Whenever we change the frequency of a CPU, we call the PRECHANGE and POSTCHANGE notifiers. They must be serialized, i.e. PRECHANGE and POSTCHANGE notifiers should strictly alternate, thereby preventing two different sets of PRECHANGE or POSTCHANGE notifiers from interleaving arbitrarily.
The following examples illustrate why this is important:
Scenario 1: ----------- A thread reading the value of cpuinfo_cur_freq, will call __cpufreq_cpu_get()->cpufreq_out_of_sync()->cpufreq_notify_transition()
The ondemand governor can decide to change the frequency of the CPU at the same time and hence it can end up sending the notifications via ->target().
If the notifiers are not serialized, the following sequence can occur: - PRECHANGE Notification for freq A (from cpuinfo_cur_freq) - PRECHANGE Notification for freq B (from target()) - Freq changed by target() to B - POSTCHANGE Notification for freq B - POSTCHANGE Notification for freq A
We can see from the above that the last POSTCHANGE Notification happens for freq A but the hardware is set to run at freq B.
Where would we break then?: adjust_jiffies() in cpufreq.c & cpufreq_callback() in arch/arm/kernel/smp.c (which also adjusts the jiffies). All the loops_per_jiffy calculations will get messed up.
Scenario 2: ----------- The governor calls __cpufreq_driver_target() to change the frequency. At the same time, if we change scaling_{min|max}_freq from sysfs, it will end up calling the governor's CPUFREQ_GOV_LIMITS notification, which will also call __cpufreq_driver_target(). And hence we end up issuing concurrent calls to ->target().
Typically, platforms have the following logic in their ->target() routines: (Eg: cpufreq-cpu0, omap, exynos, etc)
A. If new freq is more than old: Increase voltage B. Change freq C. If new freq is less than old: decrease voltage
Now, if the two concurrent calls to ->target() are X and Y, where X is trying to increase the freq and Y is trying to decrease it, we get the following race condition:
X.A: voltage gets increased for larger freq Y.A: nothing happens Y.B: freq gets decreased Y.C: voltage gets decreased X.B: freq gets increased X.C: nothing happens
Thus we can end up setting a freq which is not supported by the voltage we have set. That will probably make the clock to the CPU unstable and the system might not work properly anymore.
This patchset introduces a new set of routines cpufreq_freq_transition_begin() and cpufreq_freq_transition_end(), which will guarantee that calls to frequency transition routines are serialized. Later patches force other drivers to use these new routines.
V4: https://lkml.org/lkml/2014/3/21/23
V4->V5: - Replaced false with 0 as the variable was of int type instead of bool. - There were some discussions about requirement of a barrier, but it looks like overkill for now. So, leaving that unless we have a real problem.
Srivatsa S. Bhat (1): cpufreq: Make sure frequency transitions are serialized
Viresh Kumar (2): cpufreq: Convert existing drivers to use cpufreq_freq_transition_{begin|end} cpufreq: Make cpufreq_notify_transition & cpufreq_notify_post_transition static
drivers/cpufreq/cpufreq-nforce2.c | 4 +-- drivers/cpufreq/cpufreq.c | 52 +++++++++++++++++++++++++++++------- drivers/cpufreq/exynos5440-cpufreq.c | 4 +-- drivers/cpufreq/gx-suspmod.c | 4 +-- drivers/cpufreq/integrator-cpufreq.c | 4 +-- drivers/cpufreq/longhaul.c | 4 +-- drivers/cpufreq/pcc-cpufreq.c | 4 +-- drivers/cpufreq/powernow-k6.c | 4 +-- drivers/cpufreq/powernow-k7.c | 4 +-- drivers/cpufreq/powernow-k8.c | 4 +-- drivers/cpufreq/s3c24xx-cpufreq.c | 4 +-- drivers/cpufreq/sh-cpufreq.c | 4 +-- drivers/cpufreq/unicore2-cpufreq.c | 4 +-- include/linux/cpufreq.h | 12 ++++++--- 14 files changed, 76 insertions(+), 36 deletions(-)