Re: LTTng "TIF_KERNEL_TRACE"

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Ingo Molnar (mingo@elte.hu) wrote:
> 
> * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> 
> > Hi Ingo,
> > 
> > Looking at the current -tip tree, I notice that the 
> > TIF_SYSCALL_FTRACE flag is only implemented for x86.
> > 
> > I have TIF_KERNEL_TRACE in my lttng tree which applies to all 
> > architectures to do the exact same thing :
> > 
> > lttng-kernel-trace-thread-flag-alpha.patch
> > lttng-kernel-trace-thread-flag-arm.patch
> > lttng-kernel-trace-thread-flag-avr32.patch
> > lttng-kernel-trace-thread-flag-blackfin.patch
> > lttng-kernel-trace-thread-flag-cris.patch
> > lttng-kernel-trace-thread-flag-frv.patch
> > lttng-kernel-trace-thread-flag-h8300.patch
> > lttng-kernel-trace-thread-flag-ia64.patch
> > lttng-kernel-trace-thread-flag-m32r.patch
> > lttng-kernel-trace-thread-flag-m68k.patch
> > lttng-kernel-trace-thread-flag-mips.patch
> > lttng-kernel-trace-thread-flag-parisc.patch
> > lttng-kernel-trace-thread-flag-powerpc.patch
> > lttng-kernel-trace-thread-flag-s390.patch
> > lttng-kernel-trace-thread-flag-sh.patch
> > lttng-kernel-trace-thread-flag-sparc.patch
> > lttng-kernel-trace-thread-flag-um.patch
> > lttng-kernel-trace-thread-flag-x86.patch
> > lttng-kernel-trace-thread-flag-xtensa.patch
> > lttng-kernel-trace-thread-flag-api.patch
> > 
> > Is there any way we could get this merged ?
> > 
> > One thing I like about the name TIF_KERNEL_TRACE compared to 
> > TIF_SYSCALL_FTRACE is that it gives us a per-thread flag that 
> > could eventually be used for more kernel tracing purposes than 
> > just syscalls.
> 
> Yeah - TIF_KERNEL_TRACE indeed sounds more descriptive and less 
> restrictive. TIF_SYSCALL_FTRACE was a bit ad-hoc.
> 

Second question :

LTTng :
        read_lock(&tasklist_lock);
        do_each_thread(p, t) {
                set_tsk_thread_flag(t, TIF_KERNEL_TRACE);
        } while_each_thread(p, t);
        read_unlock(&tasklist_lock);

Ftrace:
        read_lock_irqsave(&tasklist_lock, flags);

        do_each_thread(g, t) {
                clear_tsk_thread_flag(t, TIF_SYSCALL_FTRACE);
        } while_each_thread(g, t);

        read_unlock_irqrestore(&tasklist_lock, flags);

With or without irqsave ?

Arguments against irqsave for this read lock :

- it's not used consistently for this read lock all over the kernel.
  Sometimes the read lock is taken without irqsave.
- it can be a long iteration, and therefore disables interrupts for a
  long time.

Arguments for irqsave for this read lock :

- Taking any kind of spin/rwlock with inconsistent irq disabling leads
  to races where interrupts can be disabled for an unbounded amount of
  time if a spinlock with irqoff waits on a spinlock with irqs on. This
  is a general problem with current kernel rwlock usage. See my
  "priority sifting reader-writer lock" patchset for a fix to this
  problem.

Mathieu

> 	Ingo

-- 
Mathieu Desnoyers
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