On 05/05/15 08:00, Thomas Meyer wrote:
>
>> Am 04.05.2015 um 18:22 schrieb Anton Ivanov <anton.iva...@kot-begemot.co.uk>:
>>
>> Hi Thomas,
> Hi Anton,
>
>> I read it and like it except idle sleep. See my comments inline. I
>> killed that nanosleep quite on purpose :)
> Okay. I think I understand now.
>
>> With what you did to the tramp the issue of userspace stalling
>> should go away by the way (in theory). I think that it will still show a
>> very HZ behavior, but IMHO that cannot be helped at this stage.
> Do you mean the user space processes with above statement? What else could be
> done with them if not trigger at HZ?
I do not think that you can do anything because you cannot keep the
userspace timers sync-ed to the kernel space without risking races.
Frankly, even if it just does HZ will be better than now.
>
>> A.
>>
>>
>>> On 03/05/15 16:46, Thomas Meyer wrote:
>>> Am Samstag, den 02.05.2015, 12:08 +0100 schrieb Anton Ivanov:
>>> * This is a per-cpu array. A processor only modifies its entry and it
>>> only
>>> @@ -204,8 +205,16 @@ void arch_cpu_idle(void)
>>> unsigned long long nsecs;
>>>
>>> cpu_tasks[current_thread_info()->cpu].pid = os_getpid();
>>> - nsecs = disable_timer();
>>> - idle_sleep(nsecs);
>>> +
>>> + //WHAT?
>> The way idle sleep was originally implemented is by setting a nanosleep
>> for nsecs until the next expected timer firing. After that the sleep
>> routine used to generated a timer interrupt.
>>
>> It was killing two ducks with one shot - the original timer is based on
>> VTALRM and virtual clock. So if you are idle enough to go into sleep you
>> cannot rely on the signal from itimer (as this is dependent on your CPU
>> usage) to wake you up on time. In fact, you are never going to wake up.
>>
>> Setting a nano-sleep dealt with both problems - idle sleeping and waking
>> up on time as expressed in absolute wall clock.
>>
>> If your wall clock is no longer running off VIRTUAL and is now off
>> MONOTONIC you do not need to do this. You just go to sleep without
>> disabling the next timer interrupt, the next timer interrupt happens on
>> schedule (as it is now off the right clock), you wake up from sleep. Two
>> syscalls less, same effect.
> Okay, I'll change that. In your original patch you wait maximum for one HZ
> interval, why not wait forever os_idle_sleep()?
I did not have a periodic timer. If you have a periodic timer in
addition to the one-shot (as you have done) you can wait forever - the
periodic will wake you up.
>>
>>
>>> + /* there is no benefit whatsoever in disabling a pending
>>> + * hrtimer and setting a nanowait for the same value instead
>>> + * so we do timer disable + wait only for the tracing one here
>>> + */
>>> +
>>> + nsecs = os_timer_disable();
>>> + os_idle_sleep(nsecs);
>>> + os_timer_set_interval(NULL);
>> This is wrong as nsec is time to next timer firing. You just used two
>> syscalls to achieve the same effect as leaving the timer lapse while
>> going to sleep. See above.
>>
>>> local_irq_enable();
>>> }
>>>
>>> diff --git a/arch/um/kernel/skas/clone.c b/arch/um/kernel/skas/clone.c
>>> index 289771d..c1cdc2e 100644
>>> --- a/arch/um/kernel/skas/clone.c
>>> +++ b/arch/um/kernel/skas/clone.c
>>> @@ -35,10 +35,11 @@ stub_clone_handler(void)
>>> if (err)
>>> goto out;
>>>
>>> - err = stub_syscall3(__NR_setitimer, ITIMER_VIRTUAL,
>>> - (long) &data->timer, 0);
>>> - if (err)
>>> - goto out;
>>> +// WHY? FIXME: Switch to timer_create, timer_settime needed?!
>>> +// err = stub_syscall3(__NR_setitimer, ITIMER_VIRTUAL,
>>> +// (long) &data->timer, 0);
>>> +// if (err)
>>> +// goto out;
>> Beats me - if you do not do it, userspace stalls. Though I see you do it
>> now in the tramp. I need to test that :)
> I'm not sure about that yet.
