that tries to
stay in sync with all the other CPUs.
ftrace depends heavily on this timer and it detects when this timer
"jumps". One problem is that the TSC and the gtod also drift.
When the TSC is 0.1% faster or slower than the gtod it is very noticeable
in ftrace. To help compensate for this, I've added a multiplier that
tries to keep the CPU clock updating at the same rate as the gtod.
I've tried various ways to get it to be in sync and this ended up being
the most reliable. At every scheduler tick we calculate the new multiplier:
multi = delta_gtod / delta_TSC
This means we perform a 64 bit divide at the tick (once a HZ). A shift
is used to handle the accuracy.
Other methods that failed due to dynamic HZ are:
(not used) multi += (gtod - tsc) / delta_gtod
(not used) multi += (gtod - (last_tsc + delta_tsc)) / delta_gtod
as well as other variants.
This code still allows for a slight drift between TSC and gtod, but
it keeps the damage down to a minimum.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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To read the gtod we need to grab the xtime lock for read. Reading the gtod
before the TSC can cause a bigger gab if the xtime lock is contended.
This patch simply reverses the order to read the TSC after the gtod.
The locking in the reading of the gtod handles any barriers one might
think is needed.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Reading the CPU clock should try to stay accurate within the CPU.
By reading the CPU clock from another CPU and updating the deltas can
cause unneeded jumps when reading from the local CPU.
This patch changes the code to update the last read TSC only when read
from the local CPU.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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The algorithm to calculate the 'now' of another CPU is not correct.
At each scheduler tick, each CPU records the last sched_clock and
gtod (tick_raw and tick_gtod respectively). If the TSC is somewhat the
same in speed between two clocks the algorithm would be:
tick_gtod1 + (now1 - tick_raw1) = tick_gtod2 + (now2 - tick_raw2)
To calculate now2 we would have:
now2 = (tick_gtod1 - tick_gtod2) + (tick_raw2 - tick_raw1) + now1
Currently the algorithm is:
now2 = (tick_gtod1 - tick_gtod2) + (tick_raw1 - tick_raw2) + now1
This solves most of the rest of the issues I've had with timestamps in
ftace.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Working with ftrace I would get large jumps of 11 millisecs or more with
the clock tracer. This killed the latencing timings of ftrace and also
caused the irqoff self tests to fail.
What was happening is with NO_HZ the idle would stop the jiffy counter and
before the jiffy counter was updated the sched_clock would have a bad
delta jiffies to compare with the gtod with the maximum.
The jiffies would stop and the last sched_tick would record the last gtod.
On wakeup, the sched clock update would compare the gtod + delta jiffies
(which would be zero) and compare it to the TSC. The TSC would have
correctly (with a stable TSC) moved forward several jiffies. But because the
jiffies has not been updated yet the clock would be prevented from moving
forward because it would appear that the TSC jumped too far ahead.
The clock would then virtually stop, until the jiffies are updated. Then
the next sched clock update would see that the clock was very much behind
since the delta jiffies is now correct. This would then jump the clock
forward by several jiffies.
This caused ftrace to report several milliseconds of interrupts off
latency at every resume from NO_HZ idle.
This patch adds hooks into the nohz code to disable the checking of the
maximum clock update when nohz is in effect. It resumes the max check
when nohz has updated the jiffies again.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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With keeping the max and min sched time within one jiffy of the gtod clock
was too tight. Just before a schedule tick the max could easily be hit, as
well as just after a schedule_tick the min could be hit. This caused the
clock to jump around by a jiffy.
This patch widens the minimum to
last gtod + (delta_jiffies ? delta_jiffies - 1 : 0) * TICK_NSECS
and the maximum to
last gtod + (2 + delta_jiffies) * TICK_NSECS
This keeps the minum to gtod or if one jiffy less than delta jiffies
and the maxim 2 jiffies ahead of gtod. This may cause unstable TSCs to be
a bit more sporadic, but it helps keep a clock with a stable TSC working well.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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The sched_clock code tries to keep within the gtod time by one tick (jiffy).
The current code mistakenly keeps track of the delta jiffies between
updates of the clock, where the the delta is used to compare with the
number of jiffies that have past since an update of the gtod. The gtod is
updated at each schedule tick not each sched_clock update. After one
jiffy passes the clock is updated fine. But the delta is taken from the
last update so if the next update happens before the next tick the delta
jiffies used will be incorrect.
This patch changes the code to check the delta of jiffies between ticks
and not updates to match the comparison of the updates with the gtod.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Vegard Nossum reported:
> WARNING: at kernel/lockdep.c:2738 check_flags+0x142/0x160()
which happens due to:
unsigned long long cpu_clock(int cpu)
{
unsigned long long clock;
unsigned long flags;
raw_local_irq_save(flags);
as lower level functions can take locks, we must not do that, use
proper lockdep-annotated irq save/restore.
Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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the rcutorture module relies on cpu_clock.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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with sched_clock_cpu() being reasonably in sync between cpus (max 1 jiffy
difference) use this to provide cpu_clock().
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Make sched_clock_cpu() return 0 before it has been initialized and avoid
corrupting its state due to doing so.
This fixes the weird printk timestamp jump reported.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
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this replaces the rq->clock stuff (and possibly cpu_clock()).
- architectures that have an 'imperfect' hardware clock can set
CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
- the 'jiffie' window might be superfulous when we update tick_gtod
before the __update_sched_clock() call in sched_clock_tick()
- cpu_clock() might be implemented as:
sched_clock_cpu(smp_processor_id())
if the accuracy proves good enough - how far can TSC drift in a
single jiffie when considering the filtering and idle hooks?
[ mingo@elte.hu: various fixes and cleanups ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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