Re: posix-cpu-timers revamp

Frank Mayhar
2008-03-31T20:24:58Z

Roland, I'm very much having to read between the lines of what you've
written. And, obviously, getting it wrong at least half the time. :-)

So you've cleared part of my understanding with your latest email.
Here's what I've gotten from it:

struct task_cputime {
cputime_t utime; /* User time. */
cputime_t stime; /* System time. */
unsigned long long sched_runtime; /* Scheduler time. */
};

This is for both SMP and UP, defined before signal_struct in sched.h
(since that structure refers to this one). Following that:

struct thread_group_cputime;

Which is a forward reference to the real definition later in the file.
The inline functions depend on signal_struct and task_struct, so they
have to come after:

#ifdef SMP

struct thread_group_cputime {
struct task_cputime *totals;
};

< ... inline functions ... >

#else /* SMP */

struct thread_group_cputime {
struct task_cputime totals;
};

< ... inline functions ... >

#endif

The SMP version is percpu, the UP version is just a substructure. In
signal_struct itself, delete utime & stime, add
struct thread_group_cputime cputime;

The inline functions include the ones you defined for UP plus equivalent
ones for SMP. The SMP inlines check the percpu pointer
(sig->cputime.totals) and don't update if it's NULL. One small
correction to one of your inlines, in thread_group_cputime:
*cputime = sig->cputime;
should be
*cputime = sig->cputime.totals;

A representative inline for SMP is:

static inline void account_group_system_time(struct task_struct *task,
cputime_t cputime)
{
struct task_cputime *times;

if (!sig->cputime.totals)
return;
times = per_cpu_ptr(sig->cputime.totals, get_cpu());
times->stime = cputime_add(times->stime, cputime);
put_cpu_no_resched();
}

To deal with the need for bookkeeping with multiple threads in the SMP
case (where there isn't a per-cpu structure until it's needed), I'll
allocate the per-cpu structure in __exit_signal() where the relevant
fields are updated. I'll also allocate it where I do now, in
do_setitimer(), when needed. The allocation will be a "return 0" for UP
and a call to "thread_group_times_alloc_smp()" (which lives in sched.c)
for SMP.

I'll also optimize run_posix_cpu_timers() as you suggest, and eliminate
rlim_expires.

Expect a new patch fairly soon.