> I really need to figure out when above code is called. Sadly I seem to hit
> some assertion in gdb and this kills my gdb session :-(
> I'll look into the gdb code to understand what assert it is hitting. Did you
> see similar problems while debugging the uml kernel executable?
I ended up debugging like a caveman - with printk()s. GDB is unusable
half of the time.
>
> I think start_userspace is one way of creating a new userspace process, maybe
> there are other ways to reach above code from a clone call that will start a
> new user space process?! That would be the only thing that would make
> sense... If so an posix interval timer need to be created in above code.
I never understood this part. Richard?
>
>>> +/**
>>> + * userspace() - user space control loop
>>> + * @regs: ?
>>> + *
>>> + * The main loop that traces and controls each spwaned userspace
>>> + * process, i.e.
>>> + */
>>> void userspace(struct uml_pt_regs *regs)
>>> {
>>> - struct itimerval timer;
>>> - unsigned long long nsecs, now;
>>> + unsigned long long nsecs;
>>> int err, status, op, pid = userspace_pid[0];
>>> /* To prevent races if using_sysemu changes under us.*/
>>> int local_using_sysemu;
>>> @@ -325,13 +371,11 @@ void userspace(struct uml_pt_regs *regs)
>>> /* Handle any immediate reschedules or signals */
>>> interrupt_end();
>>>
>>> - if (getitimer(ITIMER_VIRTUAL, &timer))
>>> - printk(UM_KERN_ERR "Failed to get itimer, errno = %d\n", errno);
>>> - nsecs = timer.it_value.tv_sec * UM_NSEC_PER_SEC +
>>> - timer.it_value.tv_usec * UM_NSEC_PER_USEC;
>>> - nsecs += os_nsecs();
>>> -
>>> while (1) {
>>> +
>>> + nsecs = os_timer_remain(NULL);
>>> + nsecs += os_nsecs();
>>> +
>>> /*
>>> * This can legitimately fail if the process loads a
>>> * bogus value into a segment register. It will
>>> @@ -388,31 +432,27 @@ void userspace(struct uml_pt_regs *regs)
>>> switch (sig) {
>>> case SIGSEGV:
>>> if (PTRACE_FULL_FAULTINFO) {
>>> - get_skas_faultinfo(pid,
>>> - ®s->faultinfo);
>>> - (*sig_info[SIGSEGV])(SIGSEGV, (struct siginfo *)&si,
>>> - regs);
>>> + get_skas_faultinfo(pid,®s->faultinfo);
>>> + (*sig_info[SIGSEGV])(SIGSEGV, (struct siginfo *)&si,
>>> regs);
>>> + } else {
>>> + handle_segv(pid, regs);
>>> }
>>> - else handle_segv(pid, regs);
>>> break;
>>> case SIGTRAP + 0x80:
>>> - handle_trap(pid, regs, local_using_sysemu);
>>> + handle_trap(pid, regs, local_using_sysemu);
>>> break;
>>> case SIGTRAP:
>>> relay_signal(SIGTRAP, (struct siginfo *)&si, regs);
>>> break;
>>> - case SIGVTALRM:
>>> - now = os_nsecs();
>>> - if (now < nsecs)
>>> + case SIGUSR2:
>> We need to look into this one - it was here because VTALRM can produce a
>> significant difference between the time the kernel one fires and the
>> userspace fires. If the kernel one uses MONOTONIC to an exact time and
>> the userspace uses MONOTONIC to pace it to firings from "interval" sets
>> you end up with HZ scheduling and HZ timer precision instead of the
>> scheduler and userspace timers as we know it in today's Linux.
> Okay, yes I understand now.
>
> But currently the user space posix interval timers are not created
> synchronised compared to the kernel interval timer. So there is very likely a
> gap between the firing of all timers.
> Creating all timer to fire in HZ rate at exactly the same time is the correct
> thing to do in my opinion. What do you think?
>
>
> Do we need to take care of CONFIG_NO_HZ_IDLE AND CONFIG_NO_HZ_FULL?!
Need to read them in another architecture to compare. They are new
(relative to the time the patch was done).
A.
>
> With kind regards
> Thomas
>
>
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