configtimer().
During normal operation "state->nextcallopt" will always be less than
or equal to "state->nextcall" and checking only "state->nextcallopt"
before calling "callout_process()" is sufficient. However when
"configtimer()" is called a race might happen requiring both of these
binary times to be checked.
Short description of race:
1) A configtimer() call will reset both "state->nextcall" and
"state->nextcallopt" to the same binary time.
2) If a "callout_reset()" call happens between "configtimer()" and the
next "callout_process()" call, "state->nextcallopt" will get updated
and "state->nextcall" will remain at the current time. Refer to logic
inside cpu_new_callout().
3) getnextcpuevent() only respects "state->nextcall" and returns this
value over and over again, even if it is in the past, until "now >=
state->nextcallopt" becomes true. Then these two time variables are
corrected by a "callout_process()" call and the situation goes back to
normal.
The problem manifests itself in different ways. The common factor is
the timer process(es) consume all CPU on one or more CPU cores for a
long time, blocking other kernel processes from getting execution
time. This can be seen by very high interrupt counts as displayed by
"vmstat -i | grep timer" right after boot.
When EARLY_AP_STARTUP was enabled in r310177 the likelyhood of hitting
this bug apparently increased.
Example output from "vmstat -i" before patch:
cpu0:timer 7591 69
cpu9:timer 39031773 358089
cpu4:timer 9359 85
cpu3:timer 9100 83
cpu2:timer 9620 88
Example output from "vmstat -i" after patch:
cpu0:timer 4242 34
cpu6:timer 5531 44
cpu3:timer 6450 52
cpu1:timer 4545 36
cpu9:timer 7153 58
Before the patch cpu9 in the example above, was spinning in a loop in
order to reach 39 million interrupts just a few seconds after
bootup. After the patch the timer interrupt counts are more or less
consistent.
Discussed with: mav @
Reported by: several people
MFC after: 1 week
Sponsored by: Mellanox Technologies
10c8755706