intel_pstate: Fix intel_pstate_get()

After commit 73542d67c1c8 "intel_pstate: Remove freq calculation from
intel_pstate_calc_busy()" intel_pstate_get() calls get_avg_frequency()
to compute the average frequency, which is problematic for two reasons.

First, intel_pstate_get() may be invoked before the driver reads the
CPU feedback registers for the first time and if that happens,
get_avg_frequency() will attempt to divide by zero.

Second, the get_avg_frequency() call in intel_pstate_get() is racy
with respect to intel_pstate_sample() and it may end up returning
completely meaningless values for this reason.

Moreover, after commit df42e9279b3e "intel_pstate: Move
intel_pstate_calc_busy() into get_target_pstate_use_performance()"
sample.core_pct_busy is never computed on Atom, but it is used in
intel_pstate_adjust_busy_pstate() in that case too.

To address those problems notice that if sample.core_pct_busy
was used in the average frequency computation carried out by
get_avg_frequency(), both the divide by zero problem and the
race with respect to intel_pstate_sample() would be avoided.

Accordingly, move the invocation of intel_pstate_calc_busy() from
get_target_pstate_use_performance() to intel_pstate_update_util(),
which also will take care of the uninitialized sample.core_pct_busy
on Atom, and modify get_avg_frequency() to use sample.core_pct_busy
as per the above.

Reported-by: kernel test robot <ying.huang@linux.intel.com>
Link: http://marc.info/?l=linux-kernel&m=146226437623173&w=4
Fixes: 73542d67c1c8 "intel_pstate: Remove freq calculation from intel_pstate_calc_busy()"
Fixes: df42e9279b3e "intel_pstate: Move intel_pstate_calc_busy() into get_target_pstate_use_performance()"
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 66f7f0071cab97e64f014c9165c2c8f51308292d..b230ebaae66cb7ee0def1d228ff33c778343f344 100644 (file)
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -1070,8 +1070,9 @@ static inline bool intel_pstate_sample(struct cpudata *cpu, u64 time)
 
 static inline int32_t get_avg_frequency(struct cpudata *cpu)
 {
-       return div64_u64(cpu->pstate.max_pstate_physical * cpu->sample.aperf *
-               cpu->pstate.scaling, cpu->sample.mperf);
+       return fp_toint(mul_fp(cpu->sample.core_pct_busy,
+                              int_tofp(cpu->pstate.max_pstate_physical *
+                                               cpu->pstate.scaling / 100)));
 }
 
 static inline int32_t get_target_pstate_use_cpu_load(struct cpudata *cpu)
@@ -1114,8 +1115,6 @@ static inline int32_t get_target_pstate_use_performance(struct cpudata *cpu)
        int32_t core_busy, max_pstate, current_pstate, sample_ratio;
        u64 duration_ns;
 
-       intel_pstate_calc_busy(cpu);
-
        /*
         * core_busy is the ratio of actual performance to max
         * max_pstate is the max non turbo pstate available
@@ -1199,8 +1198,11 @@ static void intel_pstate_update_util(struct update_util_data *data, u64 time,
        if ((s64)delta_ns >= pid_params.sample_rate_ns) {
                bool sample_taken = intel_pstate_sample(cpu, time);
 
-               if (sample_taken && !hwp_active)
-                       intel_pstate_adjust_busy_pstate(cpu);
+               if (sample_taken) {
+                       intel_pstate_calc_busy(cpu);
+                       if (!hwp_active)
+                               intel_pstate_adjust_busy_pstate(cpu);
+               }
        }
 }