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76031d9536
Guenter reported boot stalls on a emulated ARM 32-bit platform, which has a
24-bit wide clocksource.
It turns out that the calculated maximal idle time, which limits idle
sleeps to prevent clocksource wrap arounds, is close to the point where the
negative motion detection triggers.
max_idle_ns: 597268854 ns
negative motion tripping point: 671088640 ns
If the idle wakeup is delayed beyond that point, the clocksource
advances far enough to trigger the negative motion detection. This
prevents the clock to advance and in the worst case the system stalls
completely if the consecutive sleeps based on the stale clock are
delayed as well.
Cure this by calculating a more robust cut-off value for negative motion,
which covers 87.5% of the actual clocksource counter width. Compare the
delta against this value to catch negative motion. This is specifically for
clock sources with a small counter width as their wrap around time is close
to the half counter width. For clock sources with wide counters this is not
a problem because the maximum idle time is far from the half counter width
due to the math overflow protection constraints.
For the case at hand this results in a tripping point of 1174405120ns.
Note, that this cannot prevent issues when the delay exceeds the 87.5%
margin, but that's not different from the previous unchecked version which
allowed arbitrary time jumps.
Systems with small counter width are prone to invalid results, but this
problem is unlikely to be seen on real hardware. If such a system
completely stalls for more than half a second, then there are other more
urgent problems than the counter wrapping around.
Fixes: c163e40af9 ("timekeeping: Always check for negative motion")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/all/8734j5ul4x.ffs@tglx
Closes: https://lore.kernel.org/all/387b120b-d68a-45e8-b6ab-768cd95d11c2@roeck-us.net
49 lines
1.0 KiB
C
49 lines
1.0 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _TIMEKEEPING_INTERNAL_H
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#define _TIMEKEEPING_INTERNAL_H
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#include <linux/clocksource.h>
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#include <linux/spinlock.h>
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#include <linux/time.h>
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/*
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* timekeeping debug functions
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*/
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#ifdef CONFIG_DEBUG_FS
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DECLARE_PER_CPU(unsigned long, timekeeping_mg_floor_swaps);
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static inline void timekeeping_inc_mg_floor_swaps(void)
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{
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this_cpu_inc(timekeeping_mg_floor_swaps);
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}
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extern void tk_debug_account_sleep_time(const struct timespec64 *t);
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#else
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#define tk_debug_account_sleep_time(x)
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static inline void timekeeping_inc_mg_floor_swaps(void)
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{
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}
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#endif
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static inline u64 clocksource_delta(u64 now, u64 last, u64 mask, u64 max_delta)
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{
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u64 ret = (now - last) & mask;
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/*
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* Prevent time going backwards by checking the result against
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* @max_delta. If greater, return 0.
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*/
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return ret > max_delta ? 0 : ret;
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}
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/* Semi public for serialization of non timekeeper VDSO updates. */
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unsigned long timekeeper_lock_irqsave(void);
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void timekeeper_unlock_irqrestore(unsigned long flags);
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#endif /* _TIMEKEEPING_INTERNAL_H */
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