Hi,

On 01/17/17 20:00, John Baldwin wrote:


Does this matter for the first tick? How often is configtimer() called?

As I said, it is called at runtime when profclock is started / stopped, not
just at boot.  Those changes at runtime probably have existing callouts
active and your change will not process any callouts until the next hardclock
tick fires (but only because you are setting nextcallopt to the bogus
'next' value).



 > (One odd thing is that even in your case the first call to
handleevents(),
 > the 'now => state->nextcallout' check in handleevents() should be true
 > which resets both nextcall and nextcallopt and invokes
callout_process().)

Let me take you through the failure path, by code inspection:

I would really appreciate it if you could add traces to find out what actually
happens rather than what seems to happen by looking at the code. :-/

The problem is that once you add some prints, the problem goes away. Maybe I should try to set hz to 100 or 25 ???


0) cpu_initclocks_bsp() is called and init's nextcall and nexcallopt to SBT_MAX
   similar to your change.  If no callout is scheduled before configtimer()
   then they remain set to SBT_MAX.  Your current patch happens to trigger a
   (bogus) call to callout_process() on the first hardclock() because it
   sets nextcallopt to 'next' even though no callout is actually scheduled to
   fire at time 'next'.

1) configtimer() is called and we init nextcall and nextcallopt:

 >                 next = now + timerperiod;
...
 >                       state->nextcall = next;
 >                       state->nextcallopt = next;

These both say "the next callout() should fire at 'next' which is the time of
the next hardclock()", even though there may be no callouts scheduled (in which
case both of these fields should be set to SBT_MAX from the call to
cpu_initclocks_bsp(), or there may be callouts scheduled in which case 
'nextcall'
and 'nextcallopt' will reflect the time that those callouts are already
scheduled for and this overwrites that).

I see there are some callouts scheduled by SYSINITs, before the first configtimer(), like NFS_TIMERINIT in nfs_init(). These are setup using "dummy_timecounter" which means any nextcall values before the first configtimer should be discarded.


2) Any callout_reset() calls cpu_new_callout():

 >          */
 >         state->nextcallopt = bt_opt;
 >         if (bt >= state->nextcall)
We follow this path, because "bt" is surely based on sbinuptime() and is
greater or equal to state->nextcall. Note that state->nextcallopt is
updated to bt_opt, which is in the future.

Note, my patch should _leave_ nextcall at SBT_MAX (from cpu_initclocks_bsp())
unless there was already an earlier call to callout_reset().

Yes, there are calls to callout_reset(). See for example NFS_TIMERINIT, like mentioned above.

> IOW, it
should be a NOP for the purposes of this branch compared with your change.
(You could add a warning to print out if 'nextcall' != SBT_MAX during boot
and see if it fires for example.)

 >                 goto done;
 >         state->nextcall = bt;

3) getnextcpuevent(0) is called by the fast timercb() to setup the next
event:

 >         state = DPCPU_PTR(timerstate);
 >         /* Handle hardclock() events, skipping some if CPU is idle. */
 >         event = state->nexthard;
...
 >         /* Handle callout events. */
 >         if (event > state->nextcall)
We then go looping into this path, because state->nextcall is still
equal to "next" as in step 1) which is now in the past, until "now >=
state->nextcallopt" inside handleevents(), which clears this condition.
 >                 event = state->nextcall;
...
 >         return (event);

I'm curious if there is a callout_reset() that has set 'nextcall' to a time
that is effectively before 'now'.

Maybe add a printf like this:

Index: kern_clocksource.c
===================================================================
--- kern_clocksource.c  (revision 312301)
+++ kern_clocksource.c  (working copy)
@@ -498,12 +498,18 @@ configtimer(int start)
                CPU_FOREACH(cpu) {
                        state = DPCPU_ID_PTR(cpu, timerstate);
                        state->now = now;
+                       printf("%s: CPU %d: now %jd nextcall %jd nextcallopt %jd next 
%jd\n", __func__, cpu, state->nextcall, state->nextcall, next);
 #ifndef EARLY_AP_STARTUP
                        if (!smp_started && cpu != CPU_FIRST())
                                state->nextevent = SBT_MAX;
                        else
 #endif

In particular what I am worried about with your patch is that for
post-boot calls to configtimer() you will delay any previously-scheduled
callouts until the next hardclock.


I understand. Would a solution be to refactor callout_process(), to accept the PCPU_GET(CPUID) as an argument and be executed for all CPUs by configtimer(), instead of trying to guess state->nextcall and state->nextcallopt in configtimer() ?

--HPS
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