table of contents
CPU_LOAD_UPDATE(9) | Driver Basics | CPU_LOAD_UPDATE(9) |
NAME¶
cpu_load_update - update the rq->cpu_load[] statistics
SYNOPSIS¶
void cpu_load_update(struct rq * this_rq, unsigned long this_load, unsigned long pending_updates);
ARGUMENTS¶
this_rq
this_load
pending_updates
DESCRIPTION¶
Update rq->cpu_load[] statistics. This function is usually called every scheduler tick (TICK_NSEC).
This function computes a decaying average:
load[i]' = (1 - 1/2^i) * load[i] + (1/2^i) * load
Because of NOHZ it might not get called on every tick which gives need for the pending_updates argument.
load[i]_n = (1 - 1/2^i) * load[i]_n-1 + (1/2^i) * load_n-1 = A * load[i]_n-1 + B ; A := (1 - 1/2^i), B := (1/2^i) * load = A * (A * load[i]_n-2 + B) + B = A * (A * (A * load[i]_n-3 + B) + B) + B = A^3 * load[i]_n-3 + (A^2 + A + 1) * B = A^n * load[i]_0 + (A^(n-1) + A^(n-2) + ... + 1) * B = A^n * load[i]_0 + ((1 - A^n) / (1 - A)) * B = (1 - 1/2^i)^n * (load[i]_0 - load) + load
In the above we've assumed load_n := load, which is true for NOHZ_FULL as any change in load would have resulted in the tick being turned back on.
For regular NOHZ, this reduces to:
load[i]_n = (1 - 1/2^i)^n * load[i]_0
see decay_load_misses. For NOHZ_FULL we get to subtract and add the extra term.
COPYRIGHT¶
January 2017 | Kernel Hackers Manual 4.8. |